[PATCH] c++: Fix paren init of aggregates in unevaluated context [PR92947]

[Marek Polacek]

When I implemented C++20 parenthesized initialization of aggregates
I introduced this bogus cp_unevaluated_operand check, thus disabling
this feature in unevaluated context.  Oop.

Removing the check turned up another bug: I wasn't checking the
return value of digest_init.  So when constructible_expr called
build_new_method_call_1 to see if we can construct one type from
another, it got back a bogus INIT_EXPR that looked something like
*(struct T &) 1 = <<< error >>>.  But that isn't the error_mark_node,
so constructible_expr thought we had been successful in creating the
ctor call, and it gave the wrong answer.  Covered by paren-init17.C.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

        PR c++/92947 - Paren init of aggregates in unevaluated context.
        * call.c (build_new_method_call_1): Don't check
        cp_unevaluated_operand.  Check the return value of digest_init.

        * g++.dg/cpp2a/paren-init21.C: New test.
---
 gcc/cp/call.c                             |  3 ++-
 gcc/testsuite/g++.dg/cpp2a/paren-init21.C | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init21.C

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index fde29f8f631..51621b7dd87 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -10179,7 +10179,6 @@ build_new_method_call_1 (tree instance, tree fns, 
vec<tree, va_gc> **args,
         the two.  */
       if (DECL_CONSTRUCTOR_P (fn)
          && !(flags & LOOKUP_ONLYCONVERTING)
-         && !cp_unevaluated_operand
          && cxx_dialect >= cxx2a
          && CP_AGGREGATE_TYPE_P (basetype)
          && !user_args->is_empty ())
@@ -10194,6 +10193,8 @@ build_new_method_call_1 (tree instance, tree fns, 
vec<tree, va_gc> **args,
          else
            {
              ctor = digest_init (basetype, ctor, complain);
+             if (ctor == error_mark_node)
+               return error_mark_node;
              ctor = build2 (INIT_EXPR, TREE_TYPE (instance), instance, ctor);
              TREE_SIDE_EFFECTS (ctor) = true;
              return ctor;
diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init21.C 
b/gcc/testsuite/g++.dg/cpp2a/paren-init21.C
new file mode 100644
index 00000000000..cfd37193f66
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/paren-init21.C
@@ -0,0 +1,14 @@
+// PR c++/92947 - Paren init of aggregates in unevaluated context.
+// { dg-do compile { target c++2a } }
+
+struct A {
+  int a;
+  int b;
+};
+
+int main()
+{
+  static_assert(__is_constructible(A, int, int));
+  decltype(A(1,2)) foo;
+  bool b = noexcept(A(1,2));
+}

base-commit: 572992c8920d5339a3ac28d442c436d6daa0bfae
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA