Fortran Extensions supported by Flang¶

As a general principle, this compiler will accept by default and without complaint many legacy features, extensions to the standard language, and features that have been deleted from the standard, so long as the recognition of those features would not cause a standard-conforming program to be rejected or misinterpreted.

Other non-standard features, which do conflict with the current standard specification of the Fortran programming language, are accepted if enabled by command-line options.

Intentional violations of the standard ¶

Scalar INTEGER actual argument expressions (not variables!) are converted to the kinds of scalar INTEGER dummy arguments when the interface is explicit and the kinds differ. This conversion allows the results of the intrinsics like SIZE that (as mentioned below) may return non-default INTEGER results by default to be passed. A warning is emitted when truncation is possible. These conversions are not applied in calls to non-intrinsic generic procedures.
We are not strict on the contents of BLOCK DATA subprograms so long as they contain no executable code, no internal subprograms, and allocate no storage outside a named COMMON block. (C1415)
Delimited list-directed (and NAMELIST) character output is required to emit contiguous doubled instances of the delimiter character when it appears in the output value. When fixed-size records are being emitted, as is the case with internal output, this is not possible when the problematic character falls on the last position of a record. No two other Fortran compilers do the same thing in this situation so there is no good precedent to follow. Because it seems least wrong, we emit one copy of the delimiter as the last character of the current record and another as the first character of the next record. (The second-least-wrong alternative might be to flag a runtime error, but that seems harsh since it’s not an explicit error in the standard, and the output may not have to be usable later as input anyway.) Consequently, the output is not suitable for use as list-directed or NAMELIST input. If a later standard were to clarify this case, this behavior will change as needed to conform.

character(11) :: buffer(3)
character(10) :: quotes = '""""""""""'
write(buffer,*,delim="QUOTE") quotes
print "('>',a10,'<')", buffer
end

The name of the control variable in an implied DO loop in an array constructor or DATA statement has a scope over the value-list only, not the bounds of the implied DO loop. It is not advisable to use an object of the same name as the index variable in a bounds expression, but it will work, instead of being needlessly undefined.
If both the COUNT= and the COUNT_MAX= optional arguments are present on the same call to the intrinsic subroutine SYSTEM_CLOCK, we require that their types have the same integer kind, since the kind of these arguments is used to select the clock rate. In common with some other compilers, the clock rate varies from tenths of a second to nanoseconds depending on argument kind and platform support.
If a dimension of a descriptor has zero extent in a call to CFI_section, CFI_setpointer or CFI_allocate, the lower bound on that dimension will be set to 1 for consistency with the LBOUND() intrinsic function.
-2147483648_4 is, strictly speaking, a non-conforming literal constant on a machine with 32-bit two’s-complement integers as kind 4, because the grammar of Fortran expressions parses it as a negation of a literal constant, not a negative literal constant. This compiler accepts it with a portability warning.
Construct names like loop in loop: do j=1,n are defined to be “local identifiers” and should be distinct in the “inclusive scope” – i.e., not scoped by BLOCK constructs. As most (but not all) compilers implement BLOCK scoping of construct names, so does f18, with a portability warning.
15.6.4 paragraph 2 prohibits an implicitly typed statement function from sharing the same name as a symbol in its scope’s host, if it has one. We accept this usage with a portability warning.
A module name from a USE statement can also be used as a non-global name in the same scope. This is not conforming, but it is useful and unambiguous.
The argument to RANDOM_NUMBER may not be an assumed-size array.
NULL() without MOLD= is not allowed to be associated as an actual argument corresponding to an assumed-rank dummy argument; its rank in the called procedure would not be well-defined.
When an index variable of a FORALL or DO CONCURRENT is present in the enclosing scope, and the construct does not have an explicit type specification for its index variables, some weird restrictions in F’2023 subclause 19.4 paragraphs 6 & 8 should apply. Since this compiler properly scopes these names, violations of these restrictions elicit only portability warnings by default.
The standard defines the intrinsic functions MOD and MODULO for real arguments using expressions in terms of AINT and FLOOR. These definitions yield fairly poor results due to floating-point cancellation, and every Fortran compiler (including this one) uses better algorithms.
The rules for pairwise distinguishing the specific procedures of a generic interface are inadequate, as admitted in note C.11.6 of F’2023. Generic interfaces whose specific procedures can be easily proven by hand to be pairwise distinct (i.e., no ambiguous reference is possible) appear in real applications, but are still non-conforming under the incomplete tests in F’2023 15.4.3.4.5. These cases are compiled with optional portability warnings.
PROCEDURE(), BIND(C) :: PROC is not conforming, as there is no procedure interface. This compiler accepts it, since there is otherwise no way to declare an interoperable dummy procedure with an arbitrary interface like void (*)().
PURE functions are allowed to have dummy arguments that are neither INTENT(IN) nor VALUE, similar to PURE subroutines, with a warning. This enables atomic memory operations to be naturally represented as PURE functions, which allows their use in parallel constructs and DO CONCURRENT.
A non-definable actual argument, including the case of a vector subscript, may be associated with an ASYNCHRONOUS or VOLATILE dummy argument, F’2023 15.5.2.5 p31 notwithstanding. The effects of these attributes are scoped over the lifetime of the procedure reference, and they can by added by internal subprograms and BLOCK constructs within the procedure. Further, a dummy argument can acquire the ASYNCHRONOUS attribute implicitly simply appearing in an asynchronous data transfer statement, without the attribute being visible in the procedure’s explicit interface.
When the name of an extended derived type’s base type is the result of USE association with renaming, the name of the extended derived type’s parent component is the new name by which the base is known in the scope of the extended derived type, not the original. This interpretation has usability advantages and is what six other Fortran compilers do, but is not conforming now that J3 approved an “interp” in June 2024 to the contrary.
Arm has processors that allow a user to control what happens when an arithmetic exception is signaled, as well as processors that do not have this capability. An Arm executable will run on either type of processor, so it is effectively unknown at compile time whether or not this support will be available at runtime. The standard requires that a call to intrinsic module procedure IEEE_SUPPORT_HALTING with a constant argument has a compile time constant result in constant expression and specification expression contexts. In compilations where this information is not known at compile time, f18 generates code to determine the absence or presence of this capability at runtime. A call to IEEE_SUPPORT_HALTING in contexts that the standard requires to be constant will generate a compilation error.

