Guard against missing constraint in getModifiersTypeFromMappedType

Author: ahejlsberg

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;