c++: fix contracts with NRV

The NRV implementation was blindly replacing the operand of RETURN_EXPR,
clobbering anything that check_return_expr might have added on to the actual
initialization, such as checking the postcondition.

gcc/cp/ChangeLog:

	* semantics.cc (finalize_nrv_r): [RETURN_EXPR]: Only replace the
	INIT_EXPR.

gcc/testsuite/ChangeLog:

	* g++.dg/contracts/contracts-post7.C: New test.

1 files changed, 29 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/contracts/contracts-post7.C b/gcc/testsuite/g++.dg/contracts/contracts-post7.C
new file mode 100644
index 0000000..1c33181
--- /dev/null
+++ b/gcc/testsuite/g++.dg/contracts/contracts-post7.C
@@ -0,0 +1,29 @@
+// { dg-do run }
+// { dg-options "-std=c++2a -fcontracts" }
+
+#include <experimental/contract>
+
+using std::experimental::contract_violation;
+void handle_contract_violation(const contract_violation &violation)
+{
+  __builtin_exit (0);
+}
+
+struct A {
+  int i;
+  A(): i(42) {}
+  A(const A&);
+};
+
+A f()
+  [[ post r: r.i == 24 ]]
+{
+  A a;
+  return a;
+}
+
+int main()
+{
+  f();
+  return -1;
+}