[libcxx] MSVC fails to compile `operator valarray<template>`

[danlark]

Summary

MSVC fails to compile template operator valarray because (i believe) the type is not seen in template argument of the operator.

Minimal example:
https://gcc.godbolt.org/z/sPpQFL

I believe it is a compiler bug but workaround is not that complex so I hope it can be merged anyway

include/valarray

public:
     {
         typedef __apply_expr<value_type, value_type(*)(const value_type&)> _Op;
         typedef _UnaryOp<_Op, _ValExpr> _NewExpr;
         return __val_expr<_NewExpr>(_NewExpr(_Op(__f), __expr_));
     }
 };
 
 template<class _ValExpr>
-__val_expr<_ValExpr>::operator valarray<__val_expr::result_type>() const
+__val_expr<_ValExpr>::operator valarray<typename __val_expr<_ValExpr>::result_type>() const
 {
     valarray<result_type> __r;
     size_t __n = __expr_.size();
     if (__n)
     {
         __r.__begin_ =
             __r.__end_ =
                 static_cast<result_type*>(