c++: make -Wctad-maybe-unsupported respect complain [PR105143]

We were attempting to issue a -Wctad-maybe-unsupported warning even when
complain=tf_none, which led to a crash in the first testcase below and a
bogus error during overload resolution in the second testcase.

	PR c++/105143

gcc/cp/ChangeLog:

	* pt.cc (do_class_deduction): Check complain before attempting
	to issue a -Wctad-maybe-unsupported warning.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nodiscard1.C: New test.
	* g++.dg/warn/Wctad-maybe-unsupported4.C: New test.

3 files changed, 27 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index eeebc4c..63794a4 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30091,7 +30091,7 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
 
   /* If CTAD succeeded but the type doesn't have any explicit deduction
      guides, this deduction might not be what the user intended.  */
-  if (fndecl != error_mark_node && !any_dguides_p)
+  if (fndecl != error_mark_node && !any_dguides_p && (complain & tf_warning))
     {
       if ((!DECL_IN_SYSTEM_HEADER (fndecl)
 	   || global_dc->dc_warn_system_headers)
diff --git a/gcc/testsuite/g++.dg/cpp2a/nodiscard1.C b/gcc/testsuite/g++.dg/cpp2a/nodiscard1.C
new file mode 100644
index 0000000..c3c5094
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nodiscard1.C
@@ -0,0 +1,13 @@
+// PR c++/105143
+// { dg-do compile { target c++20 } }
+// We used to crash here with "Error reporting routines re-entered".
+
+template<class...> struct A { };
+
+template<A V> using type = int;
+
+template<A V> [[nodiscard]] type<V> get();
+
+int main() {
+  get<{}>(); // { dg-warning "nodiscard" }
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wctad-maybe-unsupported4.C b/gcc/testsuite/g++.dg/warn/Wctad-maybe-unsupported4.C
new file mode 100644
index 0000000..92ed821
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wctad-maybe-unsupported4.C
@@ -0,0 +1,13 @@
+// PR c++/105143
+// { dg-do compile { target c++17 } }
+// { dg-additional-options "-Werror=ctad-maybe-unsupported" }
+
+template<class...> struct A { };
+
+template<template<class...> class TT> auto f(...) -> decltype(TT()); // #1
+template<template<class...> class TT> void f(int); // #2
+
+int main() {
+  f<A>(0); // Calls #2 without issuing a -Wctad-maybe-unsupported
+	   // diagnostic for #1.
+}