[flang] Clean up bogus semantic error on procedure pointer assignment

[klausler]

Summary

When a procedure pointer with no interface is associated with
an EXTERNAL name with no interface information, but it is later
inferred that the procedure pointer must be a subroutine because it
appears in a CALL statement, don't complain that the EXTERNAL name
is not also known to be a subroutine.

Subroutine vs. function errors are still caught in procedure pointer
assignment compatibility checking; this fix simply ensures that those
more nuanced tests are not overridded by the attribute set equality test.

Also, leave in some code for dumping the differing attributes in legitimate
error cases that was added in the coures of debugging the specific problem.

flang/lib/Evaluate/characteristics.cpp

bool Procedure::IsCompatibleWith(
   // Ditto with elemental.
   Attrs actualAttrs{actual.attrs};
   if (!attrs.test(Attr::Pure)) {
     actualAttrs.reset(Attr::Pure);
   }
   if (!attrs.test(Attr::Elemental)) {
     actualAttrs.reset(Attr::Elemental);
   }
-  if (attrs != actualAttrs) {
+  Attrs differences{attrs ^ actualAttrs};
+  differences.reset(Attr::Subroutine); // dealt with specifically later
+  if (!differences.empty()) {
     if (whyNot) {
+      auto sep{": "s};
       *whyNot = "incompatible procedure attributes";
+      differences.IterateOverMembers([&](Attr x) {
+        *whyNot += sep + EnumToString(x);
+        sep = ", ";
+      });
     }
   } else if ((IsFunction() && actual.IsSubroutine()) ||
       (IsSubroutine() && actual.IsFunction())) {
     if (whyNot) {
       *whyNot =
           "incompatible procedures: one is a function, the other a subroutine";
     }
   } else if (functionResult && actual.functionResult &&

flang/test/Semantics/assign03.f90

subroutine s5
 
     !ERROR: Procedure pointer 'p_impure' associated with incompatible procedure designator 'f_impure2': incompatible dummy argument #1: incompatible dummy data object intents
     p_impure => f_impure2
     !ERROR: Procedure pointer 'p_pure' associated with incompatible procedure designator 'f_pure2': function results have incompatible types: INTEGER(4) vs REAL(4)
     p_pure => f_pure2
     !ERROR: Procedure pointer 'p_impure' associated with incompatible procedure designator 'f_elemental2': incompatible dummy argument #1: incompatible dummy data object attributes
     p_impure => f_elemental2
 
-    !ERROR: Procedure pointer 'sp_impure' associated with incompatible procedure designator 's_impure2': incompatible procedure attributes
+    !ERROR: Procedure pointer 'sp_impure' associated with incompatible procedure designator 's_impure2': incompatible procedure attributes: BindC
     sp_impure => s_impure2
     !ERROR: Procedure pointer 'sp_impure' associated with incompatible procedure designator 's_pure2': incompatible dummy argument #1: incompatible dummy data object intents
     sp_impure => s_pure2
     !ERROR: Procedure pointer 'sp_pure' associated with incompatible procedure designator 's_elemental2': distinct numbers of dummy arguments
     sp_pure => s_elemental2
 
     !ERROR: Function pointer 'p_impure' may not be associated with subroutine designator 's_impure1'
     p_impure => s_impure1
subroutine s12
     logical, parameter :: l6 = is_contiguous(p(:))
   end
   subroutine test3(b)
     integer, intent(inout) :: b(..)
     !ERROR: Must be a constant value
     integer, parameter :: i = rank(b)
   end subroutine
 
-
+  subroutine s13
+    external :: s_external
+    procedure(), pointer :: ptr
+    !Ok - don't emit an error about incompatible Subroutine attribute
+    ptr => s_external
+    call ptr
+  end subroutine
 end