diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index fe63d0c1c3b..2831a591fbc 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4197,6 +4197,7 @@ namespace ts {
             let props: Symbol[];
             // Flags we want to propagate to the result if they exist in all source symbols
             let commonFlags = (containingType.flags & TypeFlags.Intersection) ? SymbolFlags.Optional : SymbolFlags.None;
+            let isReadonly = false;
             for (const current of types) {
                 const type = getApparentType(current);
                 if (type !== unknownType) {
@@ -4209,6 +4210,9 @@ namespace ts {
                         else if (!contains(props, prop)) {
                             props.push(prop);
                         }
+                        if (isReadonlySymbol(prop)) {
+                            isReadonly = true;
+                        }
                     }
                     else if (containingType.flags & TypeFlags.Union) {
                         // A union type requires the property to be present in all constituent types
@@ -4238,6 +4242,7 @@ namespace ts {
                 name);
             result.containingType = containingType;
             result.declarations = declarations;
+            result.isReadonly = isReadonly;
             result.type = containingType.flags & TypeFlags.Union ? getUnionType(propTypes) : getIntersectionType(propTypes);
             return result;
         }
@@ -11975,11 +11980,9 @@ namespace ts {
             // Variables declared with 'const'
             // Get accessors without matching set accessors
             // Enum members
-            // Unions and intersections of the above
-            if (symbol.flags & SymbolFlags.SyntheticProperty) {
-                return forEach(symbol.declarations, decl => isReadonlySymbol(getSymbolOfNode(decl)));
-            }
-            return symbol.flags & SymbolFlags.Property && (getDeclarationFlagsFromSymbol(symbol) & NodeFlags.Readonly) !== 0 ||
+            // Unions and intersections of the above (unions and intersections eagerly set isReadonly on creation)
+            return symbol.isReadonly ||
+                symbol.flags & SymbolFlags.Property && (getDeclarationFlagsFromSymbol(symbol) & NodeFlags.Readonly) !== 0 ||
                 symbol.flags & SymbolFlags.Variable && (getDeclarationFlagsFromSymbol(symbol) & NodeFlags.Const) !== 0 ||
                 symbol.flags & SymbolFlags.Accessor && !(symbol.flags & SymbolFlags.SetAccessor) ||
                 (symbol.flags & SymbolFlags.EnumMember) !== 0;
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index db34bb7c6e8..7487d1db258 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2104,6 +2104,7 @@ namespace ts {
         members?: SymbolTable;                  // Class, interface or literal instance members
         exports?: SymbolTable;                  // Module exports
         globalExports?: SymbolTable;            // Conditional global UMD exports
+        /* @internal */ isReadonly?: boolean;   // readonly? (set only for intersections and unions)
         /* @internal */ id?: number;            // Unique id (used to look up SymbolLinks)
         /* @internal */ mergeId?: number;       // Merge id (used to look up merged symbol)
         /* @internal */ parent?: Symbol;        // Parent symbol