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# Fortran Extensions supported by Flang
```{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.
## 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.
## 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.
### 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]
### 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)]
```
## 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./".
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