https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102247
Bug ID: 102247 Summary: Overload resolution with brace-init is ambiguous when it shouldn't be Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: ldionne.2 at gmail dot com Target Milestone: --- The following code fails to compile when I believe it should succeed (https://godbolt.org/z/z9aved8Wb): #include <type_traits> template <class T1, class T2> struct pair { template<class U1 = T1, class U2 = T2> constexpr pair(U1&&, U2&&) { } }; struct BraceInit { BraceInit() = default; }; struct ExplicitBraceInit { explicit ExplicitBraceInit() = default; }; constexpr int f(pair<ExplicitBraceInit, ExplicitBraceInit>) { return 1; } constexpr int f(pair<BraceInit, BraceInit>) { return 2; } static_assert(f({{}, {}}) == 2, ""); Indeed, the error is <source>:15:16: error: call of overloaded 'f(<brace-enclosed initializer list>)' is ambiguous 15 | static_assert(f({{}, {}}) == 2, ""); | ~^~~~~~~~~~ <source>:12:15: note: candidate: 'constexpr int f(pair<ExplicitBraceInit, ExplicitBraceInit>)' 12 | constexpr int f(pair<ExplicitBraceInit, ExplicitBraceInit>) { return 1; } | ^ <source>:13:15: note: candidate: 'constexpr int f(pair<BraceInit, BraceInit>)' 13 | constexpr int f(pair<BraceInit, BraceInit>) { return 2; } | ^ Compiler returned: 1 I think it should succeed because `f(pair<ExplicitBraceInit, ExplicitBraceInit>)` can never be selected, since selecting it would require using the explicit constructor of `ExplicitBraceInit`. And indeed, if we try to call `f(pair<ExplicitBraceInit, ExplicitBraceInit>)` alone, we get (https://godbolt.org/z/W33Pvnnoe): <source>:15:16: error: converting to 'ExplicitBraceInit' from initializer list would use explicit constructor 'constexpr ExplicitBraceInit::ExplicitBraceInit()' 15 | static_assert(f({{}, {}}) == 2, ""); | ~^~~~~~~~~~ Compiler returned: 1 So I'm not sure why that overload is considered valid for overload resolution (leading to the ambiguity), but it seems like GCC itself doesn't think it's valid when there are no other candidates. Also note that Clang compiles this code without issue, calling the non-explicit BraceInit version as expected. This issue was uncovered while implementing http://wg21.link/P1951 in libc++.