[Bug c++/99963] [11 Regression] [concepts] template <concept> vs concept auto reports ambiguous overload
tabloid.adroit at gmail dot com
Thu Jun 23 18:47:56 GMT 2022
--- Comment #5 from Yuanfang Chen <tabloid.adroit at gmail dot com> ---
(In reply to Yuanfang Chen from comment #4)
> (In reply to Patrick Palka from comment #1)
> > Started with r11-1571. Reduced testcase that replaces the abbreviated
> > function templates with their corresponding non-abbreviated forms:
> > template <class T> concept C1 = true;
> > template <class T> concept C2 = C1<T> && true;
> > template <C1 T, C1 U> int f(T, U);
> > template <C1 T, C2 U> int f(U, T);
> > int x = f(0, 0); // error: ambiguous call
> > If I understand the wording of P2113 correctly:
> > If deduction against the other template succeeds for both transformed
> > templates, constraints can be considered as follows:
> > - ... if the corresponding template-parameters of the
> > template-parameter-lists are not equivalent ([temp.over.link]) or if the
> > function parameters that positionally correspond between the two templates
> > are not of the same type, neither template is more specialized than the other
> > then I think we're correct to reject the call as ambiguous because although
> > the second overload is more constrained than the first, their function
> > parameter lists aren't equivalent.
> IMHO, `template <C1 T, C2 U> int f(U, T);` should win over `template <C1 T,
> C1 U> int f(T, U);`.
> Based on interpreting the intent mentioned in
> https://github.com/cplusplus/nbballot/issues/119 and the second example in
> https://eel.is/c++draft/temp.fct#temp.func.order-example-6, the
> `corresponding` (of the `corresponding template-parameters of ...`)
> relationship is based on the mapping used during partial-ordering deduction.
> So the deduction between `f(T, ..)` against `f(U, ..)` builds the <T,U>
> mapping, the deduction between `f(.., U)` against `f(.., T)` builds the <U,
> T> mapping. The correspondence is [T, U] against [U, T]. So `C1 T` is less
> constrained than `C2 U`, thus the second `f` wins.
Sorry. I spoke too soon. Template parameters reordering is not allowed for the
test case. The call is indeed ambiguous.
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