Extensions, deletions, and legacy features supported by default ¶

Tabs in source
<> as synonym for .NE. and /=
$ and @ as legal characters in names
Initialization in type declaration statements using /values/
Saved variables without explicit or default initializers are zero initialized.
In a saved entity of a type with a default initializer, components without default values are zero initialized.
Kind specification with *, e.g. REAL*4
DOUBLE COMPLEX as a synonym for COMPLEX(KIND(0.D0)) – but not when spelled TYPE(DOUBLECOMPLEX).
Signed complex literal constants
DEC STRUCTURE, RECORD, with ‘%FILL’; but UNION, and MAP are not yet supported throughout compilation, and elicit a “not yet implemented” message.
Structure field access with .field
BYTE as synonym for INTEGER(KIND=1); but not when spelled TYPE(BYTE).
When kind-param is used for REAL literals, allow a matching exponent letter
Quad precision REAL literals with Q
X prefix/suffix as synonym for Z on hexadecimal literals
B, O, Z, and X accepted as suffixes as well as prefixes
Support for using bare L in FORMAT statement
Triplets allowed in array constructors
%LOC, %VAL, and %REF
Leading comma allowed before I/O item list
Empty parentheses allowed in PROGRAM P()
Missing parentheses allowed in FUNCTION F
Cray based POINTER(p,x) and LOC() intrinsic (with %LOC() as an alias)
Arithmetic IF. (Which branch should NaN take? Fall through?)
ASSIGN statement, assigned GO TO, and assigned format
PAUSE statement
Hollerith literals and edit descriptors
NAMELIST allowed in the execution part
Omitted colons on type declaration statements with attributes
COMPLEX constructor expression, e.g. (x+y,z)
+ and - before all primary expressions, e.g. x*-y
.NOT. .NOT. accepted
NAME= as synonym for FILE=
Data edit descriptors without width or other details
D lines in fixed form as comments or debug code
CARRIAGECONTROL= on the OPEN and INQUIRE statements
CONVERT= on the OPEN and INQUIRE statements
DISPOSE= on the OPEN and INQUIRE statements
Leading semicolons are ignored before any statement that could have a label
The character & in column 1 in fixed form source is a variant form of continuation line.
Character literals as elements of an array constructor without an explicit type specifier need not have the same length; the longest literal determines the length parameter of the implicit type, not the first.
Outside a character literal, a comment after a continuation marker (&) need not begin with a comment marker (!).
Classic C-style /comments/ are skipped, so multi-language header files are easier to write and use.
$ and \ edit descriptors are supported in FORMAT to suppress newline output on user prompts.
Tabs in format strings (not FORMAT statements) are allowed on output.
REAL and DOUBLE PRECISION variable and bounds in DO loops
Integer literals without explicit kind specifiers that are out of range for the default kind of INTEGER are assumed to have the least larger kind that can hold them, if one exists.
BOZ literals can be used as INTEGER values in contexts where the type is unambiguous: the right hand sides of assignments and initializations of INTEGER entities, as actual arguments to a few intrinsic functions (ACHAR, BTEST, CHAR), and as actual arguments of references to procedures with explicit interfaces whose corresponding dummy argument has a numeric type to which the BOZ literal may be converted. BOZ literals are interpreted as default INTEGER only when they appear as the first items of array constructors with no explicit type. Otherwise, they generally cannot be used if the type would not be known (e.g., IAND(X'1',X'2'), or as arguments of DIM, MOD, MODULO, and SIGN. Note that while other compilers may accept such usages, the type resolution of such BOZ literals usages is highly non portable).
BOZ literals can also be used as REAL values in some contexts where the type is unambiguous, such as initializations of REAL parameters.
EQUIVALENCE of numeric and character sequences (a ubiquitous extension), as well as of sequences of non-default kinds of numeric types with each other.
Values for whole anonymous parent components in structure constructors (e.g., EXTENDEDTYPE(PARENTTYPE(1,2,3)) rather than EXTENDEDTYPE(1,2,3) or EXTENDEDTYPE(PARENTTYPE=PARENTTYPE(1,2,3))).
Some intrinsic functions are specified in the standard as requiring the same type and kind for their arguments (viz., ATAN with two arguments, ATAN2, DIM, HYPOT, IAND, IEOR, IOR, MAX, MIN, MOD, and MODULO); we allow distinct types to be used, promoting the arguments as if they were operands to an intrinsic + operator, and defining the result type accordingly.
