[flang] Make extension explicit: exponent-letter matching kind-param

[ekieri]

Summary

As an extension for REAL literals, we allow an exponent letter which
matches an explicit kind-param. The standard requires the exponent
to be 'E' if a kind-param is present. This patch

• documents this extension in Extensions.md
• enables a portability warning if it is used with -pedantic

The test case for this, kinds05.f90, needs D125804, which makes
test_errors.py test warnings as well, to actually test the warnings.
I include it already now to keep things together, it will do no harm
(I hope ...).

We also add WARNING-directives to the test kinds04.f90 in preparation
for D125804. As the exponent-letter 'Q' does not imply the same kind
on all platforms, the emitted warnings are platform-dependent.
Therefore, the test is duplicated into two variants which are run
conditionally.

Finally, we promote the portability warning for when the exponent letter
is neither 'E' nor matching the kind-param to a standard warning.

[klausler]

flang/lib/Semantics/expression.cpp

578

I know this isn't part of your changes, but perhaps this warning should be a warning rather than a portability note.

[ekieri]

flang/lib/Semantics/expression.cpp

578

Yes, I agree.

flang/docs/Extensions.md

 * `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
 * 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`

flang/include/flang/Common/Fortran-features.h

 #include "flang/Common/idioms.h"
 
 namespace Fortran::common {
 
 ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     FixedFormContinuationWithColumn1Ampersand, LogicalAbbreviations,
     XOROperator, PunctuationInNames, OptionalFreeFormSpace, BOZExtensions,
     EmptyStatement, AlternativeNE, ExecutionPartNamelist, DECStructures,
-    DoubleComplex, Byte, StarKind, QuadPrecision, SlashInitialization,
-    TripletInArrayConstructor, MissingColons, SignedComplexLiteral,
-    OldStyleParameter, ComplexConstructor, PercentLOC, SignedPrimary, FileName,
-    Carriagecontrol, Convert, Dispose, IOListLeadingComma,
-    AbbreviatedEditDescriptor, ProgramParentheses, PercentRefAndVal,
-    OmitFunctionDummies, CrayPointer, Hollerith, ArithmeticIF, Assign,
-    AssignedGOTO, Pause, OpenACC, OpenMP, CruftAfterAmpersand, ClassicCComments,
-    AdditionalFormats, BigIntLiterals, RealDoControls,
+    DoubleComplex, Byte, StarKind, ExponentMatchingKindParam, QuadPrecision,
+    SlashInitialization, TripletInArrayConstructor, MissingColons,
+    SignedComplexLiteral, OldStyleParameter, ComplexConstructor, PercentLOC,
+    SignedPrimary, FileName, Carriagecontrol, Convert, Dispose,
+    IOListLeadingComma, AbbreviatedEditDescriptor, ProgramParentheses,
+    PercentRefAndVal, OmitFunctionDummies, CrayPointer, Hollerith, ArithmeticIF,
+    Assign, AssignedGOTO, Pause, OpenACC, OpenMP, CruftAfterAmpersand,
+    ClassicCComments, AdditionalFormats, BigIntLiterals, RealDoControls,
     EquivalenceNumericWithCharacter, EquivalenceNonDefaultNumeric,
     EquivalenceSameNonSequence, AdditionalIntrinsics, AnonymousParents,
     OldLabelDoEndStatements, LogicalIntegerAssignment, EmptySourceFile,
     ProgramReturn, ImplicitNoneTypeNever, ImplicitNoneTypeAlways,
     ForwardRefDummyImplicitNone, OpenAccessAppend, BOZAsDefaultInteger,
     DistinguishableSpecifics, DefaultSave, PointerInSeqType, NonCharacterFormat)
 
 using LanguageFeatures = EnumSet<LanguageFeature, LanguageFeature_enumSize>;

flang/lib/Semantics/expression.cpp

if (parser::IsLetter(*p)) {
         Say("Unknown exponent letter '%c'"_err_en_US, expoLetter);
       }
       break;
     }
   }
   if (letterKind) {
     defaultKind = *letterKind;
   }
-  // C716 requires 'E' as an exponent, but this is more useful
+  // C716 requires 'E' as an exponent.
+  // Extension: allow exponent-letter matching the kind-param.
   auto kind{AnalyzeKindParam(x.kind, defaultKind)};
-  if (letterKind && kind != *letterKind && expoLetter != 'e') {
+  if (letterKind && expoLetter != 'e') {
+    if (kind != *letterKind) {
       Say("Explicit kind parameter on real constant disagrees with "
-        "exponent letter '%c'"_port_en_US,
+          "exponent letter '%c'"_warn_en_US,
           expoLetter);
+    } else if (x.kind &&
+        context_.ShouldWarn(
+            common::LanguageFeature::ExponentMatchingKindParam)) {
+      Say("Explicit kind parameter together with non-'E' exponent letter "
+          "is not standard"_port_en_US);
+    }
   }
   auto result{common::SearchTypes(
       RealTypeVisitor{kind, x.real.source, GetFoldingContext()})};
   if (!result) { // C717
     Say("Unsupported REAL(KIND=%d)"_err_en_US, kind);
   }
   return AsMaybeExpr(std::move(result));
 }

