[clang-cl] `constexpr` fields cannot reference DLL exported symbols unlike with MSVC cl

[andrurogerz]

This is a minor behavioral difference between clang/clang-cl and MSVC cl when dealing with DLL exported symbols.: when compiling with MSVC, a constexpr variable is allowed to reference a symbol that is DLL-exported. When compiling the same code with clang-cl, an error is produced indicating the variable must be initialized by a constant expression.

Simplest repro:
DLL source:

__declspec(dllexport) int sum(int a, int b) {
  return a + b;
}

Client source:

__declspec(dllimport) void sum();
constexpr auto sumFn = sum;

Compiling this code with clang-cl results in:

error: constexpr variable 'sumFn' must be initialized by a constant expression
  167 |     constexpr auto sumFn = sum;
      |                    ^       ~~~
1 error generated.

Honestly, it seems like Clang is correct to disallow this and MSVC is in the wrong. However, if clang-cl is meant to be a drop-in replacement for cl on Windows then it would need to behave the same.

There is some further discussion in #144386

[efriedma-quic]

See https://reviews.llvm.org/D137107 .