rustc: Demand that if conditions have bool type

Closes #513

src/comp/middle/typeck.rs

@@ -1712,6 +1712,9 @@ fn check_expr(&@fn_ctxt fcx, &@ast::expr expr) {
         }
         case (ast::expr_if(?cond, ?thn, ?elsopt, ?a)) {
             check_expr(fcx, cond);
+            demand::simple(fcx, cond.span,
+                           ty::mk_bool(fcx.ccx.tcx),
+                           expr_ty(fcx.ccx.tcx, cond));
             check_then_else(fcx, thn, elsopt, a, expr.span);
         }
         case (ast::expr_for(?decl, ?seq, ?body, ?a)) {

src/test/compile-fail/if-typeck.rs

@@ -0,0 +1,11 @@
+// xfail-stage0
+// error-pattern:mismatched types
+// issue #513
+
+fn f() {}
+
+fn main() {
+  // f is not a bool
+  if (f) {
+  }
+}