[PATCH] c++: placeholder type constraint inside range-for [PR99869]

[Patrick Palka via Gcc-patches]

In the testcase below, during ahead-of-time deduction of a constrained
auto from do_range_for_auto_deduction, we trip over the assert inside
do_auto_deduction that expects the deduction context to be
adc_return_type or adc_variable_type, but the context in this case is
adc_unspecified.

We could safely relax the assert to also accept adc_unspecified, but it
seems the context should really be adc_variable_type in this case.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

gcc/cp/ChangeLog:

        PR c++/99869
        * parser.c (do_range_for_auto_deduction): Pass adc_variable_type
        to do_auto_deduction.

gcc/testsuite/ChangeLog:

        PR c++/99869
        * g++.dg/cpp2a/concepts-placeholder6.C: New test.
---
 gcc/cp/parser.c                                    |  4 +++-
 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C | 10 ++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d0477c42afe..808e5b61b1e 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -12878,7 +12878,9 @@ do_range_for_auto_deduction (tree decl, tree range_expr)
                                            RO_UNARY_STAR,
                                            tf_warning_or_error);
          TREE_TYPE (decl) = do_auto_deduction (TREE_TYPE (decl),
-                                               iter_decl, auto_node);
+                                               iter_decl, auto_node,
+                                               tf_warning_or_error,
+                                               adc_variable_type);
        }
     }
 }
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C
new file mode 100644
index 00000000000..18e7b56d77f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C
@@ -0,0 +1,10 @@
+// PR c++/99869
+// { dg-do compile { target c++20 } }
+
+template <class T, class U> concept same_as = __is_same(T, U);
+
+template <class>
+void f() {
+  for (same_as<int> auto c : "") {} // dg-error "constraint" }
+  for (same_as<char> auto c : "") {}
+}
-- 
2.31.1.133.g84d06cdc06