DOUBLE COMPLEX intrinsics DREAL, DCMPLX, DCONJG, and DIMAG.
The DFLOAT intrinsic function.
INT_PTR_KIND intrinsic returns the kind of c_intptr_t.
Restricted specific conversion intrinsics FLOAT, SNGL, IDINT, IFIX, DREAL, and DCMPLX accept arguments of any kind instead of only the default kind or double precision kind. Their result kinds remain as specified.
Specific intrinsics AMAX0, AMAX1, AMIN0, AMIN1, DMAX1, DMIN1, MAX0, MAX1, MIN0, and MIN1 accept more argument types than specified. They are replaced by the related generics followed by conversions to the specified result types.
When a scalar CHARACTER actual argument of the same kind is known to have a length shorter than the associated dummy argument, it is extended on the right with blanks, similar to assignment.
When a dummy argument is POINTER or ALLOCATABLE and is INTENT(IN), we relax enforcement of some requirements on actual arguments that must otherwise hold true for definable arguments.
We allow a limited polymorphic POINTER or ALLOCATABLE actual argument to be associated with a compatible monomorphic dummy argument, as our implementation, like others, supports a reallocation that would change the dynamic type
Assignment of LOGICAL to INTEGER and vice versa (but not other types) is allowed. The values are normalized to canonical .TRUE./.FALSE.. The values are also normalized for assignments of LOGICAL(KIND=K1) to LOGICAL(KIND=K2), when K1 != K2.
Static initialization of LOGICAL with INTEGER is allowed in DATA statements and object initializers. The results are not normalized to canonical .TRUE./.FALSE.. Static initialization of INTEGER with LOGICAL is also permitted.
An effectively empty source file (no program unit) is accepted and produces an empty relocatable output file.
A RETURN statement may appear in a main program.
DATA statement initialization is allowed for procedure pointers outside structure constructors.
Nonstandard intrinsic functions: ISNAN, SIZEOF
A forward reference to a default INTEGER scalar dummy argument or COMMON block variable is permitted to appear in a specification expression, such as an array bound, in a scope with IMPLICIT NONE(TYPE) if the name of the variable would have caused it to be implicitly typed as default INTEGER if IMPLICIT NONE(TYPE) were absent.
OPEN(ACCESS=’APPEND’) is interpreted as OPEN(POSITION=’APPEND’) to ease porting from Sun Fortran.
Intrinsic subroutines EXIT([status]) and ABORT()
The definition of simple contiguity in 9.5.4 applies only to arrays; we also treat scalars as being trivially contiguous, so that they can be used in contexts like data targets in pointer assignments with bounds remapping.
The CONTIGUOUS attribute can be redundantly applied to simply contiguous objects, including scalars, with a portability warning.
We support some combinations of specific procedures in generic interfaces that a strict reading of the standard would preclude when their calls must nonetheless be distinguishable. Specifically, ALLOCATABLE dummy arguments are distinguishing if an actual argument acceptable to one could not be passed to the other & vice versa because exactly one is polymorphic or exactly one is unlimited polymorphic).
External unit 0 is predefined and connected to the standard error output, and defined as ERROR_UNIT in the intrinsic ISO_FORTRAN_ENV module.
Objects in blank COMMON may be initialized.
Initialization of COMMON blocks outside of BLOCK DATA subprograms.
Multiple specifications of the SAVE attribute on the same object are allowed, with a warning.
Specific intrinsic functions BABS, IIABS, JIABS, KIABS, ZABS, and CDABS.
A POINTER component’s type need not be a sequence type when the component appears in a derived type with SEQUENCE. (This case should probably be an exception to constraint C740 in the standard.)
Format expressions that have type but are not character and not integer scalars are accepted so long as they are simply contiguous. This legacy extension supports pre-Fortran’77 usage in which variables initialized in DATA statements with Hollerith literals as modifiable formats.
At runtime, NAMELIST input will skip over NAMELIST groups with other names, and will treat text before and between groups as if they were comment lines, even if not begun with !.
Commas are required in FORMAT statements and character variables only when they prevent ambiguity.
Legacy names AND, OR, and XOR are accepted as aliases for the standard intrinsic functions IAND, IOR, and IEOR respectively.
