[flang] Fix semantics check for RETURN statement

[ekieri]

Summary

The RETURN statement is allowed in functions and subroutines, but not
in main programs. It is however a common extension, which we also
implement, to allow RETURN from main programs -- we only issue a
portability warning when -pedantic or -std=f2018 are set.

This patch fixes false positives for this portability warning, where it
was triggered also when RETURN was present in functions or subroutines.

Fixexs #55080

[PeteSteinfeld]

All builds and tests without error and looks good.

Thanks for fixing this!

flang/lib/Semantics/check-return.cpp

void ReturnStmtChecker::Leave(const parser::ReturnStmt &returnStmt) {
   // subroutine subprogram.
   const auto &scope{context_.FindScope(context_.location().value())};
   if (const auto *subprogramScope{FindContainingSubprogram(scope)}) {
     if (returnStmt.v &&
         (subprogramScope->kind() == Scope::Kind::MainProgram ||
             IsFunction(*subprogramScope->GetSymbol()))) {
       context_.Say(
           "RETURN with expression is only allowed in SUBROUTINE subprogram"_err_en_US);
-    } else if (context_.ShouldWarn(common::LanguageFeature::ProgramReturn)) {
+    } else if (subprogramScope->kind() == Scope::Kind::MainProgram &&
+        context_.ShouldWarn(common::LanguageFeature::ProgramReturn)) {
       context_.Say("RETURN should not appear in a main program"_port_en_US);
     }
   }
 }
 
 } // namespace Fortran::semantics

flang/test/Semantics/altreturn02.f90

-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: %flang_fc1 -fsyntax-only -pedantic %s  2>&1 | FileCheck %s --allow-empty
 ! Check subroutine with alt return
 
        SUBROUTINE TEST (N, *, *)
        IF ( N .EQ. 0 ) RETURN
        IF ( N .EQ. 1 ) RETURN 1
        RETURN 2
        END
+! CHECK-NOT: error:
+! CHECK-NOT: portability:

flang/test/Semantics/altreturn07.f90

+! RUN: %flang_fc1 -fsyntax-only -pedantic %s  2>&1 | FileCheck %s
+! Test extension: RETURN from main program
+
+return
+! CHECK: portability: RETURN should not appear in a main program
+end