Better error message when an invalid expectation is used

Author: ryanplusplus

spec/mach_spec.lua

@@ -750,4 +750,10 @@ describe('The mach library', function()
         end)
     end)
   end)
+
+  it('should give a helpful error message if a non-existent expectation is used', function()
+    should_fail_with("attempt to call a nil value (field 'should_be_call')", function()
+      f:should_be_call()
+    end)
+  end)
 end)

src/mach.lua

@@ -33,6 +33,9 @@ end
 local function create_expectation(_, method)
   return function(self, ...)
     local expectation = Expectation(handle_mock_calls, self)
+    if not expectation[method] then
+      error("attempt to call a nil value (field '" .. method .. "')", 2)
+    end
     return expectation[method](expectation, ...)
   end
 end