A digit count of d=0 is accepted in Ew.0, Dw.0, and Gw.0 output editing if no nonzero scale factor (kP) is in effect.
The name IMAG is accepted as an alias for the generic intrinsic function AIMAG.
The legacy extension intrinsic functions IZEXT and JZEXT are supported; ZEXT has different behavior with various older compilers, so it is not supported.
f18 doesn’t impose a limit on the number of continuation lines allowed for a single statement.
When a type-bound procedure declaration statement has neither interface nor attributes, the “::” before the bindings is optional, even if a binding has renaming with “=> proc”. The colons are not necessary for an unambiguous parse, C768 notwithstanding.
A type-bound procedure binding can be passed as an actual argument corresponding to a dummy procedure and can be used as the target of a procedure pointer assignment statement.
An explicit INTERFACE can declare the interface of a procedure pointer even if it is not a dummy argument.
A NOPASS type-bound procedure binding is required by C1529 to apply only to a scalar data-ref, but most compilers don’t enforce it and the constraint is not necessary for a correct implementation.
A label may follow a semicolon in fixed form source.
A logical dummy argument to a BIND(C) procedure, or a logical component to a BIND(C) derived type does not have to have KIND=C_BOOL since it can be converted to/from _Bool without loss of information.
The character length of the SOURCE= or MOLD= in ALLOCATE may be distinct from the constant character length, if any, of an allocated object.
When a name is brought into a scope by multiple ways, such as USE-association as well as an IMPORT from its host, it’s an error only if the resolution is ambiguous.
An entity may appear in a DATA statement before its explicit type declaration under IMPLICIT NONE(TYPE).
INCLUDE lines can start in any column, can be preceded in fixed form source by a ‘0’ in column 6, can contain spaces between the letters of the word INCLUDE, and can have a numeric character literal kind prefix on the file name.
Intrinsic procedures SIND, COSD, TAND and ATAND. Constant folding is currently not supported for these procedures but this is planned.
When a pair of quotation marks in a character literal are split by a line continuation in free form, the second quotation mark may appear at the beginning of the continuation line without an ampersand, althought one is required by the standard.
Unrestricted INTRINSIC functions are accepted for use in PROCEDURE statements in generic interfaces, as in some other compilers.
A NULL() pointer is treated as an unallocated allocatable when associated with an INTENT(IN) allocatable dummy argument.
READ(..., SIZE=n) is accepted with NML= and FMT=* with a portability warning. The Fortran standard doesn’t allow SIZE= with formatted input modes that might require look-ahead, perhaps to ease implementations.
When a file included via an INCLUDE line or #include directive has a continuation marker at the end of its last line in free form, Fortran line continuation works.
A NAMELIST input group may omit its trailing / character if it is followed by another NAMELIST input group.
A NAMELIST input group may begin with either & or $.
A comma in a fixed-width numeric input field terminates the field rather than signaling an invalid character error.
Arguments to the intrinsic functions MAX and MIN are converted when necessary to the type of the result. An OPTIONAL, POINTER, or ALLOCATABLE argument after the first two cannot be converted, as it may not be present.
A derived type that meets (most of) the requirements of an interoperable derived type can be used as such where an interoperable type is required, with warnings, even if it lacks the BIND(C) attribute.
A “mult-operand” in an expression can be preceded by a unary + or - operator.
BIND(C, NAME="...", CDEFINED) signifies that the storage for an interoperable variable will be allocated outside of Fortran, probably by a C or C++ external definition.
An automatic data object may be declared in the specification part of the main program.
A local data object may appear in a specification expression, even when it is not a dummy argument or in COMMON, so long as it is has the SAVE attribute and was initialized.
PRINT namelistname is accepted and interpreted as WRITE(*,NML=namelistname), a near-universal extension.
A character length specifier in a component or entity declaration is accepted before an array specification (ch*3(2)) as well as afterwards.

