From 47bcfb3ddbdbf5e1df805884d8534b68332256af Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Tue, 20 Dec 2016 10:27:08 -0800
Subject: [PATCH] Guard against missing constraint in
 getModifiersTypeFromMappedType

---
 src/compiler/checker.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1c22fc36207..36b952dd046 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4612,8 +4612,8 @@ namespace ts {
                     // the modifiers type is T. Otherwise, the modifiers type is {}.
                     const declaredType = <MappedType>getTypeFromMappedTypeNode(type.declaration);
                     const constraint = getConstraintTypeFromMappedType(declaredType);
-                    const extendedConstraint = constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
-                    type.modifiersType = extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
+                    const extendedConstraint = constraint && constraint.flags & TypeFlags.TypeParameter ? getConstraintOfTypeParameter(<TypeParameter>constraint) : constraint;
+                    type.modifiersType = extendedConstraint && extendedConstraint.flags & TypeFlags.Index ? instantiateType((<IndexType>extendedConstraint).type, type.mapper || identityMapper) : emptyObjectType;
                 }
             }
             return type.modifiersType;