Resolve regressions re: invalid inputs -> default values

src/style-spec/function/convert.js

@@ -133,12 +133,22 @@ function convertPropertyFunction(parameters, propertySpec) {
         firstStopType === 'boolean'
     );
 
-    expression.push([firstStopType, ['get', parameters.property]]);
+    const expectedTypeName = firstStopType.slice(0, 1).toUpperCase() + firstStopType.slice(1);
+    const checkType = ['==', expectedTypeName, ['typeof', ['get', parameters.property]]];
+    expression.push([
+        'case',
+        checkType, [firstStopType, ['get', parameters.property]],
+        null
+    ]);
 
     for (const stop of parameters.stops) {
         expression.push(stop[0], stop[1]);
     }
 
+    if (expression[0] === 'match') {
+        expression.push(parameters.default);
+    }
+
     return expression;
 }
 

src/style-spec/function/evaluation_context.js

@@ -22,6 +22,7 @@ function assert(condition, message) {
 }
 
 module.exports = () => ({
+    assert: assert,
     error: (msg) => assert(false, msg),
 
     at: function (index, arrayOrVector) {
@@ -112,7 +113,10 @@ module.exports = () => ({
     },
 
     evaluateCurve(input, stopInputs, stopOutputs, interpolation, resultType) {
+        assert(input !== null, 'Cannot evaluate function on null input value.');
+
         const stopCount = stopInputs.length;
+        if (stopInputs.length === 1) return stopOutputs[0]();
         if (input <= stopInputs[0]) return stopOutputs[0]();
         if (input >= stopInputs[stopCount - 1]) return stopOutputs[stopCount - 1]();
 

src/style-spec/function/expressions.js

@@ -327,8 +327,6 @@ const expressions: { [string]: Definition } = {
                 outputs.push(output.js);
             }
 
-            if (stops.length === 1) return {js: `${outputs[0]}`};
-
             const interpolationOptions: Object = {
                 name: interpolation.name
             };

src/style-spec/function/index.js

@@ -23,7 +23,7 @@ function createFunction(parameters, propertySpec) {
     if (compiled.result === 'success') {
         const f = function (zoom, properties) {
             const val = compiled.function({zoom}, {properties});
-            return val;
+            return val === null ? undefined : val;
         };
         f.isFeatureConstant = compiled.isFeatureConstant;
         f.isZoomConstant = compiled.isZoomConstant;