Swap the expected/actual for typechecking fields of updated records

In 'rec(a = b with c)', if a exists in c then the expected type for b is the
type of c.

Author: brson

src/comp/middle/typeck.rs

@@ -2120,8 +2120,8 @@ fn check_expr(&@fn_ctxt fcx, &@ast::expr expr) {
                         auto found = false;
                         for (ty::field bf in base_fields) {
                             if (str::eq(f.ident, bf.ident)) {
-                                demand::simple(fcx, expr.span, f.mt.ty,
-                                               bf.mt.ty);
+                                demand::simple(fcx, expr.span, bf.mt.ty,
+                                               f.mt.ty);
                                 found = true;
                             }
                         }

src/test/compile-fail/rec-extend.rs

@@ -0,0 +1,9 @@
+// xfail-stage0
+// error-pattern:expected int but found bool
+
+fn main() {
+
+  auto a = rec(foo = 0);
+
+  auto b = rec(foo = true with a);
+}