[PATCH] c++: ICE w/ dependently scoped template friend [PR119378]

[Patrick Palka]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
14/13/12 (since it was reported against GCC 12)?  This old regression is
already fixed on trunk by r15-123 which contains this change.

-- >8 --

Here we ICE during instantiation of the dependently scoped template
friend

  template<int N>
  struct<class T>
  friend class A<N>::B;

ultimately because processing_template_decl isn't set during
substitution into the A<N> scope.  Since it's naturally a partial
substitution, we need to make sure the flag is set.

For GCC 15, this is fixed similarly by r15-123.

        PR c++/119378

gcc/cp/ChangeLog:

        * pt.cc (tsubst) <case UNBOUND_CLASS_TEMPLATE>: Set
        processing_template_decl when substituting the context.

gcc/testsuite/ChangeLog:

        * g++.dg/template/friend85.C: New test.
---
 gcc/cp/pt.cc                             |  2 ++
 gcc/testsuite/g++.dg/template/friend85.C | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/template/friend85.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index ca34ed75d65..2efc7eb19e8 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -16946,8 +16946,10 @@ tsubst (tree t, tree args, tsubst_flags_t complain, 
tree in_decl)
 
     case UNBOUND_CLASS_TEMPLATE:
       {
+       ++processing_template_decl;
        tree ctx = tsubst_aggr_type (TYPE_CONTEXT (t), args, complain,
                                     in_decl, /*entering_scope=*/1);
+       --processing_template_decl;
        tree name = TYPE_IDENTIFIER (t);
        tree parm_list = DECL_TEMPLATE_PARMS (TYPE_NAME (t));
 
diff --git a/gcc/testsuite/g++.dg/template/friend85.C 
b/gcc/testsuite/g++.dg/template/friend85.C
new file mode 100644
index 00000000000..5cf83911193
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/friend85.C
@@ -0,0 +1,16 @@
+// PR c++/119378
+
+template<int N>
+struct A {
+  template<class T>
+  struct B;
+};
+
+template<class U>
+struct C {
+  template<int N>
+  template<class T>
+  friend class A<N>::B;
+};
+
+template struct C<int>;
-- 
2.49.0.rc1.37.ge969bc8759