[PATCH] c++: Fix tsubsting CLASS_PLACEHOLDER_TEMPLATE [PR95434]

[Patrick Palka via Gcc-patches]

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

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

gcc/cp/ChangeLog:

        PR c++/95434
        * pt.c (tsubst) <case TEMPLATE_TEMPLATE_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.
---
 gcc/cp/pt.c                                  | 2 ++
 gcc/testsuite/g++.dg/cpp2a/lambda-generic9.C | 9 +++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic9.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d5d3d2fd040..2fed81520e3 100644
--- 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
index 00000000000..20ceb370c38
--- /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>();
-- 
2.30.0.155.g66e871b664