Allow discriminant property to contain some non-unit types

Author: ahejlsberg

src/compiler/checker.ts

@@ -12919,6 +12919,10 @@ 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 :
@@ -14217,7 +14221,7 @@ namespace ts {
                 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) && isLiteralType(getTypeOfSymbol(prop));
+                        (<TransientSymbol>prop).isDiscriminantProperty = !!((<TransientSymbol>prop).checkFlags & CheckFlags.HasNonUniformType) && maybeUnitType(getTypeOfSymbol(prop));
                     }
                     return !!(<TransientSymbol>prop).isDiscriminantProperty;
                 }