[flang] Allow pointers to non-sequence types in sequence types

Derived types with SEQUENCE must have data components of sequence
types; but this rule is relaxed as common an extension in the case of
pointer components, whose targets' types are not really relevant
to the implementation requirements of sequence types.

Differential Revision: https://reviews.llvm.org/D117158

flang/docs/Extensions.md

@@ -200,6 +200,10 @@ end
 * 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.)
 
 ### Extensions supported when enabled by options
 

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

@@ -31,7 +31,7 @@ ENUM_CLASS(LanguageFeature, BackslashEscapes, OldDebugLines,
     OldLabelDoEndStatements, LogicalIntegerAssignment, EmptySourceFile,
     ProgramReturn, ImplicitNoneTypeNever, ImplicitNoneTypeAlways,
     ForwardRefDummyImplicitNone, OpenAccessAppend, BOZAsDefaultInteger,
-    DistinguishableSpecifics, DefaultSave)
+    DistinguishableSpecifics, DefaultSave, PointerInSeqType)
 
 using LanguageFeatures = EnumSet<LanguageFeature, LanguageFeature_enumSize>;
 

flang/lib/Semantics/resolve-names.cpp

@@ -4296,8 +4296,14 @@ bool DeclarationVisitor::Pre(const parser::DataComponentDefStmt &x) {
   if (derivedTypeInfo_.sequence) { // C740
     if (const auto *declType{GetDeclTypeSpec()}) {
       if (!declType->AsIntrinsic() && !declType->IsSequenceType()) {
-        Say("A sequence type data component must either be of an"
-            " intrinsic type or a derived sequence type"_err_en_US);
+        if (GetAttrs().test(Attr::POINTER) &&
+            context().IsEnabled(common::LanguageFeature::PointerInSeqType)) {
+          if (context().ShouldWarn(common::LanguageFeature::PointerInSeqType)) {
+            Say("A sequence type data component that is a pointer to a non-sequence type is not standard"_en_US);
+          }
+        } else {
+          Say("A sequence type data component must either be of an intrinsic type or a derived sequence type"_err_en_US);
+        }
       }
     }
   }

flang/test/Semantics/resolve31.f90

@@ -83,6 +83,8 @@ module m4
     class(*), allocatable :: typeStarField
     !ERROR: A sequence type data component must either be of an intrinsic type or a derived sequence type
     type(plainType) :: testField1
+    !Pointers are ok as an extension
+    type(plainType), pointer :: testField1p
     type(sequenceType) :: testField2
     procedure(real), pointer, nopass :: procField
   end type testType