Check that the operand in a check is a call

In addition, fix bug in fold that was turning asserts into checks.

More typechecking still needs to be done.

src/comp/middle/fold.rs

@@ -1396,7 +1396,7 @@ fn identity_fold_expr_check[ENV](&ENV e, &span sp, @expr x, ann a)
 
 fn identity_fold_expr_assert[ENV](&ENV e, &span sp, @expr x, ann a)
     -> @expr {
-    ret @respan(sp, ast.expr_check(x, a));
+    ret @respan(sp, ast.expr_assert(x, a));
 }
 
 fn identity_fold_expr_port[ENV](&ENV e, &span sp, ann a) -> @expr {

src/comp/middle/typeck.rs

@@ -9,6 +9,8 @@ import util.common;
 import util.common.span;
 import util.common.plain_ann;
 
+import util.common.log_expr_err;
+
 import middle.ty;
 import middle.ty.ann_to_type;
 import middle.ty.arg;
@@ -1955,16 +1957,25 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
         }
 
         case (ast.expr_check(?e, _)) {
-            /* FIXME */
-        /* presumably, here is where we should check that e is
-         actually a call to a predicate, where all the arguments
-        are literals or slot variables? */
             auto expr_t = check_expr(fcx, e);
             Demand.simple(fcx, expr.span, ty.mk_bool(fcx.ccx.tcx),
                           expr_ty(fcx.ccx.tcx, expr_t));
-            ret @fold.respan[ast.expr_]
-                (expr.span, ast.expr_check(expr_t,
-                                                plain_ann(fcx.ccx.tcx)));
+            /* e must be a call expr where all arguments are either
+             literals or slots */
+            alt (e.node) {
+                case (ast.expr_call(?operator, ?operands, _)) {
+                    /* operator must be a pure function */
+                    /* FIXME: need more checking */
+                    ret @fold.respan[ast.expr_]
+                        (expr.span, ast.expr_check(expr_t,
+                               plain_ann(fcx.ccx.tcx)));
+                    
+                }
+                case (_) {
+                    fcx.ccx.sess.span_err(expr.span,
+                        "Check on non-predicate");
+                }
+            }
         }
 
         case (ast.expr_assert(?e, _)) {

src/test/compile-fail/not-a-pred-2.rs

@@ -0,0 +1,9 @@
+// -*- rust -*-
+// xfail-boot
+
+// error-pattern: non-predicate
+
+fn main() {
+  check (1 == 2); // should fail to typecheck, as (a == b)
+                  // is not a manifest call
+}