c++: requires-expr and access checking [PR107179]
Like during satisfaction, we also need to avoid deferring access checks
during substitution of a requires-expr because the outcome of an access
check can determine the value of the requires-expr. Otherwise (in
deferred access checking contexts such as within a base-clause), the
requires-expr may evaluate to the wrong result, and along the way a
failed access check may leak out from it into a non-SFINAE context and
cause a hard error (as in the below testcase).
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.
(cherry picked from commit
40c34beef620ed13c4113c893ed4335ccc1b8f92)