c++: Fix up calls to immediate functions returning reference [PR99507]

build_cxx_call calls convert_from_reference at the end, so if an immediate
function returns a reference, we were constant evaluating not just that
call, but that call wrapped in an INDIRECT_REF.  That unfortunately means
it can constant evaluate to something non-addressable, so if code later
needs to take its address it will fail.

The following patch fixes that by undoing the convert_from_reference
wrapping for the cxx_constant_value evaluation and readdding it ad the end.

2021-03-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99507
	* call.c (build_over_call): For immediate evaluation of functions
	that return references, undo convert_from_reference effects before
	calling cxx_constant_value and call convert_from_reference
	afterwards.

	* g++.dg/cpp2a/consteval19.C: New test.

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 7d12fea..33278b5 100644 (file)
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -9504,6 +9504,9 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
       if (immediate_invocation_p (fndecl, nargs))
        {
          tree obj_arg = NULL_TREE;
+         /* Undo convert_from_reference called by build_cxx_call.  */
+         if (REFERENCE_REF_P (call))
+           call = TREE_OPERAND (call, 0);
          if (DECL_CONSTRUCTOR_P (fndecl))
            obj_arg = cand->first_arg ? cand->first_arg : (*args)[0];
          if (obj_arg && is_dummy_object (obj_arg))
@@ -9527,6 +9530,7 @@ 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);
+         call = convert_from_reference (call);
        }
     }
   return call;

diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval19.C b/gcc/testsuite/g++.dg/cpp2a/consteval19.C
new file mode 100644 (file)
index 0000000..d742f59
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval19.C
@@ -0,0 +1,6 @@
+// PR c++/99507
+// { dg-do compile { target c++20 } }
+
+constexpr int i{0};
+consteval const int &iref () { return i; }
+const int *a{&iref ()};