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

[Marek Polacek <polacek at redhat dot com>]

On Wed, Jun 27, 2018 at 05:47:25PM -0400, Jason Merrill wrote:
> On Thu, Jun 21, 2018 at 2:22 PM, Marek Polacek <polacek@redhat.com> wrote:
> > 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  <polacek@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);
> 
> How about adding the special handling in cp_save_expr rather than
> here, so other callers also benefit?
> 
> OK with that change.

Thanks, this is what I'll commit (bootstrap/regtest passed on x86_64):

2018-06-29  Marek Polacek  <polacek@redhat.com>

	PR c++/86184
	* tree.c (cp_save_expr): Don't call save_expr for TARGET_EXPRs.

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

diff --git gcc/cp/tree.c gcc/cp/tree.c
index e7bd79b6276..361248d4b52 100644
--- gcc/cp/tree.c
+++ gcc/cp/tree.c
@@ -4918,6 +4918,11 @@ cp_save_expr (tree expr)
      tree codes.  */
   if (processing_template_decl)
     return expr;
+
+  /* TARGET_EXPRs are only expanded once.  */
+  if (TREE_CODE (expr) == TARGET_EXPR)
+    return expr;
+
   return save_expr (expr);
 }
 
diff --git gcc/testsuite/g++.dg/ext/cond3.C gcc/testsuite/g++.dg/ext/cond3.C
index e69de29bb2d..6390dc4270b 100644
--- 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 ();
+}

	Marek