Improve type checker

checker/checker.go

@@ -321,9 +321,15 @@ func (v *visitor) BinaryNode(node *ast.BinaryNode) (reflect.Type, info) {
 			return boolType, info{}
 		}
 		if isMap(r) {
+			if l == nil {
+				return v.error(node, "cannot use nil as map key")
+			}
+			if !isAny(l) && !l.AssignableTo(r.Key()) {
+				return v.error(node, "cannot use %v (type %v) as type %v in map key", l, l, r.Key())
+			}
 			return boolType, info{}
 		}
-		if isArray(r) {
+		if isInteger(l) && isArray(r) {
 			return boolType, info{}
 		}
 		if isAny(l) && anyOf(r, isString, isArray, isMap) {

checker/checker_test.go

@@ -489,6 +489,11 @@ FuncTyped(42)
 cannot use int as argument (type string) to call FuncTyped  (1:11)
  | FuncTyped(42)
  | ..........^
+
+.0 in MapOfFoo
+cannot use float64 (type float64) as type string in map key (1:4)
+ | .0 in MapOfFoo
+ | ...^
 `
 
 func TestCheck_error(t *testing.T) {