clang incorrectly allows converting function pointers to and from void* in constant expressions

[efriedma-quic]

Consider:

void f();
constexpr auto p1 = (void*)f;
constexpr auto p2 = (void(*)())p1;

clang currently accepts, but these should be rejected. [expr.const] says an expression is not a constant expression if it contains a reinterpret_cast ... and converting a function pointer to an object pointer requires a reinterpret_cast.

gcc rejects, MSVC accepts p1.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Eli Friedman (efriedma-quic)

Consider:

void f();
constexpr auto p1 = (void*)f;
constexpr auto p2 = (void(*)())p1;

clang currently accepts, but these should be rejected. [expr.const] says an expression is not a constant expression if it contains a reinterpret_cast ... and converting a function pointer to an object pointer requires a reinterpret_cast.

gcc rejects, MSVC accepts p1.