Merge pull request #4632 from Microsoft/fixCompletionAssert

Fix completion assert

Author: mhegazy

src/compiler/checker.ts

@@ -14172,7 +14172,10 @@ namespace ts {
                 let symbols: Symbol[] = [];
                 let name = symbol.name;
                 forEach(getSymbolLinks(symbol).containingType.types, t => {
-                    symbols.push(getPropertyOfType(t, name));
+                    let symbol = getPropertyOfType(t, name);
+                    if (symbol) {
+                        symbols.push(symbol);
+                    }
                 });
                 return symbols;
             }

tests/cases/fourslash/genericTypeAliasIntersectionCompletions.ts

@@ -0,0 +1,33 @@
+/// <reference path="fourslash.ts" />
+
+//// type MixinCtor<A, B> = new () => A & B & { constructor: MixinCtor<A, B> };
+//// function merge<A, B>(a: { prototype: A }, b: { prototype: B }): MixinCtor<A, B> {
+////   let merged = function() { }
+////   Object.assign(merged.prototype, a.prototype, b.prototype);
+////   return <MixinCtor<A, B>><any>merged;
+//// }
+////
+//// class TreeNode {
+////   value: any;
+//// }
+////
+//// abstract class LeftSideNode extends TreeNode {
+////   abstract right(): TreeNode;
+////   left(): TreeNode {
+////     return null;
+////   }
+//// }
+////
+//// abstract class RightSideNode extends TreeNode {
+////   abstract left(): TreeNode;
+////   right(): TreeNode {
+////     return null;
+////   };
+//// }
+////
+//// var obj = new (merge(LeftSideNode, RightSideNode))();
+//// obj./**/
+
+goTo.marker();
+verify.completionListContains("left");
+verify.completionListContains("right");