compiler error when accessing a static constexpr value of struct

[llvmbot]

Bugzilla Link	25573
Version	3.7
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid

Extended Description

// The following code compiles in gcc (http://goo.gl/gFGc9K), but not clang (http://goo.gl/woxdi1).
#include <type_traits>
template<class T> struct Foo { Foo(T = nullptr) {} };
using U = std::is_constructible<Foo<int>>;
bool b = U::value;  // If we remove this line, it compiles in both gcc and clang. 
                    // However the value of `b` is true, which is counterintuitive.
int main() {}

[zygoloid]

The default argument instantiation is outside the immediate context of the template argument substitution, so it should be a hard error. The question is, should we have triggered default argument instantiation here? GCC's result -- claiming that Foo is default constructible despite that construction resulting in an error -- doesn't seem any better than Clang's.

[AaronBallman]

Clang and EDG reject for the same reason, GCC accepts and sets b to 0. MSVC accepts and sets b to 1. https://godbolt.org/z/q3EKM5b71

The question is, should we have triggered default argument instantiation here?

I believe Clang and EDG are correct -- is_constructible is not being passed any additional arguments, so I believe the code violates: https://eel.is/c++draft/type.traits#meta.unary.prop-9

Based on that, I believe this issue should be closed. If there's disagreement with this assessment, feel free to reopen with more information.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

Details

| | | | --- | --- | | Bugzilla Link | [25573](https://llvm.org/bz25573) | | Version | 3.7 | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@zygoloid |

Extended Description

// The following code compiles in gcc (http://goo.gl/gFGc9K), but not clang (http://goo.gl/woxdi1).
#include <type_traits>
template<class T> struct Foo { Foo(T = nullptr) {} };
using U = std::is_constructible<Foo<int>>;
bool b = U::value;  // If we remove this line, it compiles in both gcc and clang. 
                    // However the value of `b` is true, which is counterintuitive.
int main() {}