[flang] Change WhyNotModifiable to return optional

One overload of WhyNotModifiable returned an optional message while the
other returns a unique_ptr. Change the latter to be consistent with the
former and other message-returning functions in this file.

Also, reorder the if clauses to reduce the level of indentation.

Original-commit: flang-compiler/f18@864e9cfc7e
Reviewed-on: https://github.com/flang-compiler/f18/pull/1050
Tree-same-pre-rewrite: false

flang/include/flang/Semantics/tools.h

@@ -167,7 +167,7 @@ bool IsAssumedLengthExternalCharacterFunction(const Symbol &);
 // Is the symbol modifiable in this scope
 std::optional<parser::MessageFixedText> WhyNotModifiable(
     const Symbol &, const Scope &);
-std::unique_ptr<parser::Message> WhyNotModifiable(SourceName, const SomeExpr &,
+std::optional<parser::Message> WhyNotModifiable(SourceName, const SomeExpr &,
     const Scope &, bool vectorSubscriptIsOk = false);
 const Symbol *IsExternalInPureContext(const Symbol &, const Scope &);
 bool HasCoarray(const parser::Expr &);

flang/lib/Semantics/check-call.cpp

@@ -329,13 +329,12 @@ static void CheckExplicitDataArg(const characteristics::DummyDataObject &dummy,
   }
   if (reason && scope) {
     bool vectorSubscriptIsOk{isElemental || dummyIsValue};  // 15.5.2.4(21)
-    std::unique_ptr<parser::Message> why{
-        WhyNotModifiable(messages.at(), actual, *scope, vectorSubscriptIsOk)};
-    if (why.get()) {
+    if (auto why{WhyNotModifiable(
+            messages.at(), actual, *scope, vectorSubscriptIsOk)}) {
       if (auto *msg{messages.Say(
               "Actual argument associated with %s %s must be definable"_err_en_US,
               reason, dummyName)}) {
-        msg->Attach(std::move(why));
+        msg->Attach(*why);
       }
     }
   }

flang/lib/Semantics/tools.cpp

@@ -789,29 +789,23 @@ std::optional<parser::MessageFixedText> WhyNotModifiable(
   }
 }
 
-std::unique_ptr<parser::Message> WhyNotModifiable(parser::CharBlock at,
+std::optional<parser::Message> WhyNotModifiable(parser::CharBlock at,
     const SomeExpr &expr, const Scope &scope, bool vectorSubscriptIsOk) {
-  if (evaluate::IsVariable(expr)) {
-    if (auto dataRef{evaluate::ExtractDataRef(expr)}) {
-      if (!vectorSubscriptIsOk && evaluate::HasVectorSubscript(expr)) {
-        return std::make_unique<parser::Message>(
-            at, "variable has a vector subscript"_en_US);
-      } else {
-        const Symbol &symbol{dataRef->GetFirstSymbol()};
-        if (auto maybeWhy{WhyNotModifiable(symbol, scope)}) {
-          return std::make_unique<parser::Message>(symbol.name(),
-              parser::MessageFormattedText{
-                  std::move(*maybeWhy), symbol.name()});
-        }
-      }
-    } else {
-      // reference to function returning POINTER
+  if (!evaluate::IsVariable(expr)) {
+    return parser::Message{at, "Expression is not a variable"_en_US};
+  } else if (auto dataRef{evaluate::ExtractDataRef(expr)}) {
+    if (!vectorSubscriptIsOk && evaluate::HasVectorSubscript(expr)) {
+      return parser::Message{at, "Variable has a vector subscript"_en_US};
+    }
+    const Symbol &symbol{dataRef->GetFirstSymbol()};
+    if (auto maybeWhy{WhyNotModifiable(symbol, scope)}) {
+      return parser::Message{symbol.name(),
+          parser::MessageFormattedText{std::move(*maybeWhy), symbol.name()}};
     }
   } else {
-    return std::make_unique<parser::Message>(
-        at, "expression is not a variable"_en_US);
+    // reference to function returning POINTER
   }
-  return {};
+  return std::nullopt;
 }
 
 class ImageControlStmtHelper {