Improve perf of unions with many primitives

Author: ypresto

src/compiler/checker.ts

@@ -22168,8 +22168,17 @@ namespace ts {
         // constituent types keyed by the literal types of the property by that name in each constituent type.
         function getKeyPropertyName(unionType: UnionType): __String | undefined {
             const types = unionType.types;
-            // We only construct maps for large unions with non-primitive constituents.
-            if (types.length < 10 || getObjectFlags(unionType) & ObjectFlags.PrimitiveUnion) {
+            // We only construct maps for unions with many non-primitive constituents.
+            let nonPrimitiveCount = 0;
+            const isLargeNonPrimitives = types.length >= 10 && !(getObjectFlags(unionType) & ObjectFlags.PrimitiveUnion) &&
+                forEach(types, t => {
+                    if (t.flags & (TypeFlags.Primitive | TypeFlags.AnyOrUnknown | TypeFlags.Never)) {
+                        return
+                    }
+                    nonPrimitiveCount++;
+                    return nonPrimitiveCount >= 10;
+                });
+            if (!isLargeNonPrimitives) {
                 return undefined;
             }
             if (unionType.keyPropertyName === undefined) {