[over.match.class.deduct]-p6 Wrong (comments for some) examples (a7, a8)

[mrussoLuxoft]

Reporter: Mauro Russo

Reference (section label): [over.match.class.deduct] part 6

Link to reflector thread (if any):
None

Issue description:

The two examples (with a7 and a8) from [over.match.class.deduct]-p6 (draft of Nov 2024):

template <class T> struct A {
  explicit A(const T&, ...) noexcept;               // #1
  A(T&&, ...);                                      // #2
};

...

template <class T> A(const T&, const T&) -> A<T&>;  // #3
template <class T> explicit A(T&&, T&&) -> A<T>;    // #4

...

A a7 = {0, i};      // error: #3 deduces to A<int&>, #1 and #2 declare same constructor
A a8{0,i};          // error: #3 deduces to A<int&>, #1 and #2 declare same constructor

are wrong, as preliminary discussed on:
https://lists.isocpp.org/std-discussion/2024/11/2715.php (and 2718.php)
https://lists.isocpp.org/std-discussion/2024/11/2736.php

Indeed, deduction fails because guides from #2 and #3 (see the full example in standard text or on godbolt link https://godbolt.org/z/oejf5P1Tc) are non matching better each other.
The aforementioned text reads, instead, that #3 wins (successful deduction) and then the error is because, consequently, constructors #1 and #2 ambiguate, which would be true in case the deduction had led to T = int&.

Suggested resolution:

I would propose the following changes, one with the error about overload resolution between two deduction guides, and one where a deduction guide wins and two equivalent constructors are consequently generated, so that both topics of the original comments are shown. I guess it is not that important to select between copy-list-initialization (a7) or not (a8), as another example (see a5 in standard text) already shows that the explicit user-defined deduction guide is involved (as per the text of [over.match.list]).

A a7 = {0, i};      // error: #2 and #3 both match, overload resolution fails
A a8{i,i};          // error: #3 deduces to A<int&>, #1 and #2 declare same constructor

(see https://godbolt.org/z/9Ecf8943x)

Do you believe it is interesting to highlight that even #1 matches for a7 ? (but #3 wins compared to it).

Additional info:

Previously open as an issue here: cplusplus/draft#7445
There, see also cplusplus/draft#7444 if it helps.