[flang] Refine handling of short character actual arguments

[klausler]

Summary

Actual arguments whose lengths are less than the expected length
of their corresponding dummy argument are errors; but this needs
to be refined. Short actual arguments that are variables remain
errors, but those that are expressions can be (again) extended on
the right with blanks.

flang/lib/Semantics/check-call.cpp

   }
 }
 
-// When scalar CHARACTER actual arguments are known to be short,
-// we extend them on the right with spaces and a warning.
+// When a scalar CHARACTER actual argument is known to be short,
+// we extend it on the right with spaces and a warning if it is an
+// expression, and emit an error if it is a variable.
 static void CheckCharacterActual(evaluate::Expr<evaluate::SomeType> &actual,
     const characteristics::TypeAndShape &dummyType,
     characteristics::TypeAndShape &actualType,

       auto actualLength{
           ToInt64(Fold(context, common::Clone(*actualType.LEN())))};
       if (dummyLength && actualLength && *actualLength < *dummyLength) {
-        messages.Say(
-            "Actual length '%jd' is less than expected length '%jd'"_err_en_US,
-            *actualLength, *dummyLength);
-#if 0 // We used to just emit a warning, and padded the actual argument
-        auto converted{ConvertToType(dummyType.type(), std::move(actual))};
-        CHECK(converted);
-        actual = std::move(*converted);
-        actualType.set_LEN(SubscriptIntExpr{*dummyLength});
-#endif
+        if (evaluate::IsVariable(actual)) {
+          messages.Say(
+              "Actual argument variable length '%jd' is less than expected length '%jd'"_err_en_US,
+              *actualLength, *dummyLength);
+        } else {
+          messages.Say(
+              "Actual argument expression length '%jd' is less than expected length '%jd'"_warn_en_US,
+              *actualLength, *dummyLength);
+          auto converted{ConvertToType(dummyType.type(), std::move(actual))};
+          CHECK(converted);
+          actual = std::move(*converted);
+          actualType.set_LEN(SubscriptIntExpr{*dummyLength});
+        }
       }
     }
   }

flang/test/Semantics/call03.f90

   end subroutine
 
   subroutine ch2(x)
-    character(2), intent(in out) :: x
+    character(2), intent(in) :: x
   end subroutine
   subroutine pdtdefault (derivedArg)
     !ERROR: Type parameter 'n' lacks a value and has no default

     type(pdtWithDefault(3)) :: defaultVar3
     type(pdtWithDefault(4)) :: defaultVar4
     character :: ch1
-    !ERROR: Actual length '1' is less than expected length '2'
+    !ERROR: Actual argument variable length '1' is less than expected length '2'
     call ch2(ch1)
+    !WARN: Actual argument expression length '0' is less than expected length '2'
+    call ch2("")
     call pdtdefault(vardefault)
     call pdtdefault(var3)
     call pdtdefault(var4) ! error