Cache isWeakType computation

Author: ahejlsberg

Diff for: src/compiler/checker.ts

@@ -20258,15 +20258,22 @@ namespace ts {
          * and no required properties, call/construct signatures or index signatures
          */
         function isWeakType(type: Type): boolean {
-            if (type.flags & TypeFlags.Object) {
-                const resolved = resolveStructuredTypeMembers(type as ObjectType);
-                return resolved.callSignatures.length === 0 && resolved.constructSignatures.length === 0 && resolved.indexInfos.length === 0 &&
-                    resolved.properties.length > 0 && every(resolved.properties, p => !!(p.flags & SymbolFlags.Optional));
+            if (!(type.flags & (TypeFlags.Object | TypeFlags.Intersection))) {
+                return false;
             }
-            if (type.flags & TypeFlags.Intersection) {
-                return every((type as IntersectionType).types, isWeakType);
+            if (!((type as ObjectType | IntersectionType).objectFlags & ObjectFlags.IsWeakTypeComputed)) {
+                let isWeak;
+                if (type.flags & TypeFlags.Object) {
+                    const resolved = resolveStructuredTypeMembers(type as ObjectType);
+                    isWeak = resolved.callSignatures.length === 0 && resolved.constructSignatures.length === 0 && resolved.indexInfos.length === 0 &&
+                        resolved.properties.length > 0 && every(resolved.properties, p => !!(p.flags & SymbolFlags.Optional));
+                }
+                else {
+                    isWeak = every((type as IntersectionType).types, isWeakType);
+                }
+                (type as ObjectType | IntersectionType).objectFlags |= ObjectFlags.IsWeakTypeComputed | (isWeak ? ObjectFlags.IsWeakType : 0);
             }
-            return false;
+            return !!((type as ObjectType | IntersectionType).objectFlags & ObjectFlags.IsWeakType);
         }
 
         function hasCommonProperties(source: Type, target: Type, isComparingJsxAttributes: boolean) {

Diff for: src/compiler/types.ts

@@ -5360,6 +5360,12 @@ namespace ts {
         IsNeverIntersectionComputed = 1 << 25, // IsNeverLike flag has been computed
         /* @internal */
         IsNeverIntersection = 1 << 26, // Intersection reduces to never
+
+        // Flags that require TypeFlags.Object or TypeFlags.Intersection
+        /* @internal */
+        IsWeakTypeComputed = 1 << 27,
+        /* @internal */
+        IsWeakType = 1 << 28,
     }
 
     /* @internal */