c++: passing one ttp to another [PR108179]

I kept trying to improve our choice of how many levels of outer_args to add,
when really the problem was that outer_args are for PARM and for this
reverse deduction we should be adding the outer arguments for ARG.

I spent quite a while trying to get DECL_CONTEXT set consistently on
template template parameters that have gone through
reduce_template_parm_level before I realized I could just use
current_scope().

	PR c++/108179
	PR c++/104107
	PR c++/95036

gcc/cp/ChangeLog:

	* pt.cc (coerce_template_template_parms): Use args from
	DECL_CONTEXT (arg_tmpl) instead of outer_args.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp35.C: New test.

2 files changed, 25 insertions, 16 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index d75c439..ddbd733 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -8110,22 +8110,24 @@ coerce_template_template_parms (tree parm_tmpl,
 
       tree pargs = template_parms_level_to_args (parm_parms);
 
-      /* PARM, and thus the context in which we are passing ARG to it, may be
-	 at a deeper level than ARG; when trying to coerce to ARG_PARMS, we
-	 want to provide the right number of levels, so we reduce the number of
-	 levels in OUTER_ARGS before prepending them.  This is most important
-	 when ARG is a namespace-scope template, as in alias-decl-ttp2.C.
-
-	 ARG might also be deeper than PARM (ttp23).  In that case, we include
-	 all of OUTER_ARGS.  The missing levels seem potentially problematic,
-	 but I can't come up with a testcase that breaks.  */
-      if (int arg_outer_levs = TMPL_PARMS_DEPTH (arg_parms_full) - 1)
-	{
-	  auto x = make_temp_override (TREE_VEC_LENGTH (outer_args));
-	  if (TMPL_ARGS_DEPTH (outer_args) > arg_outer_levs)
-	    TREE_VEC_LENGTH (outer_args) = arg_outer_levs;
-	  pargs = add_to_template_args (outer_args, pargs);
-	}
+      /* PARM and ARG might be at different template depths, and we want to
+	 pass the right additional levels of args when coercing PARGS to
+	 ARG_PARMS in case we need to do any substitution into non-type
+	 template parameter types.
+
+	 OUTER_ARGS are not the right outer levels in this case, as they are
+	 the args we're building up for PARM, and for the coercion we want the
+	 args for ARG.  If DECL_CONTEXT isn't set for a template template
+	 parameter, we can assume that it's in the current scope.  In that case
+	 we might end up adding more levels than needed, but that shouldn't be
+	 a problem; any args we need to refer to are at the right level.  */
+      tree ctx = DECL_CONTEXT (arg_tmpl);
+      if (!ctx && DECL_TEMPLATE_TEMPLATE_PARM_P (arg_tmpl))
+	ctx = current_scope ();
+      tree scope_args = NULL_TREE;
+      if (tree tinfo = get_template_info (ctx))
+	scope_args = TI_ARGS (tinfo);
+      pargs = add_to_template_args (scope_args, pargs);
 
       pargs = coerce_template_parms (arg_parms, pargs, NULL_TREE, tf_none);
       if (pargs != error_mark_node)
diff --git a/gcc/testsuite/g++.dg/template/ttp35.C b/gcc/testsuite/g++.dg/template/ttp35.C
new file mode 100644
index 0000000..4847ea4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/ttp35.C
@@ -0,0 +1,7 @@
+// PR c++/108179
+
+template <class T, T X, template <T> class F>
+struct Foo {};
+
+template <class T0, class T1, T1 X, template <T1> class F>
+void f(Foo<T1, X, F>) {}