Extensions supported when enabled by options ¶

C-style backslash escape sequences in quoted CHARACTER literals (but not Hollerith) [-fbackslash], including Unicode escapes with \U.
Logical abbreviations .T., .F., .N., .A., .O., and .X. [-flogical-abbreviations]
.XOR. as a synonym for .NEQV. [-fxor-operator]
The default INTEGER type is required by the standard to occupy the same amount of storage as the default REAL type. Default REAL is of course 32-bit IEEE-754 floating-point today. This legacy rule imposes an artificially small constraint in some cases where Fortran mandates that something have the default INTEGER type: specifically, the results of references to the intrinsic functions SIZE, STORAGE_SIZE,LBOUND, UBOUND, SHAPE, and the location reductions FINDLOC, MAXLOC, and MINLOC in the absence of an explicit KIND= actual argument. We return INTEGER(KIND=8) by default in these cases when the -flarge-sizes option is enabled. SIZEOF and C_SIZEOF always return INTEGER(KIND=8).
Treat each specification-part like is has IMPLICIT NONE [-fimplicit-none-type-always]
Ignore occurrences of IMPLICIT NONE and IMPLICIT NONE(TYPE) [-fimplicit-none-type-never]
Old-style PARAMETER pi=3.14 statement without parentheses [-falternative-parameter-statement]
UNSIGNED type (-funsigned)

Extensions and legacy features deliberately not supported ¶

.LG. as synonym for .NE.
REDIMENSION
Allocatable COMMON
Expressions in formats
ACCEPT as synonym for READ *
TYPE as synonym for PRINT
ARRAY as synonym for DIMENSION
VIRTUAL as synonym for DIMENSION
ENCODE and DECODE as synonyms for internal I/O
IMPLICIT AUTOMATIC, IMPLICIT STATIC
Default exponent of zero, e.g. 3.14159E
Characters in defined operators that are neither letters nor digits
B suffix on unquoted octal constants
Z prefix on unquoted hexadecimal constants (dangerous)
T and F as abbreviations for .TRUE. and .FALSE. in DATA (PGI/XLF)
Use of host FORMAT labels in internal subprograms (PGI-only feature)
ALLOCATE(TYPE(derived)::…) as variant of correct ALLOCATE(derived::…) (PGI only)
Defining an explicit interface for a subprogram within itself (PGI only)
USE association of a procedure interface within that same procedure’s definition
NULL() as a structure constructor expression for an ALLOCATABLE component (PGI).
Conversion of LOGICAL to INTEGER in expressions.
Use of INTEGER data with the intrinsic logical operators .NOT., .AND., .OR., and .XOR..
IF (integer expression) THEN … END IF (PGI/Intel)
Comparison of LOGICAL with ==/.EQ. rather than .EQV. (also .NEQV.) (PGI/Intel)
Procedure pointers in COMMON blocks (PGI/Intel)
Underindexing multi-dimensional arrays (e.g., A(1) rather than A(1,1)) (PGI only)
Legacy PGI NCHARACTER type and NC Kanji character literals
Using non-integer expressions for array bounds (e.g., REAL A(3.14159)) (PGI/Intel)
Mixing INTEGER types as operands to bit intrinsics (e.g., IAND); only two compilers support it, and they disagree on sign extension.
Module & program names that conflict with an object inside the unit (PGI only).
When the same name is brought into scope via USE association from multiple modules, the name must refer to a generic interface; PGI allows a name to be a procedure from one module and a generic interface from another.
Type parameter declarations must come first in a derived type definition; some compilers allow them to follow PRIVATE, or be intermixed with the component declarations.
Wrong argument types in calls to specific intrinsics that have different names than the related generics. Some accepted exceptions are listed above in the allowed extensions. PGI, Intel, and XLF support this in ways that are not numerically equivalent. PGI converts the arguments while Intel and XLF replace the specific by the related generic.
VMS listing control directives (%LIST, %NOLIST, %EJECT)
Continuation lines on INCLUDE lines
NULL() actual argument corresponding to an ALLOCATABLE dummy data object
User (non-intrinsic) ELEMENTAL procedures may not be passed as actual arguments, in accordance with the standard; some Fortran compilers permit such usage.
Constraint C1406, which prohibits the same module name from being used in a scope for both an intrinsic and a non-intrinsic module, is implemented as a portability warning only, not a hard error.
IBM @PROCESS directive is accepted but ignored.

