[flang] Catch attempt to apply ASYNCHRONOUS attribute to a non-variable

Complain about the ASYNCHRONOUS attribute on symbols that aren't
variables.

Differential Revision: https://reviews.llvm.org/D146573

Author: klausler

flang/lib/Semantics/check-declarations.cpp

@@ -437,6 +437,11 @@ void CheckHelper::Check(const Symbol &symbol) {
           symbol.name());
     }
   }
+  if (symbol.attrs().test(Attr::ASYNCHRONOUS) &&
+      !evaluate::IsVariable(symbol)) {
+    messages_.Say(
+        "An entity may not have the ASYNCHRONOUS attribute unless it is a variable"_err_en_US);
+  }
 }
 
 void CheckHelper::CheckCommonBlock(const Symbol &symbol) {

flang/test/Semantics/resolve20.f90

@@ -54,6 +54,10 @@ subroutine forward
   !ERROR: EXTERNAL attribute not allowed on 'bar'
   external :: bar
 
+  !ERROR: An entity may not have the ASYNCHRONOUS attribute unless it is a variable
+  asynchronous :: async
+  external :: async
+
   !ERROR: PARAMETER attribute not allowed on 'm'
   parameter(m=2)
   !ERROR: PARAMETER attribute not allowed on 'foo'