From 25ac2bb57ae400621050a7e0845994336ca83b99 Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Tue, 3 Sep 2024 17:01:48 -0400
Subject: c++: ICE with TTP [PR96097]

We crash when dependent_type_p gets a TEMPLATE_TYPE_PARM outside
a template.  That happens here because in

  template <template <typename T, typename T::type TT> typename X>
  void func() {}
  template <typename U, int I>
  struct Y {};
  void g() { func<Y>(); }

when performing overload resolution for func<Y>() we have to check
if U matches T and I matches TT.  So we wind up in
coerce_template_template_parm/PARM_DECL.  TREE_TYPE (arg) is int
so we try to substitute TT's type, which is T::type.  But we have
nothing to substitute T with.  And we call make_typename_type where
ctx is still T, which checks dependent_scope_p and we trip the assert.

It should work to always perform the substitution in a template context.
If the result still contains template parameters, we cannot say if they
match.

	PR c++/96097

gcc/cp/ChangeLog:

	* pt.cc (coerce_template_template_parm): Increment
	processing_template_decl before calling tsubst.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp44.C: New test.
---
 gcc/cp/pt.cc                          |  2 ++
 gcc/testsuite/g++.dg/template/ttp44.C | 13 +++++++++++++
 2 files changed, 15 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/template/ttp44.C

(limited to 'gcc')

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index cb3164d..769e799 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -7951,7 +7951,9 @@ coerce_template_template_parm (tree parm, tree arg, tsubst_flags_t complain,
 	 i.e. the parameter list of TT depends on earlier parameters.  */
       if (!uses_template_parms (TREE_TYPE (arg)))
 	{
+	  ++processing_template_decl;
 	  tree t = tsubst (TREE_TYPE (parm), outer_args, complain, in_decl);
+	  --processing_template_decl;
 	  if (!uses_template_parms (t)
 	      && !same_type_p (t, TREE_TYPE (arg)))
 	    return false;
diff --git a/gcc/testsuite/g++.dg/template/ttp44.C b/gcc/testsuite/g++.dg/template/ttp44.C
new file mode 100644
index 0000000..2a41297
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/ttp44.C
@@ -0,0 +1,13 @@
+// PR c++/96097
+// { dg-do compile }
+
+template <template <typename T, typename T::type TT> class X>
+void func() {}
+
+template <typename U, int I>
+struct Y {};
+
+void test()
+{
+  func<Y>();
+}
-- 
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