[PATCH] c++: requires-expr substitution and access checking [PR107179]

[Patrick Palka <ppalka@redhat.com>]

Like during satisfaction, we need to check access immediately during
substitution of a requires-expr since the outcome of an access check can
determine the value of the requires-expr.  And otherwise, in contexts
where access checking is deferred (such as during substitution into a
base-clause), a failed access check may leak out from the requires-expr
into a non-SFINAE context and cause a hard error (as in the testcase
below).

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

	PR c++/107179

gcc/cp/ChangeLog:

	* constraint.cc (tsubst_requires_expr): Make sure we're not
	deferring access checks.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires31.C: New test.
---
 gcc/cp/constraint.cc                             |  3 +++
 gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C | 13 +++++++++++++
 2 files changed, 16 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 5e6a3bcf059..f6ef078171a 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2252,6 +2252,9 @@ tsubst_requires_expr (tree t, tree args, sat_info info)
 {
   local_specialization_stack stack (lss_copy);
 
+  /* We need to check access during the substitution.  */
+  deferring_access_check_sentinel acs (dk_no_deferred);
+
   /* A requires-expression is an unevaluated context.  */
   cp_unevaluated u;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C
new file mode 100644
index 00000000000..9b7e2a34889
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C
@@ -0,0 +1,13 @@
+// PR c++/107179
+// { dg-do compile { target c++20 } }
+
+template<bool B> struct bool_constant { static constexpr bool value = B; };
+
+template<typename T>
+  struct is_implicitly_default_constructible
+  : bool_constant<requires { T(); }>
+  { };
+
+struct X { private: X(); };
+
+static_assert( !is_implicitly_default_constructible<X>::value );
-- 
2.38.1.381.gc03801e19c