error: non-type template argument is not a constant expression with 32-bit address space

[ararmine]

The following C++ code is not compiled with clang x86-64 v17.0.1 with MS extensions enabled, while it compiles successfully with X86 msvc v19.
Steps to reproduce:

1. cat test.cpp

typedef void                OpcUa_Void;

typedef OpcUa_Void (*__ptr32 PfnClear)(OpcUa_Void* __ptr32);

template<class T, PfnClear fctClear>
class UaStackTypeArray
{};

OpcUa_Void ClearNothing(void* __ptr32);

template class UaStackTypeArray<bool, ClearNothing>;

2. clang test.cpp -S -fms-extensions

test.cpp:12:39: error: non-type template argument is not a constant expression
   12 | template class UaStackTypeArray<bool, ClearNothing>;
      |                                       ^~~~~~~~~~~~
test.cpp:12:39: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
1 error generated.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (ararmine)

The following C++ code is not compiled with clang x86-64 v17.0.1 with MS extensions enabled, while it compiles successfully with X86 msvc v19. Steps to reproduce:

1. cat test.cpp

typedef void                OpcUa_Void;

typedef OpcUa_Void (*__ptr32 PfnClear)(OpcUa_Void* __ptr32);

template<class T, PfnClear fctClear>
class UaStackTypeArray
{};

OpcUa_Void ClearNothing(void* __ptr32);

template class UaStackTypeArray<bool, ClearNothing>;

2. clang test.cpp -S -fms-extensions

test.cpp:12:39: error: non-type template argument is not a constant expression
   12 | template class UaStackTypeArray<bool, ClearNothing>;
      |                                       ^~~~~~~~~~~~
test.cpp:12:39: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
1 error generated.

[shafik]

This looks very similar to #12488 which was closed as won't fix.

CC @AaronBallman for second set of 👀

[zygoloid]

I think this is different -- this isn't an arbitrary reinterpret_cast conversion, it's just a CK_AddressSpaceConversion, which we could in principle permit in constant evaluation (perhaps just in this case, or perhaps in general). It'd be a bit of work to do, including IR generation changes, but probably not too hard.

I think the bigger problem is that the backend doesn't support these constants either: https://godbolt.org/z/4GW9cbenr