incorrect SFINAE constraint on (static)_simd_cast

[mattkretz]

[parallel.simd.casts] p5 and p11 allow all non-simd types T. p5 has a implicit requirement of an arithmetic type, since it must be able to represent all possible values of U (note a misspelled To, that should be T in p5). But one could argue that a UDT can fulfill this requirement. Since only vectorizable types T are valid for simd<T>, the constraint should use this term instead.

Proposed resolution:

9.4.5/5
Remarks: The function shall not participate in overload resolution unless

• every possible value of type U can be represented with type ToT, and
• either
  • is_simd_v<T> is falseT is a vectorizable type, or
  • is_simd_v<T> and T::size() == simd<U, Abi>::size() isare true.

9.4.5/11
Remarks: The function shall not participate in overload resolution unless either

• is_simd_v<T> is falseT is a vectorizable type, or
• is_simd_v<T> and T::size() == simd<U, Abi>::size() isare true.