[gcc r12-3996] c++: argument order in a variadic type trait intrinsic

[gcc r12-3996] c++: argument order in a variadic type trait intrinsic

[Patrick Palka]

https://gcc.gnu.org/g:fdf8d5bc5b52bd733037dbeeff442c59f8aa765b

commit r12-3996-gfdf8d5bc5b52bd733037dbeeff442c59f8aa765b
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Sep 30 17:29:18 2021 -0400

    c++: argument order in a variadic type trait intrinsic
    
    When parsing a variadic type trait intrinsic, we build up the list of
    trailing arguments in reverse, but we neglect to reverse the list to
    the true order afterwards.  This causes us to confuse the meaning of
    e.g. __is_xible(x, y, z) vs __is_xible(x, z, y).
    
    Note that this bug doesn't affect the library traits because they pass a
    pack expansion as the single trailing argument to __is_xible, which gets
    expanded in the correct order by tsubst_tree_list.
    
    gcc/cp/ChangeLog:
    
            * parser.c (cp_parser_trait_expr): Call nreverse on the reversed
            list of trailing arguments.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/ext/is_constructible6.C: New test.

Diff:
---
 gcc/cp/parser.c                              |  1 +
 gcc/testsuite/g++.dg/ext/is_constructible6.C | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 8430445ef8c..04f5a24cc03 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -10832,6 +10832,7 @@ cp_parser_trait_expr (cp_parser* parser, enum rid keyword)
 	    return error_mark_node;
 	  type2 = tree_cons (NULL_TREE, elt, type2);
 	}
+      type2 = nreverse (type2);
     }
 
   location_t finish_loc = cp_lexer_peek_token (parser->lexer)->location;
diff --git a/gcc/testsuite/g++.dg/ext/is_constructible6.C b/gcc/testsuite/g++.dg/ext/is_constructible6.C
new file mode 100644
index 00000000000..7fce153fa75
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/is_constructible6.C
@@ -0,0 +1,10 @@
+// Verify we respect the order of trailing arguments passed to
+// __is_constructible.
+
+struct A { };
+struct B { };
+struct C { C(A, B); };
+
+extern int n[true];
+extern int n[ __is_constructible(C, A, B)];
+extern int n[!__is_constructible(C, B, A)];