[gcc r12-8273] c++: pack init-capture of unresolved overload [PR102629]

[gcc r12-8273] c++: pack init-capture of unresolved overload [PR102629]

[Jason Merrill]

https://gcc.gnu.org/g:9ace5d4dab2ab39072b0f07089621a823580f27c

commit r12-8273-g9ace5d4dab2ab39072b0f07089621a823580f27c
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Apr 26 00:19:40 2022 -0400

    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.

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

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{});
+}