[gcc r10-10724] c++: pack init-capture of unresolved overload [PR102629]

[gcc r10-10724] c++: pack init-capture of unresolved overload [PR102629]

[Jason Merrill]

https://gcc.gnu.org/g:93ec7bf22530610ef697fd3a64a28bebd589c790

commit r10-10724-g93ec7bf22530610ef697fd3a64a28bebd589c790
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 7ce9ac234f8..fb214e8f8c9 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -12435,7 +12435,13 @@ 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{});
+}