Class template argument deduction ambiguity

Question

Is the following code example in compliance with the C++20 standard or should it be rejected due to an ambiguous deduction?

template<typename...>
struct s {
    s(int) requires true {}
    s(int) {}
};
static_assert(not noexcept(s{{}})); // clang nope, gcc ok, msvc ok

Live example

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The error message from Clang:

<source>:6:28: error: ambiguous deduction for template arguments of 's'
    6 | static_assert(not noexcept(s{{}}));
      |                            ^
<source>:3:5: note: candidate function [with $0 = <>]
    3 |     s(int) requires true {}
      |     ^
<source>:4:5: note: candidate function [with $0 = <>]
    4 |     s(int) {}
      |     ^

---

As indicated by CWG Issue 2628 that Brian Bi linked in his answer, it appears that there currently is no wording in the C++20 standard that mandates the requires-clause of a constructor to be propagated to the implictly-generated deduction guide. As a workaround, it's possible to provide a user-defined deduction guide instead:

template<typename...>
requires true
s(int) -> s<>;

template<typename...>
s(int) -> s<>;

Note that for this particular code example, the deduction guide s(int) -> s<>; would also work, since a user-defined deduction guide is preferred over an implicitly generated one. It seems the consideration here would be quality of error-reporting opposed to less code bloat.

Answer 1

This is CWG 2628: when the compiler forms the implicit deduction guide from the constructor s::s(int) requires true, that deduction guide should also have requires true, which makes it more constrained than the other deduction guide; but the wording currently doesn't specify that the deduction guide gets the constraint. It seems that GCC already does it, but Clang doesn't do it yet.