Preprocessing behavior ¶

The preprocessor is always run, whatever the filename extension may be.
We respect Fortran comments in macro actual arguments (like GNU, Intel, NAG; unlike PGI and XLF) on the principle that macro calls should be treated like function references. Fortran’s line continuation methods also work.

Standard features not silently accepted ¶

Fortran explicitly ignores type declaration statements when they attempt to type the name of a generic intrinsic function (8.2 p3). One can declare CHARACTER::COS and still get a real result from COS(3.14159), for example. f18 will complain when a generic intrinsic function’s inferred result type does not match an explicit declaration. This message is a warning.

Standard features that might as well not be ¶

f18 supports designators with constant expressions, properly constrained, as initial data targets for data pointers in initializers of variable and component declarations and in DATA statements; e.g., REAL, POINTER :: P => T(1:10:2). This Fortran 2008 feature might as well be viewed like an extension; no other compiler that we’ve tested can handle it yet.
According to 11.1.3.3p1, if a selector of an ASSOCIATE or related construct is defined by a variable, it has the TARGET attribute if the variable was a POINTER or TARGET. We read this to include the case of the variable being a pointer-valued function reference. No other Fortran compiler seems to handle this correctly for ASSOCIATE, though NAG gets it right for SELECT TYPE.
The standard doesn’t explicitly require that a named constant that appears as part of a complex-literal-constant be a scalar, but most compilers emit an error when an array appears. f18 supports them with a portability warning.
f18 does not enforce a blanket prohibition against generic interfaces containing a mixture of functions and subroutines. We allow both to appear, unlike several other Fortran compilers. This is especially desirable when two generics of the same name are combined due to USE association and the mixture may be inadvertent.
Since Fortran 90, INCLUDE lines have been allowed to have a numeric kind parameter prefix on the file name. No other Fortran compiler supports them that I can find.
A SEQUENCE derived type is required (F’2023 C745) to have at least one component. No compiler enforces this constraint; this compiler emits a warning.
Many compilers disallow a VALUE assumed-length character dummy argument, which has been standard since F’2008. We accept this usage with an optional portability warning.
The ASYNCHRONOUS attribute can be implied by usage in data transfer I/O statements. Only one other compiler supports this correctly. This compiler does, apart from objects in asynchronous NAMELIST I/O, for which an actual asynchronous runtime implementation seems unlikely.

Behavior in cases where the standard is ambiguous or indefinite ¶

When an inner procedure of a subprogram uses the value or an attribute of an undeclared name in a specification expression and that name does not appear in the host, it is not clear in the standard whether that name is an implicitly typed local variable of the inner procedure or a host association with an implicitly typed local variable of the host. For example:

module module
 contains
  subroutine host(j)
    ! Although "m" never appears in the specification or executable
    ! parts of this subroutine, both of its contained subroutines
    ! might be accessing it via host association.
    integer, intent(in out) :: j
    call inner1(j)
    call inner2(j)
   contains
    subroutine inner1(n)
      integer(kind(m)), intent(in) :: n
      m = n + 1
    end subroutine
    subroutine inner2(n)
      integer(kind(m)), intent(out) :: n
      n = m + 2
    end subroutine
  end subroutine
end module

program demo
  use module
  integer :: k
  k = 0
  call host(k)
  print *, k, " should be 3"
end

Other Fortran compilers disagree in their interpretations of this example; some seem to treat the references to m as if they were host associations to an implicitly typed variable (and print 3), while others seem to treat them as references to implicitly typed local variables, and load uninitialized values.

