diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 9c30cb87a4d..b254bfc1031 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -1425,9 +1425,9 @@ namespace ts {
     }
 
     function directoryOfCombinedPath(fileName: string, basePath: string) {
-        // Use the `identity` function to avoid canonicalizing the path, as it must remain noncanonical
+        // Use the `getNormalizedAbsolutePath` function to avoid canonicalizing the path, as it must remain noncanonical
         // until consistient casing errors are reported
-        return getDirectoryPath(toPath(fileName, basePath, identity));
+        return getDirectoryPath(getNormalizedAbsolutePath(fileName, basePath));
     }
 
     /**
@@ -1452,8 +1452,7 @@ namespace ts {
         Debug.assert((json === undefined && sourceFile !== undefined) || (json !== undefined && sourceFile === undefined));
         const errors: Diagnostic[] = [];
 
-        const getCanonicalFileName = createGetCanonicalFileName(host.useCaseSensitiveFileNames);
-        const parsedConfig = parseConfig(json, sourceFile, host, basePath, configFileName, getCanonicalFileName, resolutionStack, errors);
+        const parsedConfig = parseConfig(json, sourceFile, host, basePath, configFileName, resolutionStack, errors);
         const { raw } = parsedConfig;
         const options = extend(existingOptions, parsedConfig.options || {});
         options.configFilePath = configFileName;
@@ -1547,7 +1546,10 @@ namespace ts {
         raw: any;
         options?: CompilerOptions;
         typeAcquisition?: TypeAcquisition;
-        extendedConfigPath?: Path;
+        /**
+         * Note that the case of the config path has not yet been normalized, as no files have been imported into the project yet
+         */
+        extendedConfigPath?: string;
     }
 
     function isSuccessfulParsedTsconfig(value: ParsedTsconfig) {
@@ -1564,12 +1566,11 @@ namespace ts {
             host: ParseConfigHost,
             basePath: string,
             configFileName: string,
-            getCanonicalFileName: GetCanonicalFileName,
-            resolutionStack: Path[],
+            resolutionStack: string[],
             errors: Push<Diagnostic>,
     ): ParsedTsconfig {
         basePath = normalizeSlashes(basePath);
-        const resolvedPath = toPath(configFileName || "", basePath, getCanonicalFileName);
+        const resolvedPath = getNormalizedAbsolutePath(configFileName || "", basePath);
 
         if (resolutionStack.indexOf(resolvedPath) >= 0) {
             errors.push(createCompilerDiagnostic(Diagnostics.Circularity_detected_while_resolving_configuration_Colon_0, [...resolutionStack, resolvedPath].join(" -> ")));
@@ -1577,14 +1578,13 @@ namespace ts {
         }
 
         const ownConfig = json ?
-            parseOwnConfigOfJson(json, host, basePath, getCanonicalFileName, configFileName, errors) :
-            parseOwnConfigOfJsonSourceFile(sourceFile, host, basePath, getCanonicalFileName, configFileName, errors);
+            parseOwnConfigOfJson(json, host, basePath, configFileName, errors) :
+            parseOwnConfigOfJsonSourceFile(sourceFile, host, basePath, configFileName, errors);
 
         if (ownConfig.extendedConfigPath) {
             // copy the resolution stack so it is never reused between branches in potential diamond-problem scenarios.
             resolutionStack = resolutionStack.concat([resolvedPath]);
-            const extendedConfig = getExtendedConfig(sourceFile, ownConfig.extendedConfigPath, host, basePath, getCanonicalFileName,
-                resolutionStack, errors);
+            const extendedConfig = getExtendedConfig(sourceFile, ownConfig.extendedConfigPath, host, basePath, resolutionStack, errors);
             if (extendedConfig && isSuccessfulParsedTsconfig(extendedConfig)) {
                 const baseRaw = extendedConfig.raw;
                 const raw = ownConfig.raw;
@@ -1612,7 +1612,6 @@ namespace ts {
         json: any,
         host: ParseConfigHost,
         basePath: string,
-        getCanonicalFileName: GetCanonicalFileName,
         configFileName: string | undefined,
         errors: Push<Diagnostic>
     ): ParsedTsconfig {
@@ -1625,7 +1624,7 @@ namespace ts {
         // It should be removed in future releases - use typeAcquisition instead.
         const typeAcquisition = convertTypeAcquisitionFromJsonWorker(json.typeAcquisition || json.typingOptions, basePath, errors, configFileName);
         json.compileOnSave = convertCompileOnSaveOptionFromJson(json, basePath, errors);
-        let extendedConfigPath: Path;
+        let extendedConfigPath: string;
 
         if (json.extends) {
             if (!isString(json.extends)) {
@@ -1633,7 +1632,7 @@ namespace ts {
             }
             else {
                 const newBase = configFileName ? directoryOfCombinedPath(configFileName, basePath) : basePath;
-                extendedConfigPath = getExtendsConfigPath(json.extends, host, newBase, getCanonicalFileName, errors, createCompilerDiagnostic);
+                extendedConfigPath = getExtendsConfigPath(json.extends, host, newBase, errors, createCompilerDiagnostic);
             }
         }
         return { raw: json, options, typeAcquisition, extendedConfigPath };
@@ -1643,13 +1642,12 @@ namespace ts {
         sourceFile: JsonSourceFile,
         host: ParseConfigHost,
         basePath: string,
-        getCanonicalFileName: GetCanonicalFileName,
         configFileName: string | undefined,
         errors: Push<Diagnostic>
     ): ParsedTsconfig {
         const options = getDefaultCompilerOptions(configFileName);
         let typeAcquisition: TypeAcquisition, typingOptionstypeAcquisition: TypeAcquisition;
-        let extendedConfigPath: Path;
+        let extendedConfigPath: string;
 
