Take the address of a dllimport slot at run time

[yosh-matsuda]

Take the address of a dllimport slot at run time

NB_SLOT_ALIAS initializes a constexpr pointer with the address of a CPython function (nb_backend.h:615):

#define NB_SLOT_ALIAS(ret, name, args, target) \
    inline constexpr ret (*name) args = ⌖

Windows declares a CPython function with __declspec(dllimport), so its address is the content of an import slot that the loader fills, and not a constant expression. Clang rejects the initializer:

// The declaration that Python.h produces on Windows
extern "C" __declspec(dllimport) int PyObject_Vectorcall(int);
inline constexpr int (*vectorcall)(int) = &PyObject_Vectorcall;

> clang-cl /std:c++17 /c repro.cpp
repro.cpp(2,24): error: constexpr variable 'vectorcall' must be initialized by a constant expression

Every Windows target of Clang fails, both --target=x86_64-pc-windows-msvc and --target=x86_64-w64-windows-gnu, so no extension builds with clang-cl. MSVC accepts the initializer as an extension, which is why the windows-2022 job of the CI stays green.

The macro arrived with the vector call slots in 9219167 and reached its first release in 3.0.0.

Fix

Make the pointer const instead of constexpr:

-    inline constexpr ret (*name) args = ⌖
+    inline ret (*const name) args = ⌖

Tests

The error appears at compile time on one platform, so the test is a CI job. .github/workflows/ci.yml gains a clang-cl job.

[wjakob]

Thank you. I will trim back some of the comments of these changes, I think they are overkill. The commit history and PR description is enough to look into the detailed rationale.