[PATCH] c++: ICE during aggr CTAD for member tmpl [PR105476]

[Patrick Palka via Gcc-patches]

Here we're crashing from maybe_aggr_guide ultimately because
processing_template_decl isn't set when partially instantiating the
guide's parameter list.  This causes us to prematurely force completion
of the dependent type Visitor_functior<Fn>, which fails and results in
an unexpected error_mark_node.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/12.2?

        PR c++/105476

gcc/cp/ChangeLog:

        * pt.cc (maybe_aggr_guide): Set processing_template_decl when
        partially instantiating the guide's parameter list.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/class-deduction-aggr13.C: New test.
        * g++.dg/cpp2a/class-deduction-aggr13a.C: New test.
---
 gcc/cp/pt.cc                                   |  6 +++++-
 .../g++.dg/cpp2a/class-deduction-aggr13.C      | 11 +++++++++++
 .../g++.dg/cpp2a/class-deduction-aggr13a.C     | 18 ++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13a.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index ac002907a41..2bec47dc295 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -29569,7 +29569,11 @@ maybe_aggr_guide (tree tmpl, tree init, 
vec<tree,va_gc> *args)
         PARMS, so that its template level is properly reduced and we don't get
         mismatches when deducing types using the guide with PARMS.  */
       if (member_template_p)
-       parms = tsubst (parms, DECL_TI_ARGS (tmpl), complain, init);
+       {
+         ++processing_template_decl;
+         parms = tsubst (parms, DECL_TI_ARGS (tmpl), complain, init);
+         --processing_template_decl;
+       }
     }
   else if (TREE_CODE (init) == TREE_LIST)
     {
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13.C 
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13.C
new file mode 100644
index 00000000000..aa8032c60b5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13.C
@@ -0,0 +1,11 @@
+// PR c++/105476
+// { dg-do compile { target c++20 } }
+
+template<class> struct Visitor_functor;
+
+template<class> struct Events {
+  template<class... Fn> struct Visitor : Visitor_functor<Fn>::type_t... { };
+};
+
+using ev_t = Events<int>;
+ev_t::Visitor v = { {} }; // { dg-error "too many initializers" }
diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13a.C 
b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13a.C
new file mode 100644
index 00000000000..69ae5dd4b60
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-aggr13a.C
@@ -0,0 +1,18 @@
+// PR c++/105476
+// { dg-do compile { target c++20 } }
+// A valid version of class-deduction-aggr13.C.
+
+template<class> struct Visitor_functor;
+
+template<> struct Visitor_functor<int> {
+  using type_t = int;
+};
+
+template<class T> struct Events {
+  template<class Fn=T> struct Visitor {
+    Visitor_functor<Fn>::type_t t;
+  };
+};
+
+using ev_t = Events<int>;
+ev_t::Visitor v = { {} };
-- 
2.36.0.44.g0f828332d5