A conversion function template shall not have a deduced return type

[llvmbot]

Bugzilla Link	52434
Version	trunk
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@zygoloid

Extended Description

This code is invalid:

struct S {
    template<class=void>
    operator auto() const { return 2; }
};

int main() {
    S s;
    [[maybe_unused]] int d = s;
}

because the standard explicitly prohibits conversion function template having a deduced return type:
https://timsong-cpp.github.io/cppwp/n4861/class.conv.fct#6

But now Clang accepts the program:
https://gcc.godbolt.org/z/rMK4zboEd
which is a bug.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [52434](https://llvm.org/bz52434) | | Version | trunk | | OS | Windows NT | | Reporter | LLVM Bugzilla Contributor | | CC | @zygoloid |

Extended Description

This code is invalid:

struct S {
    template<class=void>
    operator auto() const { return 2; }
};

int main() {
    S s;
    [[maybe_unused]] int d = s;
}

because the standard explicitly prohibits conversion function template having a deduced return type:
https://timsong-cpp.github.io/cppwp/n4861/class.conv.fct#6

But now Clang accepts the program:
https://gcc.godbolt.org/z/rMK4zboEd
which is a bug.

[llvmbot]

@llvm/issue-subscribers-c-14

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [52434](https://llvm.org/bz52434) | | Version | trunk | | OS | Windows NT | | Reporter | LLVM Bugzilla Contributor | | CC | @zygoloid |

Extended Description

This code is invalid:

struct S {
    template<class=void>
    operator auto() const { return 2; }
};

int main() {
    S s;
    [[maybe_unused]] int d = s;
}

because the standard explicitly prohibits conversion function template having a deduced return type:
https://timsong-cpp.github.io/cppwp/n4861/class.conv.fct#6

But now Clang accepts the program:
https://gcc.godbolt.org/z/rMK4zboEd
which is a bug.