Use union types in the return type of functions in the error case

Fixes #6663

(cherry picked from commit da6e82f639)

src/compiler/checker.ts

@@ -10433,7 +10433,8 @@ namespace ts {
                     }
                     else {
                         error(func, Diagnostics.No_best_common_type_exists_among_return_expressions);
-                        return unknownType;
+                        // Defer to unioning the return types so we get a) downstream errors earlier and b) better Salsa experience
+                        return getUnionType(types);
                     }
                 }
 

tests/cases/fourslash/getJavaScriptCompletions19.ts

@@ -0,0 +1,25 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: file.js
+//// function fn() {
+//// 	if (foo) {
+//// 		return 0;
+//// 	} else {
+//// 		return '0';
+//// 	}
+//// }
+//// let x = fn();
+//// if(typeof x === 'string') {
+//// 	x/*str*/
+//// } else {
+//// 	x/*num*/
+//// }
+
+goTo.marker('str');
+edit.insert('.');
+verify.completionListContains('substr', undefined, undefined, 'method');
+
+goTo.marker('num');
+edit.insert('.');
+verify.completionListContains('toFixed', undefined, undefined, 'method');