https://gcc.gnu.org/g:7f7101cdc372c47e070e356e3e8fe980f5a1676e
commit r15-10838-g7f7101cdc372c47e070e356e3e8fe980f5a1676e Author: Patrick Palka <[email protected]> Date: Tue Feb 17 11:21:45 2026 -0500 c++: void(concept-id) evaluation [PR121822] Similar to r16-7056-g22f51c0f5e62a4, here the expression within the decltype void(Derived<T>) is non-dependent enough that we instantiate/fold it immediately, during which however convert_to_void tries to evaluate the concept-id, which fails. When in an unevaluated context such as decltype I don't think convert_to_void should be evaluating concept-ids. PR c++/121822 gcc/cp/ChangeLog: * cvt.cc (convert_to_void): Don't evaluate a concept-id in an unevaluated context. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/concepts-decltype6.C: New test. Reviewed-by: Jason Merrill <[email protected]> (cherry picked from commit 44a51e8c7037eacd4315df0e84bf51e6e4021088) Diff: --- gcc/cp/cvt.cc | 2 +- gcc/testsuite/g++.dg/cpp2a/concepts-decltype6.C | 16 ++++++++++++++++ 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc index f663a6d08c89..e3a15b900a19 100644 --- a/gcc/cp/cvt.cc +++ b/gcc/cp/cvt.cc @@ -1204,7 +1204,7 @@ convert_to_void (tree expr, impl_conv_void implicit, tsubst_flags_t complain) /* Explicitly evaluate void-converted concept checks since their satisfaction may produce ill-formed programs. */ - if (concept_check_p (expr)) + if (concept_check_p (expr) && !cp_unevaluated_operand) expr = evaluate_concept_check (expr); if (VOID_TYPE_P (TREE_TYPE (expr))) diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype6.C b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype6.C new file mode 100644 index 000000000000..03ecc7b0a07c --- /dev/null +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype6.C @@ -0,0 +1,16 @@ +// PR c++/121822 +// { dg-do compile { target c++20 } } + +template<class...> +using void_t = void; + +template<class T> +concept Derived = requires { typename T::derived_type; }; + +template<class T, class = void> +struct Wrapper; + +template<class T> +struct Wrapper<T, void_t<decltype(void(Derived<T>))>> { }; + +Wrapper<int> x;
