Use intersection of return types

Author: Andy Hanson

src/compiler/checker.ts

@@ -17149,7 +17149,7 @@ namespace ts {
                 /*typeParameters*/ undefined, // Before calling this we tested for `!candidates.some(c => !!c.typeParameters)`.
                 thisParameter,
                 parameters,
-                /*resolvedReturnType*/ unknownType,
+                /*resolvedReturnType*/ getIntersectionType(candidates.map(getReturnTypeOfSignature)),
                 /*typePredicate*/ undefined,
                 minArgumentCount,
                 hasRestParameter,

tests/cases/fourslash/completionsCombineOverloads_returnType.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////interface A { a: number }
+////interface B { b: number }
+////declare function f(n: number): A;
+////declare function f(s: string): B;
+////f()./**/
+
+verify.completionsAt("", ["a", "b"]);