Defer creation of map object in createUnionOrIntersectionProperty

Author: ahejlsberg

src/compiler/checker.ts

@@ -10353,7 +10353,8 @@ namespace ts {
         }
 
         function createUnionOrIntersectionProperty(containingType: UnionOrIntersectionType, name: __String): Symbol | undefined {
-            const propSet = createMap<Symbol>();
+            let singleProp: Symbol | undefined;
+            let propSet: Map<Symbol> | undefined;
             let indexTypes: Type[] | undefined;
             const isUnion = containingType.flags & TypeFlags.Union;
             const excludeModifiers = isUnion ? ModifierFlags.NonPublicAccessibilityModifier : 0;
@@ -10373,9 +10374,18 @@ namespace ts {
                         else {
                             optionalFlag &= prop.flags;
                         }
-                        const id = "" + getSymbolId(prop);
-                        if (!propSet.has(id)) {
-                            propSet.set(id, prop);
+                        if (!singleProp) {
+                            singleProp = prop;
+                        }
+                        else if (prop !== singleProp) {
+                            if (!propSet) {
+                                propSet = createMap<Symbol>();
+                                propSet.set("" + getSymbolId(singleProp), singleProp);
+                            }
+                            const id = "" + getSymbolId(prop);
+                            if (!propSet.has(id)) {
+                                propSet.set(id, prop);
+                            }
                         }
                         checkFlags |= (isReadonlySymbol(prop) ? CheckFlags.Readonly : 0) |
                             (!(modifiers & ModifierFlags.NonPublicAccessibilityModifier) ? CheckFlags.ContainsPublic : 0) |
@@ -10402,13 +10412,13 @@ namespace ts {
                     }
                 }
             }
-            if (!propSet.size) {
+            if (!singleProp) {
                 return undefined;
             }
-            const props = arrayFrom(propSet.values());
-            if (props.length === 1 && !(checkFlags & CheckFlags.ReadPartial) && !indexTypes) {
-                return props[0];
+            if (!propSet && !(checkFlags & CheckFlags.ReadPartial) && !indexTypes) {
+                return singleProp;
             }
+            const props = propSet ? arrayFrom(propSet.values()) : [singleProp];
             let declarations: Declaration[] | undefined;
             let firstType: Type | undefined;
             let nameType: Type | undefined;