         const optionsIterator: JsonConversionNotifier = {
             onSetValidOptionKeyValueInParent(parentOption: string, option: CommandLineOption, value: CompilerOptionsValue) {
@@ -1670,7 +1668,6 @@ namespace ts {
                             <string>value,
                             host,
                             newBase,
-                            getCanonicalFileName,
                             errors,
                             (message, arg0) =>
                                 createDiagnosticForNodeInSourceFile(sourceFile, valueNode, message, arg0)
@@ -1712,7 +1709,6 @@ namespace ts {
         extendedConfig: string,
         host: ParseConfigHost,
         basePath: string,
-        getCanonicalFileName: GetCanonicalFileName,
         errors: Push<Diagnostic>,
         createDiagnostic: (message: DiagnosticMessage, arg1?: string) => Diagnostic) {
         extendedConfig = normalizeSlashes(extendedConfig);
@@ -1721,9 +1717,9 @@ namespace ts {
             errors.push(createDiagnostic(Diagnostics.A_path_in_an_extends_option_must_be_relative_or_rooted_but_0_is_not, extendedConfig));
             return undefined;
         }
-        let extendedConfigPath = toPath(extendedConfig, basePath, getCanonicalFileName);
+        let extendedConfigPath = getNormalizedAbsolutePath(extendedConfig, basePath);
         if (!host.fileExists(extendedConfigPath) && !endsWith(extendedConfigPath, Extension.Json)) {
-            extendedConfigPath = `${extendedConfigPath}.json` as Path;
+            extendedConfigPath = `${extendedConfigPath}.json`;
             if (!host.fileExists(extendedConfigPath)) {
                 errors.push(createDiagnostic(Diagnostics.File_0_does_not_exist, extendedConfig));
                 return undefined;
@@ -1734,11 +1730,10 @@ namespace ts {
 
     function getExtendedConfig(
         sourceFile: JsonSourceFile,
-        extendedConfigPath: Path,
+        extendedConfigPath: string,
         host: ts.ParseConfigHost,
         basePath: string,
-        getCanonicalFileName: GetCanonicalFileName,
-        resolutionStack: Path[],
+        resolutionStack: string[],
         errors: Push<Diagnostic>,
     ): ParsedTsconfig | undefined {
         const extendedResult = readJsonConfigFile(extendedConfigPath, path => host.readFile(path));
@@ -1752,14 +1747,14 @@ namespace ts {
 
         const extendedDirname = getDirectoryPath(extendedConfigPath);
         const extendedConfig = parseConfig(/*json*/ undefined, extendedResult, host, extendedDirname,
-            getBaseFileName(extendedConfigPath), getCanonicalFileName, resolutionStack, errors);
+            getBaseFileName(extendedConfigPath), resolutionStack, errors);
         if (sourceFile) {
             sourceFile.extendedSourceFiles.push(...extendedResult.extendedSourceFiles);
         }
 
         if (isSuccessfulParsedTsconfig(extendedConfig)) {
             // Update the paths to reflect base path
-            const relativeDifference = convertToRelativePath(extendedDirname, basePath, getCanonicalFileName);
+            const relativeDifference = convertToRelativePath(extendedDirname, basePath, identity);
             const updatePath = (path: string) => isRootedDiskPath(path) ? path : combinePaths(relativeDifference, path);
             const mapPropertiesInRawIfNotUndefined = (propertyName: string) => {
                 if (raw[propertyName]) {
diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index fee21fcdb2d..42199c895b1 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -6795,4 +6795,48 @@ namespace ts.projectSystem {
             }
         });
     });
+
+    describe("tsserverProjectSystem forceConsistentCasingInFileNames", () => {
+        it("works when extends is specified with a case insensitive file system", () => {
+            const rootPath = "/Users/username/dev/project";
+            const file1: FileOrFolder = {
+                path: `${rootPath}/index.ts`,
+                content: 'import {x} from "file2";',
+            };
+            const file2: FileOrFolder = {
+                path: `${rootPath}/file2.js`,
+                content: "",
+            };
+            const file2Dts: FileOrFolder = {
+                path: `${rootPath}/types/file2/index.d.ts`,
+                content: "export declare const x: string;",
+            };
+            const tsconfigAll: FileOrFolder = {
+                path: `${rootPath}/tsconfig.all.json`,
+                content: JSON.stringify({
+                    compilerOptions: {
+                        baseUrl: ".",
+                        paths: { file2: ["./file2.js"] },
+                        typeRoots: ["./types"],
+                        forceConsistentCasingInFileNames: true,
+                    },
+                }),
+            };
+            const tsconfig: FileOrFolder = {
+                path: `${rootPath}/tsconfig.json`,
+                content: JSON.stringify({ extends: "./tsconfig.all.json" }),
+            };
+
+            const host = createServerHost([file1, file2, file2Dts, libFile, tsconfig, tsconfigAll], { useCaseSensitiveFileNames: false });
+            const session = createSession(host);
+
+            openFilesForSession([file1], session);
+            const projectService = session.getProjectService();
+
+            checkNumberOfProjects(projectService, { configuredProjects: 1 });
+
+            const diagnostics = configuredProjectAt(projectService, 0).getLanguageService().getCompilerOptionsDiagnostics();
+            assert.deepEqual(diagnostics, []);
+        });
+    });
 }