C++ PATCH for c++/86184, rejects-valid with ?: and omitted operand

[Marek Polacek]

The following testcase is rejected because, for this line:

  bool b = X() ?: false;

arg2 is missing and arg1 is a TARGET_EXPR.  A TARGET_EXPR is a class
prvalue so we wrap it in a SAVE_EXPR.  Later when building 'this' we
call build_this (SAVE_EXPR <TARGET_EXPR <...>>) which triggers lvalue_error:
 5856       cp_lvalue_kind kind = lvalue_kind (arg);
 5857       if (kind == clk_none)
 5858         {
 5859           if (complain & tf_error)
 5860             lvalue_error (input_location, lv_addressof);
because all SAVE_EXPRs are non-lvalue.

Since
a) cp_build_addr_expr_1 can process xvalues and class prvalues,
b) TARGET_EXPRs are only evaluated once (gimplify_target_expr),
I thought we could do the following.  The testcase ensures that
with the omitted operand we only construct X once.

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

2018-06-21  Marek Polacek  <pola...@redhat.com>

        PR c++/86184
        * call.c (build_conditional_expr_1): Don't wrap TARGET_EXPRs
        in a SAVE_EXPR.

        * g++.dg/ext/cond3.C: New test.

--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -4806,6 +4806,10 @@ build_conditional_expr_1 (location_t loc, tree arg1, 
tree arg2, tree arg3,
       /* Make sure that lvalues remain lvalues.  See g++.oliva/ext1.C.  */
       if (lvalue_p (arg1))
        arg2 = arg1 = cp_stabilize_reference (arg1);
+      else if (TREE_CODE (arg1) == TARGET_EXPR)
+       /* TARGET_EXPRs are only expanded once, don't wrap it in a SAVE_EXPR,
+          rendering it clk_none of clk_class.  */
+       arg2 = arg1;
       else
        arg2 = arg1 = cp_save_expr (arg1);
     }
--- gcc/testsuite/g++.dg/ext/cond3.C
+++ gcc/testsuite/g++.dg/ext/cond3.C
@@ -0,0 +1,20 @@
+// PR c++/86184
+// { dg-do run }
+// { dg-options "" }
+
+int j;
+struct X {
+  X() { j++; }
+  operator bool() { return true; }
+};
+
+/* Only create X once.  */
+bool b = X() ?: false;
+bool b2 = X() ? X() : false;
+
+int
+main ()
+{
+  if (j != 3)
+    __builtin_abort ();
+}