https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105425
Bug ID: 105425
Summary: ambiguous template instantiation with hana::when
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ed at catmur dot uk
Target Milestone: ---
Simplified:
template<bool> struct when;
template<int> constexpr int B = 1;
template<class, class> struct A;
template<class T, bool K> struct A<T, when<K>> {}; // fallback
template<class C> struct A<C, when<B<0> == 1>> {};
A<int, when<true>> a;
"B<0> == 1" is sufficiently complex (?) to trigger the bug (it doesn't need to
be dependent on C) but "B<0>" on its own isn't.
This affects hana since it uses the when<true> pattern heavily.
Per godbolt 11.3.0 is good, 12.0.1 20220426 is bad.
