From 0ff597c3a831ef39481714f3ad39c3b1e878aaaa Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Sat, 2 Jun 2018 17:42:07 -0700
Subject: [PATCH] Improve typeof x === "function" narrowing for non-unions

---
 src/compiler/checker.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index dc43faac435..9306907b418 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -14112,11 +14112,14 @@ namespace ts {
                 if (operator === SyntaxKind.ExclamationEqualsToken || operator === SyntaxKind.ExclamationEqualsEqualsToken) {
                     assumeTrue = !assumeTrue;
                 }
+                if (type.flags & TypeFlags.Any && literal.text === "function") {
+                    return type;
+                }
                 if (assumeTrue && !(type.flags & TypeFlags.Union)) {
                     // We narrow a non-union type to an exact primitive type if the non-union type
                     // is a supertype of that primitive type. For example, type 'any' can be narrowed
                     // to one of the primitive types.
-                    const targetType = typeofTypesByName.get(literal.text);
+                    const targetType = literal.text === "function" ? globalFunctionType : typeofTypesByName.get(literal.text);
                     if (targetType) {
                         if (isTypeSubtypeOf(targetType, type)) {
                             return targetType;