https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107340
Bug ID: 107340 Summary: std::unordered_map and completeness of nested classes Product: gcc Version: 11.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: jens.maurer at gmx dot net Target Milestone: --- Consider: #include <unordered_map> struct X { struct Key { int x = 0; bool operator==(const Key&) const = default; std::size_t operator()(const Key& k) const noexcept { return k.x; } }; int a[sizeof(Key)]; std::unordered_map<Key, std::size_t, Key> m; }; X x; When compiled with "g++ -std=c++20", it yields: a1.cc:24:3: error: use of deleted function ‘X::X()’ (plus lots of details) since gcc 11.3 and 12.x; gcc 11.2 compiled this code fine. Analysis: The nested class "X::Key" is complete at its closing brace, also evidenced by the array data member declaration. Yet, "m" attains a deleted default constructor for no apparent reason. Moving class "Key" to namespace scope or providing a separate, but still nested, "X::KeyHash" class works around the issue.