Disallow interpolation for variable-length numeric arrays (#5519)

* Disallow interpolation for variable-length numeric arrays

* Add separate unit test for piecewise-constant check

This fix causes the 'curve' parsing validation to preempt
createExpression()'s check for piecewise-constant zoom function using
'step' interpolation, because `line-dasharray`'s style spec type,
`array<number>`, already isn't interpolatable.

Adding this unit test to make sure we always enforce step curves, even
for a theoretical style property whose base type is interpolatable.

src/style-spec/expression/definitions/curve.js

@@ -140,7 +140,12 @@ class Curve implements Expression {
         if (interpolation.name !== 'step' &&
             outputType.kind !== 'number' &&
             outputType.kind !== 'color' &&
-            !(outputType.kind === 'array' && outputType.itemType.kind === 'number')) {
+            !(
+                outputType.kind === 'array' &&
+                outputType.itemType.kind === 'number' &&
+                typeof outputType.N === 'number'
+            )
+        ) {
             return context.error(`Type ${toString(outputType)} is not interpolatable, and thus cannot be used as a ${interpolation.name} curve's output type.`);
         }
 

test/integration/expression-tests/curve/exponential-uninterpolatable-numeric-array/test.json

@@ -0,0 +1,23 @@
+{
+  "expression": [
+    "curve",
+    ["exponential", 2],
+    ["number", ["get", "x"]],
+    1,
+    ["array", "number", ["get", "array"]],
+    3,
+    ["array", "number", ["get", "array_two"]]
+  ],
+  "inputs": [],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [
+        {
+          "key": "",
+          "error": "Type array<number> is not interpolatable, and thus cannot be used as a exponential curve's output type."
+        }
+      ]
+    }
+  }
+}

test/unit/style-spec/expression.test.js

@@ -0,0 +1,21 @@
+'use strict';
+
+const test = require('mapbox-gl-js-test').test;
+const {createExpression} = require('../../../src/style-spec/expression');
+
+test('createExpression', (t) => {
+    test('require piecewise-constant zoom curves to use "step" interpolation', (t) => {
+        const expression = createExpression([
+            'curve', ['linear'], ['zoom'], 0, 0, 10, 10
+        ], {
+            type: 'number',
+            function: 'piecewise-constant'
+        });
+        t.equal(expression.result, 'error');
+        t.equal(expression.errors.length, 1);
+        t.equal(expression.errors[0].message, 'interpolation type must be "step" for this property');
+        t.end();
+    });
+
+    t.end();
+});

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

@@ -184,7 +184,7 @@
     "line": 934
   },
   {
-    "message": "layers[52].paint.line-dasharray: interpolation type must be \"step\" for this property",
+    "message": "layers[52].paint.line-dasharray: Type array<number> is not interpolatable, and thus cannot be used as a linear curve's output type.",
     "line": 943
   }
 ]