c++: redeclare_class_template and ttps [PR110523]

Now that we cache level-lowered ttps we can end up processing the same
ttp multiple times via (multiple calls to) redeclare_class_template, so
we can't assume a ttp's DECL_CONTEXT is initially empty.

	PR c++/110523

gcc/cp/ChangeLog:

	* pt.cc (redeclare_class_template): Relax the ttp DECL_CONTEXT
	assert, and downgrade it to a checking assert.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp37.C: New test.

2 files changed, 17 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d7d774f..076f788 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -6388,7 +6388,8 @@ redeclare_class_template (tree type, tree parms, tree cons)
 	 DECL_CONTEXT of the template for which they are a parameter.  */
       if (TREE_CODE (parm) == TEMPLATE_DECL)
 	{
-	  gcc_assert (DECL_CONTEXT (parm) == NULL_TREE);
+	  gcc_checking_assert (DECL_CONTEXT (parm) == NULL_TREE
+			       || DECL_CONTEXT (parm) == tmpl);
 	  DECL_CONTEXT (parm) = tmpl;
 	}
     }
diff --git a/gcc/testsuite/g++.dg/template/ttp37.C b/gcc/testsuite/g++.dg/template/ttp37.C
new file mode 100644
index 0000000..c5f4e99
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/ttp37.C
@@ -0,0 +1,15 @@
+// PR c++/110523
+
+template<template<class> class>
+class basic_json;
+
+template<class>
+struct json_pointer {
+  template<template<class> class>
+  friend class basic_json;
+};
+
+template struct json_pointer<int>;
+template struct json_pointer<char>;
+template struct json_pointer<long>;
+template struct json_pointer<void>;