From a282cbb07e18f0ef1bfd21633f8b461fa961e42f Mon Sep 17 00:00:00 2001
From: Nathan Shively-Sanders <nathansa@microsoft.com>
Date: Mon, 7 Aug 2017 10:56:18 -0700
Subject: [PATCH] Weak type errors for signature-only types too

Now source types that only have a call signature (like functions) or construct
signature will get a weak type error too. This is really good for
catching uncalled functions:

```ts
functionTakingWeakType(returnWeakType);
// OOPS. Forgot to call `returnWeakType()`. That's an error!
```
---
 src/compiler/checker.ts | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index a8af11e7803..990bfd546a3 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8938,7 +8938,9 @@ namespace ts {
                     !(target.flags & TypeFlags.Union) &&
                     !isIntersectionConstituent &&
                     source !== globalObjectType &&
-                    getPropertiesOfType(source).length > 0 &&
+                    (getPropertiesOfType(source).length > 0 ||
+                     getSignaturesOfType(source, SignatureKind.Call).length > 0 ||
+                     getSignaturesOfType(source, SignatureKind.Construct).length > 0) &&
                     isWeakType(target) &&
                     !hasCommonProperties(source, target)) {
                     if (reportErrors) {