[flang] Check that DO index variables are definable

[klausler]

Summary

We're letting immutable objects appear as DO index variables;
catch and diagnose this error.

flang/lib/Semantics/check-do-forall.cpp

 //===-- lib/Semantics/check-do-forall.cpp ---------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "check-do-forall.h"
+#include "definable.h"
 #include "flang/Common/template.h"
 #include "flang/Evaluate/call.h"
 #include "flang/Evaluate/expression.h"
 #include "flang/Evaluate/tools.h"
 #include "flang/Parser/message.h"
 #include "flang/Parser/parse-tree-visitor.h"
 #include "flang/Parser/tools.h"
 #include "flang/Semantics/attr.h"
private:
   }
 
   void CheckDoVariable(const parser::ScalarName &scalarName) {
     const parser::CharBlock &sourceLocation{scalarName.thing.source};
     if (const Symbol * symbol{scalarName.thing.symbol}) {
       if (!IsVariableName(*symbol)) {
         context_.Say(
             sourceLocation, "DO control must be an INTEGER variable"_err_en_US);
+      } else if (auto why{WhyNotDefinable(sourceLocation,
+                     context_.FindScope(sourceLocation), DefinabilityFlags{},
+                     *symbol)}) {
+        context_
+            .Say(sourceLocation,
+                "'%s' may not be used as a DO variable"_err_en_US,
+                symbol->name())
+            .Attach(std::move(*why));
       } else {
         const DeclTypeSpec *symType{symbol->GetType()};
         if (!symType) {
           SayBadDoControl(sourceLocation);
         } else {
           if (!symType->IsNumeric(TypeCategory::Integer)) {
             CheckDoControl(
                 sourceLocation, symType->IsNumeric(TypeCategory::Real));

flang/test/Semantics/definable03.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+subroutine sub(j)
+  integer, intent(in) :: j
+  !ERROR: 'j' may not be used as a DO variable
+  !BECAUSE: 'j' is an INTENT(IN) dummy argument
+  do j = 1, 10
+  end do
+end