From c110f57deb0c423c4e160ab5bce74c35187cb039 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Wed, 10 Oct 2018 15:11:35 -0700
Subject: [PATCH] Discriminant must include at least one unit type and no
 instantiable types

---
 src/compiler/checker.ts | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9578f7a106b..795b893f69e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -12919,10 +12919,6 @@ namespace ts {
                 isUnitType(type);
         }
 
-        function maybeUnitType(type: Type): boolean {
-            return type.flags & TypeFlags.Union ? some((<UnionType>type).types, isUnitType) : isUnitType(type);
-        }
-
         function getBaseTypeOfLiteralType(type: Type): Type {
             return type.flags & TypeFlags.EnumLiteral ? getBaseTypeOfEnumLiteralType(<LiteralType>type) :
                 type.flags & TypeFlags.StringLiteral ? stringType :
@@ -14216,12 +14212,26 @@ namespace ts {
             return undefined;
         }
 
+        function isDiscriminantType(type: Type): boolean {
+            if (type.flags & TypeFlags.Union) {
+                if (type.flags & (TypeFlags.Boolean | TypeFlags.EnumLiteral)) {
+                    return true;
+                }
+                let combined = 0;
+                for (const t of (<UnionType>type).types) combined |= t.flags;
+                if (combined & TypeFlags.Unit && !(combined & TypeFlags.Instantiable)) {
+                    return true;
+                }
+            }
+            return false;
+        }
+
         function isDiscriminantProperty(type: Type | undefined, name: __String) {
             if (type && type.flags & TypeFlags.Union) {
                 const prop = getUnionOrIntersectionProperty(<UnionType>type, name);
                 if (prop && getCheckFlags(prop) & CheckFlags.SyntheticProperty) {
                     if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
-                        (<TransientSymbol>prop).isDiscriminantProperty = !!((<TransientSymbol>prop).checkFlags & CheckFlags.HasNonUniformType) && maybeUnitType(getTypeOfSymbol(prop));
+                        (<TransientSymbol>prop).isDiscriminantProperty = !!((<TransientSymbol>prop).checkFlags & CheckFlags.HasNonUniformType) && isDiscriminantType(getTypeOfSymbol(prop));
                     }
                     return !!(<TransientSymbol>prop).isDiscriminantProperty;
                 }