From b96f6cd84ca1f920c2c44b899d49d45b24c71be1 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 1 Sep 2016 06:47:18 -0700
Subject: [PATCH] Union type instead of best common supertype for multiple
 return statements

---
 src/compiler/checker.ts              | 16 ++--------------
 src/compiler/diagnosticMessages.json |  8 --------
 2 files changed, 2 insertions(+), 22 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index b59e4eae607..e49ab907d71 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12477,20 +12477,8 @@ namespace ts {
                         return isAsync ? createPromiseReturnType(func, voidType) : voidType;
                     }
                 }
-                // When yield/return statements are contextually typed we allow the return type to be a union type.
-                // Otherwise we require the yield/return expressions to have a best common supertype.
-                type = contextualSignature ? getUnionType(types, /*subtypeReduction*/ true) : getCommonSupertype(types);
-                if (!type) {
-                    if (funcIsGenerator) {
-                        error(func, Diagnostics.No_best_common_type_exists_among_yield_expressions);
-                        return createIterableIteratorType(unknownType);
-                    }
-                    else {
-                        error(func, Diagnostics.No_best_common_type_exists_among_return_expressions);
-                        // Defer to unioning the return types so we get a) downstream errors earlier and b) better Salsa experience
-                        return isAsync ? createPromiseReturnType(func, getUnionType(types, /*subtypeReduction*/ true)) : getUnionType(types, /*subtypeReduction*/ true);
-                    }
-                }
+                // Return a union of the return expression types.
+                type = getUnionType(types, /*subtypeReduction*/ true);
 
                 if (funcIsGenerator) {
                     type = createIterableIteratorType(type);
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index b8978b32571..33cd602e5a7 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1067,10 +1067,6 @@
         "category": "Error",
         "code": 2353
     },
-    "No best common type exists among return expressions.": {
-        "category": "Error",
-        "code": 2354
-    },
     "A function whose declared type is neither 'void' nor 'any' must return a value.": {
         "category": "Error",
         "code": 2355
@@ -1635,10 +1631,6 @@
         "category": "Error",
         "code": 2503
     },
-    "No best common type exists among yield expressions.": {
-        "category": "Error",
-        "code": 2504
-    },
     "A generator cannot have a 'void' type annotation.": {
         "category": "Error",
         "code": 2505