diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5c187d6ad8a..4bd2dd54908 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -9012,7 +9012,7 @@ namespace ts {
                             neverType;
                 }
             }
-            return getUnionTypeFromSortedList(typeSet, includes & TypeFlags.NotPrimitiveUnion ? 0 : TypeFlags.UnionOfPrimitiveTypes, aliasSymbol, aliasTypeArguments);
+            return getUnionTypeFromSortedList(typeSet, !(includes & TypeFlags.NotPrimitiveUnion), aliasSymbol, aliasTypeArguments);
         }
 
         function getUnionTypePredicate(signatures: ReadonlyArray<Signature>): TypePredicate | undefined {
@@ -9052,7 +9052,7 @@ namespace ts {
         }
 
         // This function assumes the constituent type list is sorted and deduplicated.
-        function getUnionTypeFromSortedList(types: Type[], unionOfUnitTypes: TypeFlags, aliasSymbol?: Symbol, aliasTypeArguments?: ReadonlyArray<Type>): Type {
+        function getUnionTypeFromSortedList(types: Type[], primitiveTypesOnly: boolean, aliasSymbol?: Symbol, aliasTypeArguments?: ReadonlyArray<Type>): Type {
             if (types.length === 0) {
                 return neverType;
             }
@@ -9063,9 +9063,10 @@ namespace ts {
             let type = unionTypes.get(id);
             if (!type) {
                 const propagatedFlags = getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
-                type = <UnionType>createType(TypeFlags.Union | propagatedFlags | unionOfUnitTypes);
+                type = <UnionType>createType(TypeFlags.Union | propagatedFlags);
                 unionTypes.set(id, type);
                 type.types = types;
+                type.primitiveTypesOnly = primitiveTypesOnly;
                 /*
                 Note: This is the alias symbol (or lack thereof) that we see when we first encounter this union type.
                 For aliases of identical unions, eg `type T = A | B; type U = A | B`, the symbol of the first alias encountered is the aliasSymbol.
@@ -9157,13 +9158,16 @@ namespace ts {
         // other unions and return true. Otherwise, do nothing and return false.
         function intersectUnionsOfPrimitiveTypes(types: Type[]) {
             let unionTypes: UnionType[] | undefined;
-            const index = findIndex(types, t => (t.flags & TypeFlags.UnionOfPrimitiveTypes) !== 0);
+            const index = findIndex(types, t => !!(t.flags & TypeFlags.Union) && (<UnionType>t).primitiveTypesOnly);
+            if (index < 0) {
+                return false;
+            }
             let i = index + 1;
             // Remove all but the first union of primitive types and collect them in
             // the unionTypes array.
             while (i < types.length) {
                 const t = types[i];
-                if (t.flags & TypeFlags.UnionOfPrimitiveTypes) {
+                if (t.flags & TypeFlags.Union && (<UnionType>t).primitiveTypesOnly) {
                     (unionTypes || (unionTypes = [<UnionType>types[index]])).push(<UnionType>t);
                     orderedRemoveItemAt(types, i);
                 }
@@ -9190,7 +9194,7 @@ namespace ts {
                 }
             }
             // Finally replace the first union with the result
-            types[index] = getUnionTypeFromSortedList(result, TypeFlags.UnionOfPrimitiveTypes);
+            types[index] = getUnionTypeFromSortedList(result, /*primitiveTypesOnly*/ true);
             return true;
         }
 
@@ -9232,7 +9236,7 @@ namespace ts {
                 return typeSet[0];
             }
             if (includes & TypeFlags.Union) {
-                if (includes & TypeFlags.UnionOfPrimitiveTypes && intersectUnionsOfPrimitiveTypes(typeSet)) {
+                if (intersectUnionsOfPrimitiveTypes(typeSet)) {
                     // When the intersection creates a reduced set (which might mean that *all* union types have
                     // disappeared), we restart the operation to get a new set of combined flags. Once we have
                     // reduced we'll never reduce again, so this occurs at most once.
@@ -11809,12 +11813,14 @@ namespace ts {
             }
 
             // Keep this up-to-date with the same logic within `getApparentTypeOfContextualType`, since they should behave similarly
-            function findMatchingDiscriminantType(source: Type, target: UnionOrIntersectionType) {
-                const sourceProperties = getPropertiesOfObjectType(source);
-                if (sourceProperties) {
-                    const sourcePropertiesFiltered = findDiscriminantProperties(sourceProperties, target);
-                    if (sourcePropertiesFiltered) {
-                        return discriminateTypeByDiscriminableItems(target, map(sourcePropertiesFiltered, p => ([() => getTypeOfSymbol(p), p.escapedName] as [() => Type, __String])), isRelatedTo);
+            function findMatchingDiscriminantType(source: Type, target: Type) {
+                if (target.flags & TypeFlags.Union) {
+                    const sourceProperties = getPropertiesOfObjectType(source);
+                    if (sourceProperties) {
+                        const sourcePropertiesFiltered = findDiscriminantProperties(sourceProperties, target);
+                        if (sourcePropertiesFiltered) {
+                            return discriminateTypeByDiscriminableItems(<UnionType>target, map(sourcePropertiesFiltered, p => ([() => getTypeOfSymbol(p), p.escapedName] as [() => Type, __String])), isRelatedTo);
+                        }
                     }
                 }
                 return undefined;
@@ -14846,7 +14852,7 @@ namespace ts {
             if (type.flags & TypeFlags.Union) {
                 const types = (<UnionType>type).types;
                 const filtered = filter(types, f);
-                return filtered === types ? type : getUnionTypeFromSortedList(filtered, type.flags & TypeFlags.UnionOfPrimitiveTypes);
+                return filtered === types ? type : getUnionTypeFromSortedList(filtered, (<UnionType>type).primitiveTypesOnly);
             }
             return f(type) ? type : neverType;
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 46a5ed91275..0ca4de3f537 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -3838,13 +3838,11 @@ namespace ts {
         /* @internal */
         FreshLiteral            = 1 << 27,  // Fresh literal or unique type
         /* @internal */
-        UnionOfPrimitiveTypes   = 1 << 28,  // Type is union of primitive types
+        ContainsWideningType    = 1 << 28,  // Type is or contains undefined or null widening type
         /* @internal */
-        ContainsWideningType    = 1 << 29,  // Type is or contains undefined or null widening type
+        ContainsObjectLiteral   = 1 << 29,  // Type is or contains object literal type
         /* @internal */
-        ContainsObjectLiteral   = 1 << 30,  // Type is or contains object literal type
-        /* @internal */
-        ContainsAnyFunctionType = 1 << 31,  // Type is or contains the anyFunctionType
+        ContainsAnyFunctionType = 1 << 30,  // Type is or contains the anyFunctionType
 
         /* @internal */
         AnyOrUnknown = Any | Unknown,
@@ -4077,7 +4075,10 @@ namespace ts {
         couldContainTypeVariables: boolean;
     }
 
-    export interface UnionType extends UnionOrIntersectionType { }
+    export interface UnionType extends UnionOrIntersectionType {
+        /* @internal */
+        primitiveTypesOnly: boolean;
+    }
 
     export interface IntersectionType extends UnionOrIntersectionType {
         /* @internal */