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

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.

	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.

2 files changed, 25 insertions, 1 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index a77b316..3cf1d7a 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 0000000..f3c3899
--- /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{});
+}