flang/test/Semantics/kinds04_q10.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
-! C716 If both kind-param and exponent-letter appear, exponent-letter 
-! shall be E.
-! C717 The value of kind-param shall specify an approximation method that 
+! C716 If both kind-param and exponent-letter appear, exponent-letter
+! shall be E. (As an extension we also allow an exponent-letter which matches
+! the kind-param)
+! C717 The value of kind-param shall specify an approximation method that
 ! exists on the processor.
+!
+! This test is for x86_64, where exponent-letter 'q' is for
+! 10-byte extended precision
+! REQUIRES: x86-registered-target
 subroutine s(var)
   real :: realvar1 = 4.0E6_4
   real :: realvar2 = 4.0D6
   real :: realvar3 = 4.0Q6
   real :: realvar4 = 4.0D6_8
-  real :: realvar5 = 4.0Q6_16
-  real :: realvar6 = 4.0E6_8
-  real :: realvar7 = 4.0E6_10
-  real :: realvar8 = 4.0E6_16
+  real :: realvar5 = 4.0Q6_10
+  !WARNING: Explicit kind parameter on real constant disagrees with exponent letter 'q'
+  real :: realvar6 = 4.0Q6_16
+  real :: realvar7 = 4.0E6_8
+  real :: realvar8 = 4.0E6_10
+  real :: realvar9 = 4.0E6_16
   !ERROR: Unsupported REAL(KIND=32)
-  real :: realvar9 = 4.0E6_32
+  real :: realvar10 = 4.0E6_32
 
   double precision :: doublevar1 = 4.0E6_4
   double precision :: doublevar2 = 4.0D6
   double precision :: doublevar3 = 4.0Q6
   double precision :: doublevar4 = 4.0D6_8
+  !WARNING: Explicit kind parameter on real constant disagrees with exponent letter 'q'
   double precision :: doublevar5 = 4.0Q6_16
   double precision :: doublevar6 = 4.0E6_8
   double precision :: doublevar7 = 4.0E6_10
   double precision :: doublevar8 = 4.0E6_16
   !ERROR: Unsupported REAL(KIND=32)
   double precision :: doublevar9 = 4.0E6_32
 end subroutine s

flang/test/Semantics/kinds04_q16.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
-! C716 If both kind-param and exponent-letter appear, exponent-letter 
-! shall be E.
-! C717 The value of kind-param shall specify an approximation method that 
+! C716 If both kind-param and exponent-letter appear, exponent-letter
+! shall be E. (As an extension we also allow an exponent-letter which matches
+! the kind-param)
+! C717 The value of kind-param shall specify an approximation method that
 ! exists on the processor.
+!
+! This test is for non-x86_64, where exponent-letter 'q' is for
+! 16-byte quadruple precision
+! UNSUPPORTED: x86-registered-target
 subroutine s(var)
   real :: realvar1 = 4.0E6_4
   real :: realvar2 = 4.0D6
   real :: realvar3 = 4.0Q6
   real :: realvar4 = 4.0D6_8
-  real :: realvar5 = 4.0Q6_16
-  real :: realvar6 = 4.0E6_8
-  real :: realvar7 = 4.0E6_10
-  real :: realvar8 = 4.0E6_16
+  !WARNING: Explicit kind parameter on real constant disagrees with exponent letter 'q'
+  real :: realvar5 = 4.0Q6_10
+  real :: realvar6 = 4.0Q6_16
+  real :: realvar7 = 4.0E6_8
+  real :: realvar8 = 4.0E6_10
+  real :: realvar9 = 4.0E6_16
   !ERROR: Unsupported REAL(KIND=32)
-  real :: realvar9 = 4.0E6_32
+  real :: realvar10 = 4.0E6_32
 
   double precision :: doublevar1 = 4.0E6_4
   double precision :: doublevar2 = 4.0D6
   double precision :: doublevar3 = 4.0Q6
   double precision :: doublevar4 = 4.0D6_8
   double precision :: doublevar5 = 4.0Q6_16
   double precision :: doublevar6 = 4.0E6_8
   double precision :: doublevar7 = 4.0E6_10
   double precision :: doublevar8 = 4.0E6_16
   !ERROR: Unsupported REAL(KIND=32)
   double precision :: doublevar9 = 4.0E6_32
 end subroutine s

flang/test/Semantics/kinds05.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
+! Check that we get portability warnings for the extensions
+!  - exponent-letter 'Q'
+!  - matching but non-'E' exponent letter together with kind-param
+
+subroutine s
+  real :: realvar1 = 4.0
+  real :: realvar2 = 4.0D6
+  real :: realvar3 = 4.0_8
+  real :: realvar4 = 4.0E6_4
+  real :: realvar5 = 4.0E6_8
+  !WARNING: nonstandard usage: Q exponent
+  real :: realvar6 = 4.0Q6
+  !WARNING: Explicit kind parameter together with non-'E' exponent letter is not standard
+  real :: realvar7 = 4.0D6_8
+  !WARNING: Explicit kind parameter on real constant disagrees with exponent letter 'd'
+  real :: realvar8 = 4.0D6_4
+end subroutine s