[gcc r11-7963] c++: placeholder type constraint inside range-for [PR99869]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:260caabe10cde0158b87968a3bac85575301dafa

commit r11-7963-g260caabe10cde0158b87968a3bac85575301dafa
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Apr 2 19:46:24 2021 -0400

    c++: placeholder type constraint inside range-for [PR99869]
    
    In the testcase below, during ahead-of-time deduction of a constrained
    auto inside a range-based for loop, we trip over an assert within
    do_auto_deduction which expects the deduction context to be
    adc_return_type or adc_variable_type, but do_range_for_auto_deduction
    calls do_auto_deduction with the context defaulted to adc_unspecified.
    
    We could safely relax the assert to also accept adc_unspecified, but it
    seems the deduction context should really be adc_variable_type here.
    
    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.

Diff:
---
 gcc/cp/parser.c                                    |  4 +++-
 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder6.C | 10 ++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

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..fe1e3927129
--- /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 : "") {}
+}