[gcc r11-6614] c++: Fix ICE with CTAD in concept [PR98611]

[Patrick Palka]

https://gcc.gnu.org/g:e0bec6ceac47752616dd9fe0801344ed45db2fd3

commit r11-6614-ge0bec6ceac47752616dd9fe0801344ed45db2fd3
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Jan 12 09:34:41 2021 -0500

    c++: Fix ICE with CTAD in concept [PR98611]
    
    This patch teaches cp_walk_subtrees to visit the template represented
    by a CTAD placeholder, which would otherwise be not visited during
    find_template_parameters.  The template may be a template template
    parameter (as in the first testcase), or it may implicitly use the
    template parameters of an enclosing class template (as in the second
    testcase), and in either case we need to visit this tree to record the
    template parameters used therein for later satisfaction.
    
    gcc/cp/ChangeLog:
    
            PR c++/98611
            * tree.c (cp_walk_subtrees) <case TEMPLATE_TYPE_PARM>: Visit
            the template of a CTAD placeholder.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/98611
            * g++.dg/cpp2a/concepts-ctad1.C: New test.
            * g++.dg/cpp2a/concepts-ctad2.C: New test.

Diff:
---
 gcc/cp/tree.c                               |  5 ++++-
 gcc/testsuite/g++.dg/cpp2a/concepts-ctad1.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-ctad2.C | 13 +++++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index c536eb581a7..d339036e88e 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -5173,12 +5173,15 @@ cp_walk_subtrees (tree *tp, int *walk_subtrees_p, walk_tree_fn func,
   result = NULL_TREE;
   switch (code)
     {
+    case TEMPLATE_TYPE_PARM:
+      if (template_placeholder_p (*tp))
+	WALK_SUBTREE (CLASS_PLACEHOLDER_TEMPLATE (*tp));
+      /* Fall through.  */
     case DEFERRED_PARSE:
     case TEMPLATE_TEMPLATE_PARM:
     case BOUND_TEMPLATE_TEMPLATE_PARM:
     case UNBOUND_CLASS_TEMPLATE:
     case TEMPLATE_PARM_INDEX:
-    case TEMPLATE_TYPE_PARM:
     case TYPEOF_TYPE:
     case UNDERLYING_TYPE:
       /* None of these have subtrees other than those already walked
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ctad1.C b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad1.C
new file mode 100644
index 00000000000..ec2e4b014d7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad1.C
@@ -0,0 +1,16 @@
+// PR c++/98611
+// { dg-do compile { target c++20 } }
+
+template <class T, class U>
+concept IsSame = __is_same(T, U);
+
+template <class T, template <class...> class _Class>
+concept IsInstantiationOf = requires(T object) {
+ { _Class{object} } -> IsSame<T>;
+};
+
+template <class T> struct Degrees {};
+static_assert(IsInstantiationOf<Degrees<int>, Degrees>);
+
+template <class T> struct NotDegrees {};
+static_assert(!IsInstantiationOf<Degrees<int>, NotDegrees>);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ctad2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad2.C
new file mode 100644
index 00000000000..0d7f9790777
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ctad2.C
@@ -0,0 +1,13 @@
+// PR c++/98611
+// { dg-do compile { target c++20 } }
+
+template <class>
+struct S {
+  template <class T> struct Tmpl { Tmpl(T); };
+
+  template <class T>
+    requires requires (T object) { Tmpl{object}; }
+  static int f(T);
+};
+
+int a = S<int>::f(0);