c++: Fix tsubsting CLASS_PLACEHOLDER_TEMPLATE [PR95434]

Here, during partial instantiation of the generic lambda, we do
tsubst_copy on the CLASS_PLACEHOLDER_TEMPLATE for U{0} which yields a
(level-lowered) TEMPLATE_TEMPLATE_PARM rather than the corresponding
TEMPLATE_DECL.  This later confuses do_class_deduction which expects
that a CLASS_PLACEHOLDER_TEMPLATE is always a TEMPLATE_DECL.

gcc/cp/ChangeLog:

	PR c++/95434
	* pt.c (tsubst) <case TEMPLATE_TYPE_PARM>: If tsubsting
	CLASS_PLACEHOLDER_TEMPLATE yields a TEMPLATE_TEMPLATE_PARM,
	adjust to its TEMPLATE_TEMPLATE_PARM_TEMPLATE_DECL.

gcc/testsuite/ChangeLog:

	PR c++/95434
	* g++.dg/cpp2a/lambda-generic9.C: New test.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index bcf4c6efe1195505585b4d9249898dc89ab8137a..373f82796047226910eea33d00967eababbfc331 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -15688,6 +15688,8 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
                    else if (tree pl = CLASS_PLACEHOLDER_TEMPLATE (t))
                      {
                        pl = tsubst_copy (pl, args, complain, in_decl);
+                       if (TREE_CODE (pl) == TEMPLATE_TEMPLATE_PARM)
+                         pl = TEMPLATE_TEMPLATE_PARM_TEMPLATE_DECL (pl);
                        CLASS_PLACEHOLDER_TEMPLATE (r) = pl;
                      }
                  }

diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic9.C b/gcc/testsuite/g++.dg/cpp2a/lambda-generic9.C
new file mode 100644 (file)
index 0000000..20ceb37
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic9.C
@@ -0,0 +1,9 @@
+// PR c++/95434
+// { dg-do compile { target c++20 } }
+
+template <class>
+void f() {
+  [] <template <class> class U> { U{0}; };
+}
+
+template void f<int>();