From 63387bb5e00d14642c6ca52470297dd5b14c8211 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Fri, 18 Nov 2016 06:13:04 -0800
Subject: [PATCH] Error on circular constraints in mapped types

---
 src/compiler/checker.ts | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5ea185556f7..f6689471cd8 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6059,6 +6059,9 @@ namespace ts {
                 type.aliasSymbol = getAliasSymbolForTypeNode(node);
                 type.aliasTypeArguments = getAliasTypeArgumentsForTypeNode(node);
                 links.resolvedType = type;
+                // Eagerly resolve the constraint type which forces an error if the constraint type circularly
+                // references itself through one or more type aliases.
+                getConstraintTypeFromMappedType(type);
             }
             return links.resolvedType;
         }