.. contents:: :local:
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.
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.BLOCK DATA subprograms so long as they contain no executable code, no internal subprograms, and allocate no storage outside a named COMMON block. (C1415)character(11) :: buffer(3)
character(10) :: quotes = '""""""""""'
write(buffer,*,delim="QUOTE") quotes
print "('>',a10,'<')", buffer
end
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.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.<> as synonym for .NE. and /=$ and @ as legal characters in names/values/*, e.g. REAL*4DOUBLE COMPLEX as a synonym for COMPLEX(KIND(0.D0)) -- but not when spelled TYPE(DOUBLECOMPLEX).STRUCTURE, RECORD, with ‘%FILL’; but UNION, and MAP are not yet supported throughout compilation, and elicit a “not yet implemented” message..fieldBYTE as synonym for INTEGER(KIND=1); but not when spelled TYPE(BYTE).QX prefix/suffix as synonym for Z on hexadecimal literalsB, O, Z, and X accepted as suffixes as well as prefixes%LOC, %VAL, and %REFPROGRAM P()FUNCTION FPOINTER(p,x) and LOC() intrinsic (with %LOC() as an alias)IF. (Which branch should NaN take? Fall through?)ASSIGN statement, assigned GO TO, and assigned formatPAUSE statementNAMELIST allowed in the execution part(x+y,z)+ and - before all primary expressions, e.g. x*-y.NOT. .NOT. acceptedNAME= as synonym for FILE=D lines in fixed form as comments or debug codeCARRIAGECONTROL= on the OPEN and INQUIRE statementsCONVERT= on the OPEN and INQUIRE statementsDISPOSE= on the OPEN and INQUIRE statements& in column 1 in fixed form source is a variant form of continuation line.FORMAT statements) are allowed on output.IAND(X'1',X'2')).EXTENDEDTYPE(PARENTTYPE(1,2,3)) rather than EXTENDEDTYPE(1,2,3) or EXTENDEDTYPE(PARENTTYPE=PARENTTYPE(1,2,3))).+ operator, and defining the result type accordingly.POINTER or ALLOCATABLE and is INTENT(IN), we relax enforcement of some requirements on actual arguments that must otherwise hold true for definable arguments.LOGICAL to INTEGER and vice versa (but not other types) is allowed. The values are normalized.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.RETURN statement may appear in a main program.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).ERROR_UNIT in the intrinsic ISO_FORTRAN_ENV module.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.)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 !.AND, OR, and XOR are accepted as aliases for the standard intrinsic functions IAND, IOR, and IEOR respectively.IMAG is accepted as an alias for the generic intrinsic function AIMAG.IZEXT and JZEXT are supported; ZEXT has different behavior with various older compilers, so it is not supported.INTERFACE can declare the interface of a procedure pointer even if it is not a dummy argument.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..T., .F., .N., .A., .O., and .X. [-flogical-abbreviations].XOR. as a synonym for .NEQV. [-fxor-operator]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).IMPLICIT NONE [-fimplicit-none-type-always]IMPLICIT NONE and IMPLICIT NONE(TYPE) [-fimplicit-none-type-never]PARAMETER pi=3.14 statement without parentheses [-falternative-parameter-statement].LG. as synonym for .NE.REDIMENSIONCOMMONACCEPT as synonym for READ *TYPE as synonym for PRINTARRAY as synonym for DIMENSIONVIRTUAL as synonym for DIMENSIONENCODE and DECODE as synonyms for internal I/OIMPLICIT AUTOMATIC, IMPLICIT STATIC3.14159EB suffix on unquoted octal constantsZ prefix on unquoted hexadecimal constants (dangerous)T and F as abbreviations for .TRUE. and .FALSE. in DATA (PGI/XLF).NOT., .AND., .OR., and .XOR..NCHARACTER type and NC Kanji character literalsPRIVATE, or be intermixed with the component declarations.%LIST, %NOLIST, %EJECT)INCLUDE linesNULL() actual argument corresponding to an ALLOCATABLE dummy data objectELEMENTAL procedures may not be passed as actual arguments, in accordance with the standard; some Fortran compilers permit such usage.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.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.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.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 variabless, 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.
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.
EXTENDS_TYPE_OF() returns .TRUE. if both of its arguments have the same type, a case that is technically implementation-defined.