include 'trace' layout attributes set in subplot containers

... during validation.

This solution is not-strict as it could be, but at least fixes
the false-negative case of #3022.

src/plot_api/validate.js

@@ -281,16 +281,23 @@ function crawl(objIn, objOut, schema, list, base, path) {
 
 // the 'full' layout schema depends on the traces types presents
 function fillLayoutSchema(schema, dataOut) {
+    var layoutSchema = schema.layout.layoutAttributes;
+
     for(var i = 0; i < dataOut.length; i++) {
-        var traceType = dataOut[i].type,
-            traceLayoutAttr = schema.traces[traceType].layoutAttributes;
+        var traceOut = dataOut[i];
+        var traceSchema = schema.traces[traceOut.type];
+        var traceLayoutAttr = traceSchema.layoutAttributes;
 
         if(traceLayoutAttr) {
-            Lib.extendFlat(schema.layout.layoutAttributes, traceLayoutAttr);
+            if(traceOut.subplot) {
+                Lib.extendFlat(layoutSchema[traceSchema.attributes.subplot.dflt], traceLayoutAttr);
+            } else {
+                Lib.extendFlat(layoutSchema, traceLayoutAttr);
+            }
         }
     }
 
-    return schema.layout.layoutAttributes;
+    return layoutSchema;
 }
 
 // validation error codes

test/jasmine/tests/validate_test.js

@@ -539,4 +539,36 @@ describe('Plotly.validate', function() {
         var out = Plotly.validate(shapeMock.data, shapeMock.layout);
         expect(out).toBeUndefined();
     });
+
+    it('should work with *trace* layout attributes', function() {
+        var out = Plotly.validate([{
+            type: 'bar',
+            y: [1, 2, 1]
+        }, {
+            type: 'barpolar',
+            r: [1, 2, 3]
+        }, {
+            type: 'scatterpolar',
+            theta: [0, 90, 200],
+            subplot: 'polar2'
+        }], {
+            bargap: 0.3,
+            polar: {bargap: 0.2},
+            polar2: {bargap: 0.05},
+            polar3: {bargap: 0.4}
+        });
+
+        expect(out.length).toBe(1);
+        assertErrorContent(
+            out[0], 'unused', 'layout', null, ['polar3'], 'polar3',
+            'In layout, container polar3 did not get coerced'
+        );
+
+        // Plotly.validate should be more strict here.
+        //
+        // It should log an 'unused' warning for `polar2.bargap`,
+        // but trace layout attribute set in subplot containers are considered
+        // valid as long as one trace on that graph has those trace layout
+        // attributes.
+    });
 });