[gcc r11-9988] c++: pack init-capture of unresolved overload [PR102629]

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:f0484f60e6409ef6e837e4712d212a5d827767ba

commit r11-9988-gf0484f60e6409ef6e837e4712d212a5d827767ba
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.c (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.c                                    |  8 +++++++-
 gcc/testsuite/g++.dg/cpp2a/lambda-pack-init7.C | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index c98acbec481..a8fa2347c8a 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -12642,7 +12642,13 @@ 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{});
+}