c++: Fix up default initialization with consteval default ctor [PR96994]

> > The following testcase is miscompiled (in particular the a and i
> > initialization).  The problem is that build_special_member_call due to
> > the immediate constructors (but not evaluated in constant expression mode)
> > doesn't create a CALL_EXPR, but returns a TARGET_EXPR with CONSTRUCTOR
> > as the initializer for it,
>
> That seems like the bug; at the end of build_over_call, after you
>
> >        call = cxx_constant_value (call, obj_arg);
>
> You need to build an INIT_EXPR if obj_arg isn't a dummy.

That works.  obj_arg is NULL if it is a dummy from the earlier code.

2020-10-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96994
	* call.c (build_over_call): If obj_arg is non-NULL, return INIT_EXPR
	setting obj_arg to call.

	* g++.dg/cpp2a/consteval18.C: New test.

2 files changed, 28 insertions, 0 deletions

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index dce229c..d67e8fe 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -9212,6 +9212,8 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 		}
 	    }
 	  call = cxx_constant_value (call, obj_arg);
+	  if (obj_arg && !error_operand_p (call))
+	    call = build2 (INIT_EXPR, void_type_node, obj_arg, call);
 	}
     }
   return call;
diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval18.C b/gcc/testsuite/g++.dg/cpp2a/consteval18.C
new file mode 100644
index 0000000..586fede
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval18.C
@@ -0,0 +1,26 @@
+// PR c++/96994
+// { dg-do run { target c++20 } }
+
+struct A { consteval A () { i = 1; } consteval A (int x) : i (x) {} int i = 0; };
+struct B { constexpr B () { i = 1; } constexpr B (int x) : i (x) {} int i = 0; };
+A const a;
+constexpr A b;
+B const c;
+A const constinit d;
+A const e = 2;
+constexpr A f = 3;
+B const g = 4;
+A const constinit h = 5;
+A i;
+B j;
+A k = 6;
+B l = 7;
+static_assert (b.i == 1 && f.i == 3);
+
+int
+main()
+{
+  if (a.i != 1 || c.i != 1 || d.i != 1 || e.i != 2 || g.i != 4 || h.i != 5
+      || i.i != 1 || j.i != 1 || k.i != 6 || l.i != 7)
+    __builtin_abort ();
+}