[gcc r13-2964] c++: fix class-valued ?: extension

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:bbdb5612f6661f2c64b0c0f1d2291cb59fde2b40

commit r13-2964-gbbdb5612f6661f2c64b0c0f1d2291cb59fde2b40
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Sep 23 09:07:22 2022 -0400

    c++: fix class-valued ?: extension
    
    When the gimplifier encounters the same TARGET_EXPR twice, it evaluates
    TARGET_EXPR_INITIAL the first time and clears it so that the later
    evaluation is just the temporary.  With this testcase, using the extension
    to treat an omitted middle operand as repeating the first operand, that led
    to doing a bitwise copy of the S(1) temporary on return rather than properly
    calling the copy constructor.
    
    We can't use S(1) to initialize the return value here anyway, because we
    need to materialize it into a temporary so we can convert it to bool and
    determine which arm we're evaluating.  So let's just treat the middle
    operand as an xvalue.
    
            PR c++/93046
    
    gcc/cp/ChangeLog:
    
            * call.cc (build_conditional_expr): For a?:c extension, treat
            a reused class prvalue as an xvalue.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/ext/cond4.C: Add runtime test.

Diff:
---
 gcc/cp/call.cc                   |  5 +++++
 gcc/testsuite/g++.dg/ext/cond4.C | 17 ++++++++++++++---
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 9fad3cb950b..bd04a1d309a 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -5402,6 +5402,11 @@ build_conditional_expr (const op_location_t &loc,
 	  arg1 = cp_stabilize_reference (arg1);
 	  arg2 = arg1 = prevent_lifetime_extension (arg1);
 	}
+      else if (TREE_CODE (arg1) == TARGET_EXPR)
+	/* arg1 can't be a prvalue result of the conditional
+	   expression, since it needs to be materialized for the
+	   conversion to bool, so treat it as an xvalue in arg2.  */
+	arg2 = move (TARGET_EXPR_SLOT (arg1));
       else
 	arg2 = arg1 = cp_save_expr (arg1);
     }
diff --git a/gcc/testsuite/g++.dg/ext/cond4.C b/gcc/testsuite/g++.dg/ext/cond4.C
index d2853f40387..86993306aa5 100644
--- a/gcc/testsuite/g++.dg/ext/cond4.C
+++ b/gcc/testsuite/g++.dg/ext/cond4.C
@@ -1,10 +1,14 @@
 // PR c++/93046
-// { dg-do compile }
+// { dg-do run }
 // { dg-options "" }
 
+int c;
+
 struct S {
-  S (int);
-  operator bool ();
+  int i;
+  S (int i) : i(i) { ++c; }
+  S (const S &s): i(s.i) { ++c; }
+  operator bool () { return i; }
 };
 
 S
@@ -12,3 +16,10 @@ foo ()
 {
   return S (1) ? : S (2);
 }
+
+int main()
+{
+  S s = foo();
+  if (s.i != 1 || c != 2)
+    __builtin_abort ();
+}