[flang][msvc] Avoid templated initializer list initialization of vector. NFC.

[Meinersbur]

Summary

The Microsoft compiler emits an error when populating the vector with a single element of a templated argument using the brace syntax. The error is:

constant.h(102,1): error C2664: 'std::vector<Fortran::evaluate::value::Complex<...>, ...>::vector(std::initializer_list<_Ty>,const _Alloc &)': cannot convert argument 1 from 'initializer list' to 'std::initializer_list<_Ty>'

To work around this error, we replace the templated constructor with one for the expected type. Conversion to the element type has to be done by the caller.

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.

[klausler]

flang/include/flang/Evaluate/constant.h

102

This change will restrict this constructor to work only with scalar items, I think.

[Meinersbur]

Just avoiding the template is sufficient.

[Meinersbur]

flang/include/flang/Evaluate/constant.h

102

I was trying to find out when this could be the case. However, I think it just moved any implicit conversion from type A to ELEMENT from happening from inside the ctor to the code that calls the ctor.

flang/include/flang/Evaluate/constant.h

class ConstantBase : public ConstantBounds {
   static_assert(RESULT::category != TypeCategory::Character);
 
 public:
   using Result = RESULT;
   using Element = ELEMENT;
 
   template <typename A>
   ConstantBase(const A &x, Result res = Result{}) : result_{res}, values_{x} {}
-  template <typename A, typename = common::NoLvalue<A>>
-  ConstantBase(A &&x, Result res = Result{})
+  ConstantBase(ELEMENT &&x, Result res = Result{})
       : result_{res}, values_{std::move(x)} {}
   ConstantBase(
       std::vector<Element> &&, ConstantSubscripts &&, Result = Result{});
 
   DEFAULT_CONSTRUCTORS_AND_ASSIGNMENTS(ConstantBase)
   ~ConstantBase();
 
   bool operator==(const ConstantBase &) const;
   bool empty() const { return values_.empty(); }
   std::size_t size() const { return values_.size(); }