[gcc r10-9484] c++: Fix up calls to immediate functions returning reference [PR99507]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:3ef0d71f64408b527169a740ea7dc5f4d169b0b6

commit r10-9484-g3ef0d71f64408b527169a740ea7dc5f4d169b0b6
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Mar 12 10:11:24 2021 +0100

    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.
    
    (cherry picked from commit 425afe1f0c907e6469cef1672160c9c95177e71a)

Diff:
---
 gcc/cp/call.c                            | 4 ++++
 gcc/testsuite/g++.dg/cpp2a/consteval19.C | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 33a95cbc958..aa8d84fa90c 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -9178,6 +9178,9 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	      || !current_binding_level->immediate_fn_ctx_p))
 	{
 	  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))
@@ -9201,6 +9204,7 @@ 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
index 00000000000..d742f59070f
--- /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 ()};