[flang][msvc] Workaround 'forgotten' symbols in FoldOperation. NFC.

[Meinersbur]

Summary

This resolves an issue where the Microsoft compiler 'forgets' symbols when using constexpr in a lambda in a templated function. The symbols are:

1. The implicit lambda captures context and convert. Fix by making them explicit captures. The error message was:

fold-implementation.h(1220): error C2065: 'convert': undeclared identifier

2. The function template argument FROMCAT. Fix by storing it in a temporary constexpr variable inside the function. The error message was:

fold-implementation.h(1216): error C2065: 'FROMCAT': undeclared identifier

This patch is part of the series to make flang compilable with MS Visual Studio http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html.

[Meinersbur]

Using the switch /Zc:lambda also allows to compile this code. The switch is not documented in the Visual Studio help, but cl /help documents it as:

/Zc:arg1[,arg2] C++ language conformance, where arguments can be:
  [...]
  lambda[-]             better lambda support by using the newer lambda processor (off by default)

[klausler]

If the magic MSVC option works around this compiler bug, and perhaps others, it seems to me like a cleaner way to avoid their problems.

flang/lib/Evaluate/fold-implementation.h

1160

Please conform to our conventional use of braced initialization, and consider adding a comment that the new constexpr is a MSVC work-around, or it'll be susceptible to being removed as redundant.

[Meinersbur]

• Address @klausler's review

[Meinersbur]

/Zc:lambda fixes D88052 and this compiler bug. This flag was actually suggested by Microsoft in the msvc bug report for D88052.

I created all the reviews to compile flang with msvc (some tests are failing though), these two are the only patches necessary to compile without /Zc:lambda. I am happy to create patch that adds /Zc:lambda when compiling flang if needed, but considering that its state of support is unclear to me, I am not sure whether it is worth it over two smaller workarounds. I would also have to check that whether other problems appear when compiling all of flang with that flag.

[Meinersbur]

Indeed, switchin on /Zc:lambda globally results in a different compile failure:

flang\lib\Evaluate\intrinsics-library-templates.h(199,63): error C2664: 'Fortran::evaluate::value::Real<Fortran::evaluate::value::Integer<64,fal
se,32,unsigned int,unsigned __int64>,53> (Fortran::evaluate::FoldingContext &,Fortran::evaluate::GenericFunctionPointer,const Fortran::evaluate::value::Real<Fortran::evaluate::value
::Integer<64,false,32,unsigned int,unsigned __int64>,53> &,const Fortran::evaluate::value::Real<Fortran::evaluate::value::Integer<64,false,32,unsigned int,unsigned __int64>,53> &)':
 cannot convert argument 3 from 'const ConstantContainer<T>' to 'const Fortran::evaluate::value::Real<Fortran::evaluate::value::Integer<64,false,32,unsigned int,unsigned __int64>,53
> &'
          with
          [
              T=Fortran::evaluate::Type<Fortran::common::TypeCategory::Real,8>
          ]
flang\lib\Evaluate\intrinsics-library-templates.h(173,36): message : Reason: cannot convert from 'const ConstantContainer<T>' to 'const Fortran:
:evaluate::value::Real<Fortran::evaluate::value::Integer<64,false,32,unsigned int,unsigned __int64>,53>'
          with
          [
              T=Fortran::evaluate::Type<Fortran::common::TypeCategory::Real,8>
          ]

flang/lib/Evaluate/fold-implementation.h

 
 template <typename TO, TypeCategory FROMCAT>
 Expr<TO> FoldOperation(
     FoldingContext &context, Convert<TO, FROMCAT> &&convert) {
   if (auto array{ApplyElementwise(context, convert)}) {
     return *array;
   }
   return std::visit(
-      [&](auto &kindExpr) -> Expr<TO> {
+      [&context, &convert](auto &kindExpr) -> Expr<TO> {
         using Operand = ResultType<decltype(kindExpr)>;
+        // This variable is a workaround for msvc which emits an error when
+        // using the FROMCAT template parameter below.
+        TypeCategory constexpr FromCat{FROMCAT};
         char buffer[64];
         if (auto value{GetScalarConstantValue<Operand>(kindExpr)}) {
           if constexpr (TO::category == TypeCategory::Integer) {
             if constexpr (Operand::category == TypeCategory::Integer) {
               auto converted{Scalar<TO>::ConvertSigned(*value)};
               if (converted.overflow) {
                 context.messages().Say(
                     "INTEGER(%d) to INTEGER(%d) conversion overflowed"_en_US,
return std::visit(
             if (auto converted{ConvertString<Scalar<TO>>(std::move(*value))}) {
               return ScalarConstantToExpr(std::move(*converted));
             }
           } else if constexpr (TO::category == TypeCategory::Logical &&
               Operand::category == TypeCategory::Logical) {
             return Expr<TO>{value->IsTrue()};
           }
         } else if constexpr (std::is_same_v<Operand, TO> &&
-            FROMCAT != TypeCategory::Character) {
+            FromCat != TypeCategory::Character) {
           return std::move(kindExpr); // remove needless conversion
         }
         return Expr<TO>{std::move(convert)};
       },
       convert.left().u);
 }
 
 template <typename T>