In f18, we chose to emit an error message for this case since the standard is unclear, the usage is not portable, and the issue can be easily resolved by adding a declaration.

In subclause 7.5.6.2 of Fortran 2018 the standard defines a partial ordering of the final subroutine calls for finalizable objects, their non-parent components, and then their parent components. (The object is finalized, then the non-parent components of each element, and then the parent component.) Some have argued that the standard permits an implementation to finalize the parent component before finalizing an allocatable component in the context of deallocation, and the next revision of the language may codify this option. In the interest of avoiding needless confusion, this compiler implements what we believe to be the least surprising order of finalization. Specifically: all non-parent components are finalized before the parent, allocatable or not; all finalization takes place before any deallocation; and no object or subobject will be finalized more than once.
When RECL= is set via the OPEN statement for a sequential formatted input file, it functions as an effective maximum record length. Longer records, if any, will appear as if they had been truncated to the value of RECL=. (Other compilers ignore RECL=, signal an error, or apply effective truncation to some forms of input in this situation.) For sequential formatted output, RECL= serves as a limit on record lengths that raises an error when it is exceeded.
When a DATA statement in a BLOCK construct could be construed as either initializing a host-associated object or declaring a new local initialized object, f18 interprets the standard’s classification of a DATA statement as being a “declaration” rather than a “specification” construct, and notes that the BLOCK construct is defined as localizing names that have specifications in the BLOCK construct. So this example will elicit an error about multiple initialization:

subroutine subr
  integer n = 1
  block
    data n/2/
  end block
end subroutine

Other Fortran compilers disagree with each other in their interpretations of this example. The precedent among the most commonly used compilers agrees with f18’s interpretation: a DATA statement without any other specification of the name refers to the host-associated object.

Many Fortran compilers allow a non-generic procedure to be USE-associated into a scope that also contains a generic interface of the same name but does not have the USE-associated non-generic procedure as a specific procedure.

module m1
 contains
  subroutine foo(n)
    integer, intent(in) :: n
  end subroutine
end module

module m2
  use m1, only: foo
  interface foo
    module procedure noargs
  end interface
 contains
  subroutine noargs
  end subroutine
end module

This case elicits a warning from f18, as it should not be treated any differently than the same case with the non-generic procedure of the same name being defined in the same scope rather than being USE-associated into it, which is explicitly non-conforming in the standard and not allowed by most other compilers. If the USE-associated entity of the same name is not a procedure, most compilers disallow it as well.

Fortran 2018 19.3.4p1: “A component name has the scope of its derived-type definition. Outside the type definition, it may also appear …” which seems to imply that within its derived-type definition, a component name is in its scope, and at least shadows any entity of the same name in the enclosing scope and might be read, thanks to the “also”, to mean that a “bare” reference to the name could be used in a specification inquiry. However, most other compilers do not allow a component to shadow exterior symbols, much less appear in specification inquiries, and there are application codes that expect exterior symbols whose names match components to be visible in a derived-type definition’s default initialization expressions, and so f18 follows that precedent.
19.3.1p1 “Within its scope, a local identifier of an entity of class (1) or class (4) shall not be the same as a global identifier used in that scope…” is read so as to allow the name of a module, submodule, main program, or BLOCK DATA subprogram to also be the name of an local entity in its scope, with a portability warning, since that global name is not actually capable of being “used” in its scope.
In the definition of the ASSOCIATED intrinsic function (16.9.16), its optional second argument TARGET= is required to be “allowable as the data-target or proc-target in a pointer assignment statement (10.2.2) in which POINTER is data-pointer-object or proc-pointer-object.” Some Fortran compilers interpret this to require that the first argument (POINTER=) be a valid left-hand side for a pointer assignment statement – in particular, it cannot be NULL(), but also it is required to be modifiable. As there is no good reason to disallow (say) an INTENT(IN) pointer here, or even NULL() as a well-defined case that is always .FALSE., this compiler doesn’t require the POINTER= argument to be a valid left-hand side for a pointer assignment statement, and we emit a portability warning when it is not.
F18 allows a USE statement to reference a module that is defined later in the same compilation unit, so long as mutual dependencies do not form a cycle. This feature forestalls any risk of such a USE statement reading an obsolete module file from a previous compilation and then overwriting that file later.
F18 allows OPTIONAL dummy arguments to interoperable procedures unless they are VALUE (C865).
F18 processes the NAMELIST group declarations in a scope after it has resolved all of the names in that scope. This means that names that appear before their local declarations do not resolve to host associated objects and do not elicit errors about improper redeclarations of implicitly typed entities.
Standard Fortran allows forward references to derived types, which can lead to ambiguity when combined with host association. Some Fortran compilers resolve the type name to the host type, others to the forward-referenced local type; this compiler diagnoses an error.

