[pushed] c++: pack init-capture of unresolved overload [PR102629]

[Jason Merrill via Gcc-patches]

Here we were failing to diagnose that the initializer for the capture pack
is an unresolved overload.  It turns out that the reason we didn't recognize
the deduction failure in do_auto_deduction was that the individual 'auto' in
the expansion of the capture pack was still marked as a parameter pack, so
we were deducing it to an empty pack instead of failing.

Tested x86_64-pc-linux-gnu, applying to trunk.

        PR c++/102629

gcc/cp/ChangeLog:

        * pt.cc (gen_elem_of_pack_expansion_instantiation): Clear
        TEMPLATE_TYPE_PARAMETER_PACK on auto.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/lambda-pack-init7.C: New test.
---
 gcc/cp/pt.cc                                   |  8 +++++++-
 gcc/testsuite/g++.dg/cpp2a/lambda-pack-init7.C | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-pack-init7.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index a77b3166818..3cf1d7af8d2 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -12682,7 +12682,13 @@ gen_elem_of_pack_expansion_instantiation (tree pattern,
     t = tsubst_expr (pattern, args, complain, in_decl,
                     /*integral_constant_expression_p=*/false);
   else
-    t = tsubst (pattern, args, complain, in_decl);
+    {
+      t = tsubst (pattern, args, complain, in_decl);
+      if (is_auto (t) && !ith_elem_is_expansion)
+       /* When expanding the fake auto... pack expansion from add_capture, we
+          need to mark that the expansion is no longer a pack.  */
+       TEMPLATE_TYPE_PARAMETER_PACK (t) = false;
+    }
 
   /*  If the Ith argument pack element is a pack expansion, then
       the Ith element resulting from the substituting is going to
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init7.C 
b/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init7.C
new file mode 100644
index 00000000000..f3c3899e97a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-pack-init7.C
@@ -0,0 +1,18 @@
+// PR c++/102629
+// { dg-do compile { target c++20 } }
+
+template <class T> T&& forward(T&);
+template <class T> T&& forward(T&&);
+
+struct S {};
+
+template <typename... Args>
+void foo(Args&&... args) {
+  [...args = forward<Args> /*(args)*/] { // { dg-error "" }
+    [](auto...) { } (forward<Args>(args)...);
+  };
+}
+
+void bar( ) {
+  foo(S{});
+}

base-commit: 7d31c678d68d7b6820a958584619ca763b0eb9c5
-- 
2.27.0