[flang][msvc] Define implicit conversion from UnsignedInt128 to int64_t.

[Meinersbur]

Summary

The custom implementation of UnsignedInt128 has an implicit conversion operator to unit64_t, but not int64_t. Considering that the former is already truncating, and C++ implicitly converts uint64_t to int64_t, UnsignedInt128 should also support an implicit conversion to int64_t. An analogous conversion would be from uint32_t to int16_t.

Without the conversion operator overload, the msvc emits the following error:

descriptor-io.h(44): error C2440: 'static_cast': cannot convert from 'A' to 'int64_t'
        with
        [
            A=Fortran::common::uint128_t
        ]

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]

Your "analogous conversion" from uint32 to int64 can't lose information, though.

[Meinersbur]

This is not really msvc specific, but also occurs with gcc when compiling with AVOID_NATIVE_UINT128_T.

[Meinersbur]

In D88509#2301638, @klausler wrote:

Your "analogous conversion" from uint32 to int64 can't lose information, though.

I intended "conversion from uint32_t to int16_t". My mistake.

flang/include/flang/Common/uint128.h

public:
   constexpr UnsignedInt128 &operator=(UnsignedInt128 &&) = default;
 
   constexpr UnsignedInt128 operator+() const { return *this; }
   constexpr UnsignedInt128 operator~() const { return {~high_, ~low_}; }
   constexpr UnsignedInt128 operator-() const { return ~*this + 1; }
   constexpr bool operator!() const { return !low_ && !high_; }
   constexpr explicit operator bool() const { return low_ || high_; }
   constexpr explicit operator std::uint64_t() const { return low_; }
+  constexpr explicit operator std::int64_t() const { return low_; }
   constexpr explicit operator int() const { return static_cast<int>(low_); }
 
   constexpr std::uint64_t high() const { return high_; }
   constexpr std::uint64_t low() const { return low_; }
 
   constexpr UnsignedInt128 operator++(/*prefix*/) {
     *this += 1;
     return *this;