[clang] void is apparently referenceable

[cjdb]

The following shouldn't be well-formed, but it is in Clang.

template<class T>
using with_reference = T&;

template<class T>
concept can_reference = requires {
  typename with_reference<T>;
};

template<class T>
concept dereferenceable = requires(T& t) {
  { *t } -> can_reference;
};

static_assert(dereferenceable<void*>);

Clang invocation: clang++ -std=c++20 -pedantic -pedantic-errors file.cpp

Version:

clang version 15.0.0 (https://github.com/llvm/llvm-project.git 186a13f647acac623898bcffa4b966624d2358ba)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/compiler-explorer/clang-trunk/bin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[cjdb]

I don't think this is exclusively a C++20 problem; it's just that my minimal repro exposes this using C++20.

[erichkeane]

This seems related to: #57487

Apparently this should just result in a failed concept check (NOT be ill-formed).

[ahatanak]

The static assert fails if I replace T->isVoidType() in Sema::BuildReferenceType with T.getNonReferenceType()->isVoidType(). Type void & is being passed to Sema::BuildReferenceType and the error isn't detected there because it's a reference to void.

llvm-project/clang/lib/Sema/SemaType.cpp

Line 2249 in 682c956

if (T->isVoidType()) {

The error message doesn't look quite correct though.

$ clang++ -std=c++20 -c -o /dev/null file.cpp
file.cpp:14:1: error: static assertion failed
static_assert(dereferenceable<void*>);
^ ~~~~~~~~~~~~~~~~~~~~~~
file.cpp:14:15: note: because 'void ' does not satisfy 'dereferenceable'
static_assert(dereferenceable<void>);
^
file.cpp:11:5: note: because 'void &' does not satisfy 'can_reference':
{ *t } -> can_reference;
^
file.cpp:2:1: note: because 'typename with_reference' would be invalid: cannot form a reference to 'void'
using with_reference = T&;
^
1 error generated.

[erichkeane]

The bug I thought was related was not, so I'll look into this separately.