[gcc r14-8886] c++: NTTP type CTAD w/ tmpl from current inst [PR113649]

[gcc r14-8886] c++: NTTP type CTAD w/ tmpl from current inst [PR113649]

[Patrick Palka]

https://gcc.gnu.org/g:465b9c51f5c79ce3fbb36f1abc7194b39ad9cae7

commit r14-8886-g465b9c51f5c79ce3fbb36f1abc7194b39ad9cae7
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Feb 8 09:10:44 2024 -0500

    c++: NTTP type CTAD w/ tmpl from current inst [PR113649]
    
    Since template argument coercion happens relative to the most general
    template (for a class template at least), during NTTP type CTAD we might
    need to consider outer arguments particularly if the CTAD template is from
    the current instantiation (and so depends on outer template parameters).
    
    This patch makes do_class_deduction substitute as many levels of outer
    template arguments into a CTAD template (from the current instantiation)
    as it can take.
    
            PR c++/113649
    
    gcc/cp/ChangeLog:
    
            * pt.cc (do_class_deduction): Add outer_targs parameter.
            Substitute outer arguments into the CTAD template.
            (do_auto_deduction): Pass outer_targs to do_class_deduction.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/nontype-class65.C: New test.
    
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/pt.cc                                 | 21 +++++++++++++++++++--
 gcc/testsuite/g++.dg/cpp2a/nontype-class65.C | 16 ++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 55c23b642bcc..f9abb21a9a21 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -30682,7 +30682,7 @@ ctad_template_p (tree tmpl)
    type.  */
 
 static tree
-do_class_deduction (tree ptype, tree tmpl, tree init,
+do_class_deduction (tree ptype, tree tmpl, tree init, tree outer_targs,
 		    int flags, tsubst_flags_t complain)
 {
   /* We should have handled this in the caller.  */
@@ -30744,6 +30744,23 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
   if (type_dependent_expression_p (init))
     return ptype;
 
+  if (outer_targs)
+    {
+      int args_depth = TMPL_ARGS_DEPTH (outer_targs);
+      int parms_depth = TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (tmpl));
+      if (parms_depth > 1)
+	{
+	  /* Substitute outer arguments into this CTAD template from the
+	     current instantiation.  */
+	  int want = std::min (args_depth, parms_depth - 1);
+	  outer_targs = strip_innermost_template_args (outer_targs,
+						       args_depth - want);
+	  tmpl = tsubst (tmpl, outer_targs, complain, NULL_TREE);
+	  if (tmpl == error_mark_node)
+	    return error_mark_node;
+	}
+    }
+
   /* Don't bother with the alias rules for an equivalent template.  */
   tmpl = get_underlying_template (tmpl);
 
@@ -30999,7 +31016,7 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 
   if (tree ctmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
     /* C++17 class template argument deduction.  */
-    return do_class_deduction (type, ctmpl, init, flags, complain);
+    return do_class_deduction (type, ctmpl, init, outer_targs, flags, complain);
 
   if (init == NULL_TREE || TREE_TYPE (init) == NULL_TREE)
     /* Nothing we can do with this, even in deduction context.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class65.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class65.C
new file mode 100644
index 000000000000..8397ea5a886e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class65.C
@@ -0,0 +1,16 @@
+// PR c++/113649
+// { dg-do compile { target c++20 } }
+
+template<class... Args>
+struct A {
+  template<class Ret>
+  struct Fun { constexpr Fun(Ret(*)(Args...)) { } };
+
+  template<Fun f>
+  struct B { using type = decltype(f); };
+};
+
+bool f(char, long);
+
+using type = A<char, long>::B<&f>::type;
+using type = A<char, long>::Fun<bool>;