module m
  type ambiguous; integer n; end type
 contains
  subroutine s
    type(ambiguous), pointer :: ptr
    type ambiguous; real a; end type
  end
end

When an intrinsic procedure appears in the specification part of a module only in function references, but not an explicit INTRINSIC statement, its name is not brought into other scopes by a USE statement.
The subclause on rounding in formatted I/O (13.7.2.3.8 in F’2023) only discusses rounding for decimal-to/from-binary conversions, omitting any mention of rounding for hexadecimal conversions. As other compilers do apply rounding, so does this one.
For real MAXVAL, MINVAL, MAXLOC, and MINLOC, NaN values are essentially ignored unless there are some unmasked array entries and all of them are NaNs.
When INDEX is used as an unrestricted specific intrinsic function in the context of an actual procedure, as the explicit interface in a PROCEDURE declaration statement, or as the target of a procedure pointer assignment, its interface has exactly two dummy arguments (STRING= and SUBSTRING=), and includes neither BACK= nor KIND=. This is how INDEX as an unrestricted specific intrinsic function was documented in FORTRAN ‘77 and Fortran ‘90; later revisions of the standard deleted the argument information from the section on unrestricted specific intrinsic functions. At least one other compiler (XLF) seems to expect that the interface for INDEX include an optional BACK= argument, but it doesn’t actually work.
Allocatable components of array and structure constructors are deallocated after use without calling final subroutines. The standard does not specify when and how deallocation of array and structure constructors allocatable components should happen. All compilers free the memory after use, but the behavior when the allocatable component is a derived type with finalization differ, especially when dealing with nested array and structure constructors expressions. Some compilers call final routine for the allocatable components of each constructor sub-expressions, some call it only for the allocatable component of the top level constructor, and some only deallocate the memory. Deallocating only the memory offers the most flexibility when lowering such expressions, and it is not clear finalization is desirable in such context (Fortran interop 1.6.2 in F2018 standards require array and structure constructors not to be finalized, so it also makes sense not to finalize their allocatable components when releasing their storage).
F’2023 19.4 paragraph 5: “If integer-type-spec appears in data-implied-do or ac-implied-do-control it has the specified type and type parameters; otherwise it has the type and type parameters that it would have if it were the name of a variable in the innermost executable construct or scoping unit that includes the DATA statement or array constructor, and this type shall be integer type.” Reading “would have if it were” as being the subjunctive, this would mean that an untyped implied DO index variable should be implicitly typed according to the rules active in the enclosing scope. But all other Fortran compilers interpret the “would have if it were” as meaning “has if it is” – i.e., if the name is visible in the enclosing scope, the type of that name is used as the type of the implied DO index. So this is an error, not a simple application of the default implicit typing rule:

character j
print *, [(j,j=1,10)]

The Fortran standard doesn’t mention integer overflow explicitly. In many cases, however, integer overflow makes programs non-conforming. F18 follows other widely-used Fortran compilers. Specifically, f18 assumes integer overflow never occurs in address calculations and increment of do-variable unless the option -fwrapv is enabled.

De Facto Standard Features ¶

EXTENDS_TYPE_OF() returns .TRUE. if both of its arguments have the same type, a case that is technically implementation-defined.
ENCODING= is not in the list of changeable modes on an I/O unit, but every Fortran compiler allows the encoding to be changed on an open unit.
A NAMELIST input item that references a scalar element of a vector or contiguous array can be used as the initial element of a storage sequence. For example, “&GRP A(1)=1. 2. 3./” is treated as if had been “&GRP A(1:)=1. 2. 3./”.