[flang][msvc] Disambiguate injected class name.

[Meinersbur]

Summary

The identifier Expr within the scope of the Expr class (including its temple specializations) refers to the current template/instantiation (see https://en.cppreference.com/w/cpp/language/injected-class-name for details). The MapTemplate template expect a non-instantiated template as the first template argument, not the concrete instantiation of Expr.

At least msvc interprets Expr as the injected class name, whereas gcc and clang use the global flang::evaluate::Expr template. Disambiguate by explicitly using the namespace.

This patch is part of the series to make flang compilable with MS Visual Studio.

[DavidTruby]

LGTM!

Interesting that the compilers don't use the same lookup rule here. I wonder which is correct (my gut feeling is actually that MSVC is).

flang/include/flang/Evaluate/expression.h

 // kind, represented as a discriminated union over Expr<Type<CAT, K>>
 // for each supported kind K in the category.
 template <TypeCategory CAT>
 class Expr<SomeKind<CAT>> : public ExpressionBase<SomeKind<CAT>> {
 public:
   using Result = SomeKind<CAT>;
   EVALUATE_UNION_CLASS_BOILERPLATE(Expr)
   int GetKind() const;
-  common::MapTemplate<Expr, CategoryTypes<CAT>> u;
+  common::MapTemplate<evaluate::Expr, CategoryTypes<CAT>> u;
 };
 
 template <> class Expr<SomeCharacter> : public ExpressionBase<SomeCharacter> {
 public:
   using Result = SomeCharacter;
   EVALUATE_UNION_CLASS_BOILERPLATE(Expr)
   int GetKind() const;
   std::optional<Expr<SubscriptInteger>> LEN() const;