diff --git a/src/compiler/builder.ts b/src/compiler/builder.ts
index 6784485d21cac..a4f279d6b4116 100644
--- a/src/compiler/builder.ts
+++ b/src/compiler/builder.ts
@@ -68,7 +68,6 @@ import {
     Program,
     ProjectReference,
     ReadBuildProgramHost,
-    ReadonlyCollection,
     RepopulateDiagnosticChainInfo,
     RepopulateModuleNotFoundDiagnosticChain,
     returnFalse,
@@ -307,8 +306,8 @@ function getPendingEmitKind(
 }
 
 function hasSameKeys(
-    map1: ReadonlyCollection<string> | undefined,
-    map2: ReadonlyCollection<string> | undefined,
+    map1: ReadonlyMap<string, unknown> | ReadonlySet<string> | undefined,
+    map2: ReadonlyMap<string, unknown> | ReadonlySet<string> | undefined,
 ): boolean {
     // Has same size and every key is present in both maps
     return map1 === map2 || map1 !== undefined && map2 !== undefined && map1.size === map2.size && !forEachKey(map1, key => !map2.has(key));
diff --git a/src/compiler/corePublic.ts b/src/compiler/corePublic.ts
index ed4b500aaec63..291f11a71182a 100644
--- a/src/compiler/corePublic.ts
+++ b/src/compiler/corePublic.ts
@@ -22,17 +22,6 @@ export interface SortedArray<T> extends Array<T> {
     " __sortedArrayBrand": any;
 }
 
-/**
- * Common read methods for ES6 Map/Set.
- *
- * @internal
- */
-export interface ReadonlyCollection<K> {
-    readonly size: number;
-    has(key: K): boolean;
-    keys(): IterableIterator<K>;
-}
-
 /** @internal */
 export type EqualityComparer<T> = (a: T, b: T) => boolean;
 
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 1b972e990f7fb..20f8d6c261def 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -479,7 +479,6 @@ import {
     PunctuationSyntaxKind,
     QualifiedName,
     QuestionQuestionEqualsToken,
-    ReadonlyCollection,
     ReadonlyTextRange,
     removeTrailingDirectorySeparator,
     RequireOrImportCall,
@@ -734,7 +733,7 @@ export function forEachEntry<K, V, U>(map: ReadonlyMap<K, V>, callback: (value:
  *
  * @internal
  */
-export function forEachKey<K, T>(map: ReadonlyCollection<K>, callback: (key: K) => T | undefined): T | undefined {
+export function forEachKey<K, T>(map: ReadonlyMap<K, unknown> | ReadonlySet<K>, callback: (key: K) => T | undefined): T | undefined {
     const iterator = map.keys();
     for (const key of iterator) {
         const result = callback(key);
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index c3d16e0da5b25..df85b7c2b526b 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -99,7 +99,6 @@ import {
     ProjectPackageJsonInfo,
     ProjectReference,
     ReadMapFile,
-    ReadonlyCollection,
     removeFileExtension,
     removeIgnoredPath,
     removeMinAndVersionNumbers,
@@ -4345,7 +4344,7 @@ export class ProjectService {
     }
 
     /** @internal */
-    loadAncestorProjectTree(forProjects?: ReadonlyCollection<string>) {
+    loadAncestorProjectTree(forProjects?: ReadonlyMap<string, boolean> | ReadonlySet<string>) {
         forProjects ??= new Set(
             mapDefinedIterator(this.configuredProjects.entries(), ([key, project]) => !project.isInitialLoadPending() ? key : undefined),
         );
@@ -4364,7 +4363,7 @@ export class ProjectService {
         }
     }
 
-    private ensureProjectChildren(project: ConfiguredProject, forProjects: ReadonlyCollection<string>, seenProjects: Set<NormalizedPath>) {
+    private ensureProjectChildren(project: ConfiguredProject, forProjects: ReadonlyMap<string, boolean> | ReadonlySet<string>, seenProjects: Set<NormalizedPath>) {
         if (!tryAddToSet(seenProjects, project.canonicalConfigFilePath)) return;
 
         // If this project disables child load ignore it
diff --git a/src/services/codefixes/convertToEsModule.ts b/src/services/codefixes/convertToEsModule.ts
index 02d1a5f930114..64e75a5b68686 100644
--- a/src/services/codefixes/convertToEsModule.ts
+++ b/src/services/codefixes/convertToEsModule.ts
@@ -68,7 +68,6 @@ import {
     PropertyAccessExpression,
     QuotePreference,
     rangeContainsRange,
-    ReadonlyCollection,
     ScriptTarget,
     some,
     SourceFile,
@@ -396,7 +395,7 @@ function convertReExportAll(reExported: StringLiteralLike, checker: TypeChecker)
     // `module.exports = require("x");` ==> `export * from "x"; export { default } from "x";`
     const moduleSpecifier = reExported.text;
     const moduleSymbol = checker.getSymbolAtLocation(reExported);
-    const exports = moduleSymbol ? moduleSymbol.exports! : emptyMap as ReadonlyCollection<__String>;
+    const exports = moduleSymbol ? moduleSymbol.exports! : emptyMap as ReadonlyMap<__String, never>;
     return exports.has(InternalSymbolName.ExportEquals) ? [[reExportDefault(moduleSpecifier)], true] :
         !exports.has(InternalSymbolName.Default) ? [[reExportStar(moduleSpecifier)], false] :
         // If there's some non-default export, must include both `export *` and `export default`.
diff --git a/src/testRunner/unittests/helpers/baseline.ts b/src/testRunner/unittests/helpers/baseline.ts
index 7f5cc3e7390cd..ff7901bd529ed 100644
--- a/src/testRunner/unittests/helpers/baseline.ts
+++ b/src/testRunner/unittests/helpers/baseline.ts
@@ -127,7 +127,7 @@ function baselineProgram(baseline: string[], [program, builderProgram]: CommandL
     baseline.push("");
 }
 
-export function generateSourceMapBaselineFiles(sys: ts.System & { writtenFiles: ts.ReadonlyCollection<ts.Path>; }) {
+export function generateSourceMapBaselineFiles(sys: ts.System & { writtenFiles: Map<ts.Path, unknown> | Set<ts.Path>; }) {
     const mapFileNames = ts.mapDefinedIterator(sys.writtenFiles.keys(), f => f.endsWith(".map") ? f : undefined);
     for (const mapFile of mapFileNames) {
         const text = Harness.SourceMapRecorder.getSourceMapRecordWithSystem(sys, mapFile);