[flang] Catch C721 violations in defined interfaces

[klausler]

Summary

Fortran's constraint C721 allows an assumed-length CHARACTER
entity to be declared in a very limited set of circumstances that
does not include an explicit external interface definition.

flang/lib/Semantics/check-declarations.cpp

       }
     }
   }
+  if (details.isInterface() && !details.isDummy() && details.isFunction() &&
+      IsAssumedLengthCharacter(details.result())) { // C721
+    messages_.Say(details.result().name(),
+        "A function interface may not declare an assumed-length CHARACTER(*) result"_err_en_US);
+  }
 }
 
 void CheckHelper::CheckDerivedType(

   } else if (!proc.functionResult.has_value()) {
     msg = "%s procedure '%s' must be a function"_err_en_US;
   } else if (proc.functionResult->IsAssumedLengthCharacter()) {
+    const auto *subpDetails{specific.detailsIf<SubprogramDetails>()};
+    if (subpDetails && !subpDetails->isDummy() && subpDetails->isInterface()) {
+      // Error is caught by more general test for interfaces with
+      // assumed-length character function results
+      return true;
+    }
     msg = "%s function '%s' may not have assumed-length CHARACTER(*)"
           " result"_err_en_US;
   } else if (auto m{CheckNumberOfArgs(kind, proc.dummyArguments.size())}) {

flang/test/Semantics/call01.f90

   interface
     character*(*) function f2()
     end function
+    !ERROR: A function interface may not declare an assumed-length CHARACTER(*) result
     character*(*) function f4()
     end function
   end interface

   print *, f2()
   !ERROR: Assumed-length character function must be defined with a length to be called
   print *, f3()
-  !ERROR: Assumed-length character function must be defined with a length to be called
-  print *, f4()
   print *, fp1()
   print *, fp2()
 end subroutine

flang/test/Semantics/resolve67.f90

 
 module m3
   interface operator(/)
-    !ERROR: OPERATOR(/) function 'divide' may not have assumed-length CHARACTER(*) result
+    !ERROR: A function interface may not declare an assumed-length CHARACTER(*) result
     character(*) function divide(x, y)
       character(*), intent(in) :: x, y
     end

       end interface
     end
   end interface
+ contains
+  subroutine s(alcf1, alcf2)
+    interface
+      character(*) function alcf1(x, y)
+        character(*), intent(in) :: x, y
+      end function
+      character(*) function alcf2(x, y)
+        character(*), intent(in) :: x, y
+      end function
+    end interface
+    interface operator(+)
+      !ERROR: OPERATOR(+) function 'alcf1' may not have assumed-length CHARACTER(*) result
+      procedure alcf1
+    end interface
+    !ERROR: OPERATOR(-) function 'alcf2' may not have assumed-length CHARACTER(*) result
+    generic :: operator(-) => alcf2
+  end subroutine
 end
 
 module m4