[PATCH] c++: missing SFINAE during alias CTAD [PR115296]

[Patrick Palka]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk/14?

-- >8 --

During the alias CTAD transformation, if substitution failed for some
guide we should just discard the guide silently.  We currently do
discard the guide, but not silently, which causes us to reject the
below testcase due to forming a too-large array type when transforming
the user-defined deduction guides.

This patch fixes this by passing complain=tf_none instead of
=tf_warning_or_error in the couple of spots where we expect subsitution
to possibly fail and so subsequently check for error_mark_node.

        PR c++/115296

gcc/cp/ChangeLog:

        * pt.cc (alias_ctad_tweaks): Pass complain=tf_none to
        tsubst_decl and tsubst_constraint_info.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/class-deduction-alias23.C: New test.
---
 gcc/cp/pt.cc                                  |  4 ++--
 .../g++.dg/cpp2a/class-deduction-alias23.C    | 20 +++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d1316483e24..a382dce8788 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30451,7 +30451,7 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
              /* Parms are to have DECL_CHAIN tsubsted, which would be skipped
                 if cp_unevaluated_operand.  */
              cp_evaluated ev;
-             g = tsubst_decl (DECL_TEMPLATE_RESULT (f), targs, complain,
+             g = tsubst_decl (DECL_TEMPLATE_RESULT (f), targs, tf_none,
                               /*use_spec_table=*/false);
            }
          if (g == error_mark_node)
@@ -30478,7 +30478,7 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
            {
              if (tree outer_targs = outer_template_args (f))
                ci = tsubst_constraint_info (ci, outer_targs, complain, 
in_decl);
-             ci = tsubst_constraint_info (ci, targs, complain, in_decl);
+             ci = tsubst_constraint_info (ci, targs, tf_none, in_decl);
            }
          if (ci == error_mark_node)
            continue;
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C 
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C
new file mode 100644
index 00000000000..e5766586761
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-alias23.C
@@ -0,0 +1,20 @@
+// PR c++/115296
+// { dg-do compile { target c++20 } }
+
+using size_t = decltype(sizeof(0));
+
+template<class T, size_t N = -1ULL>
+struct span { span(T); };
+
+template<class T, size_t N>
+span(T(&)[N]) -> span<T, N>; // { dg-bogus "array exceeds maximum" }
+
+template<class T, size_t N>
+requires (sizeof(T(&)[N]) != 42) // { dg-bogus "array exceeds maximum" }
+span(T*) -> span<T, N>;
+
+template<class T>
+using array_view = span<T>;
+
+span x = 0;
+array_view y = 0;
-- 
2.45.2.746.g06e570c0df