Bootstrapped and regtested on x86_64-pc-linux-gnu, does this
look OK for trunk/backports? Not sure if we want to do this for
every choice of impl_conv_void, or just for ICV_CAST?
-- >8 --
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 covert_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.
---
gcc/cp/cvt.cc | 2 +-
gcc/testsuite/g++.dg/cpp2a/concepts-decltype6.C | 16 ++++++++++++++++
2 files changed, 17 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-decltype6.C
diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index fe9b9dc6dd18..4042938da5e9 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1210,7 +1210,7 @@ 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);
/* Detect using expressions of consteval-only types outside manifestly
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..3af2697b95d1
--- /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;
--
2.53.0.80.g6fcee47852
