tighten validation to disallow expressions as stop values

src/style-spec/validate/validate_function.js

@@ -1,6 +1,7 @@
 
 import ValidationError from '../error/validation_error';
 import getType from '../util/get_type';
+import extend from '../util/extend';
 import validate from './validate';
 import validateObject from './validate_object';
 import validateArray from './validate_array';
@@ -138,10 +139,15 @@ export default function validateFunction(options) {
             }, value));
         }
 
+        // Remove `expression` from the value spec because expressions
+        // and functions are not allowed as values within functions.
+        const stopValueSpec = extend({}, functionValueSpec);
+        delete stopValueSpec.expression;
+
         return errors.concat(validate({
             key: `${key}[1]`,
             value: value[1],
-            valueSpec: functionValueSpec,
+            valueSpec: stopValueSpec,
             style: options.style,
             styleSpec: options.styleSpec
         }));

test/unit/style-spec/fixture/functions.input.json

@@ -967,6 +967,25 @@
         "layout": {
           "line-join": [ "coalesce", ["feature-state", "myState"], "bevel"]
         }
+    }, {
+      "id": "invalid function - expression as stop value",
+      "type": "line",
+      "source": "source",
+      "source-layer": "layer",
+      "paint": {
+        "line-width": {
+          "base": 1,
+          "stops": [
+            [
+              6,
+              [
+                "literal",
+                3
+              ]
+            ]
+          ]
+        }
       }
+    }
   ]
 }

test/unit/style-spec/fixture/functions.output.json

@@ -198,5 +198,9 @@
   {                                                                                                                     
     "message": "layers[55].layout.line-join: \"feature-state\" data expressions are not supported with layout properties.",
     "line": 968
+  },
+  {
+    "message": "layers[56].paint.line-width.stops[0][1]: number expected, array found",
+    "line": 981
   }
-]
+]