Transfer all declared symbols onto the type from the interface that extends another

Author: Andarist

src/compiler/checker.ts

@@ -12583,7 +12583,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         const baseTypes = getBaseTypes(source);
         if (baseTypes.length) {
             if (source.symbol && members === getMembersOfSymbol(source.symbol)) {
-                members = createSymbolTable(source.declaredProperties);
+                members = createSymbolTable(arrayFrom(members.values()));
             }
             setStructuredTypeMembers(type, members, callSignatures, constructSignatures, indexInfos);
             const thisArgument = lastOrUndefined(typeArguments);

tests/cases/fourslash/quickInfoOnPropDeclaredUsingIndexSignatureOnInterfaceWithBase.ts

@@ -0,0 +1,12 @@
+/// <reference path="fourslash.ts" />
+
+// https://github.com/microsoft/TypeScript/issues/55251
+
+//// interface P {}
+//// interface B extends P {
+////   [k: string]: number;
+//// }
+//// declare const b: B;
+//// b.t/*1*/est = 10;
+
+verify.quickInfoAt("1", "(index) B[string]: number");