https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99963
--- 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.
