From: Patrick Palka <ppalka@redhat.com>
Date: Sat, 20 Sep 2025 14:45:22 +0000 (-0400)
Subject: c++: find_template_parameters and NTTPs [PR121981]
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=396e9118849c4b918eaf3edcfa60d36e2b973019;p=thirdparty%2Fgcc.git

c++: find_template_parameters and NTTPs [PR121981]

Here the normal form of the two immediately-declared D<<placeholder>, V>
constraints is the same, so we rightfully share the normal form between
them.  We first compute the normal form from the context of auto deduction
for W in which case the placeholder has level 2 where the set of
in-scope template parameters has depth 2 (a dummy level is added from
normalize_placeholder_type_constraints).

Naturally the atomic constraint only depends on the template parameter
V of depth 1 index 0.  The depth 2 of current_template_parms however
means that find_template_parameters when it sees V within the atomic
constraint will recurse into its TREE_TYPE, an auto of level 2, and mark
the atomic constraint as also depending on the template parameter of
depth 2 index 0, which is clearly wrong.  Later during constraint
checking for B we ICE within the satisfaction cache since we lack two
levels of template arguments supposedly needed by the cached atomic
constraint.

I think when find_template_parameters sees an NTTP, it doesn't need to
walk its TREE_TYPE because NTTP substitution is done obliviously with
respect to its type -- only the corresponding NTTP argument matters,
not other template arguments possibly used within its type.  This is
most clearly true for (unconstrained) auto NTTPs as in the testcase, but
also true for other NTTPs.  Doing so fixes the testcase because we no
longer record any depth 2 when walking V within the atomic constraint.

	PR c++/121981

gcc/cp/ChangeLog:

	* pt.cc (any_template_parm_r) <case TEMPLATE_TYPE_PARM>:
	Don't walk TREE_TYPE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-placeholder15.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index b7cb807d8da..494b6be7a24 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -11125,7 +11125,10 @@ any_template_parm_r (tree t, void *data)
       break;
 
     case TEMPLATE_PARM_INDEX:
-      WALK_SUBTREE (TREE_TYPE (t));
+      /* No need to consider template parameters within the type of an NTTP:
+	 substitution into an NTTP is done directly with the corresponding
+	 template argument, and its type only comes into play earlier during
+	 coercion.  */
       break;
 
     case TEMPLATE_DECL:
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C
new file mode 100644
index 00000000000..e6571e9f18c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder15.C
@@ -0,0 +1,17 @@
+// PR c++/121981
+// { dg-do compile { target c++20 } }
+
+template<auto V>
+concept C = requires { V; };
+
+template<class T, auto V>
+concept D = C<V>;
+
+template<auto V, D<V> auto W>
+struct A { };
+
+template<auto V, D<V> T>
+struct B { };
+
+A<0, 1> a;
+B<0, int> b;