diff --git a/.vscode/launch.json b/.vscode/launch.json
index 35c5c3194ec3..1c2b4b3dd803 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -18,6 +18,20 @@
           "sourceMaps": true,
           "console": "integratedTerminal",
           "internalConsoleOptions": "neverOpen"
+      },
+      {
+        "type": "node",
+        "request": "launch",
+        "name": "Run currently opened typescript-estree test",
+        "cwd": "${workspaceFolder}/packages/typescript-estree/",
+        "program": "${workspaceFolder}/node_modules/jest/bin/jest.js",
+        "args": [
+            "--runInBand",
+            "${relativeFile}"
+        ],
+        "sourceMaps": true,
+        "console": "integratedTerminal",
+        "internalConsoleOptions": "neverOpen"
       }
   ]
 }
diff --git a/packages/parser/README.md b/packages/parser/README.md
index 042fb4f5f540..9fcb9d043633 100644
--- a/packages/parser/README.md
+++ b/packages/parser/README.md
@@ -66,7 +66,9 @@ The following additional configuration options are available by specifying them
     ];
     ```
 
-  - Note that if you use project references, TypeScript will not automatically use project references to resolve files. This means that you will have to add each referenced tsconfig to the `project` field either separately, or via a glob.
+  - If you use project references, TypeScript will not automatically use project references to resolve files. This means that you will have to add each referenced tsconfig to the `project` field either separately, or via a glob.
+
+  - TypeScript will ignore files with duplicate filenames in the same folder (for example, `src/file.ts` and `src/file.js`). TypeScript purposely ignore all but one of the files, only keeping the one file with the highest priority extension (the extension priority order (from highest to lowest) is `.ts`, `.tsx`, `.d.ts`, `.js`, `.jsx`). For more info see #955.
 
   - Note that if this setting is specified and `createDefaultProgram` is not, you must only lint files that are included in the projects as defined by the provided `tsconfig.json` files. If your existing configuration does not include all of the files you would like to lint, you can create a separate `tsconfig.eslint.json` as follows:
 
diff --git a/packages/typescript-estree/src/WatchCompilerHostOfConfigFile.ts b/packages/typescript-estree/src/WatchCompilerHostOfConfigFile.ts
new file mode 100644
index 000000000000..7fb4663b985c
--- /dev/null
+++ b/packages/typescript-estree/src/WatchCompilerHostOfConfigFile.ts
@@ -0,0 +1,37 @@
+// These types are internal to TS.
+// They have been trimmed down to only include the relevant bits
+// We use some special internal TS apis to help us do our parsing flexibly
+
+import * as ts from 'typescript'; // leave this as * as ts so people using util package don't need syntheticDefaultImports
+
+// https://github.com/microsoft/TypeScript/blob/b84e65db4ea5c39dbaa2ccd6594efe4653318251/src/compiler/watchUtilities.ts#L6-L18
+interface DirectoryStructureHost {
+  readDirectory?(
+    path: string,
+    extensions?: ReadonlyArray<string>,
+    exclude?: ReadonlyArray<string>,
+    include?: ReadonlyArray<string>,
+    depth?: number,
+  ): string[];
+}
+
+// https://github.com/microsoft/TypeScript/blob/b84e65db4ea5c39dbaa2ccd6594efe4653318251/src/compiler/watchUtilities.ts#L25-L35
+interface CachedDirectoryStructureHost extends DirectoryStructureHost {
+  readDirectory(
+    path: string,
+    extensions?: ReadonlyArray<string>,
+    exclude?: ReadonlyArray<string>,
+    include?: ReadonlyArray<string>,
+    depth?: number,
+  ): string[];
+}
+
+// https://github.com/microsoft/TypeScript/blob/5d36aab06f12b0a3ba6197eb14e98204ec0195fb/src/compiler/watch.ts#L548-L554
+interface WatchCompilerHostOfConfigFile<T extends ts.BuilderProgram>
+  extends ts.WatchCompilerHostOfConfigFile<T> {
+  onCachedDirectoryStructureHostCreate(
+    host: CachedDirectoryStructureHost,
+  ): void;
+}
+
+export { WatchCompilerHostOfConfigFile };
diff --git a/packages/typescript-estree/src/tsconfig-parser.ts b/packages/typescript-estree/src/tsconfig-parser.ts
index 364dd0543c8e..e6ef356b6f30 100644
--- a/packages/typescript-estree/src/tsconfig-parser.ts
+++ b/packages/typescript-estree/src/tsconfig-parser.ts
@@ -1,6 +1,7 @@
 import path from 'path';
 import * as ts from 'typescript'; // leave this as * as ts so people using util package don't need syntheticDefaultImports
 import { Extra } from './parser-options';
+import { WatchCompilerHostOfConfigFile } from './WatchCompilerHostOfConfigFile';
 
 //------------------------------------------------------------------------------
 // Environment calculation
@@ -13,6 +14,8 @@ export const defaultCompilerOptions: ts.CompilerOptions = {
   allowNonTsExtensions: true,
   allowJs: true,
   checkJs: true,
+  noEmit: true,
+  // extendedDiagnostics: true,
 };
 
 /**
@@ -115,7 +118,7 @@ export function calculateProjectParserOptions(
       ts.createSemanticDiagnosticsBuilderProgram,
       diagnosticReporter,
       /*reportWatchStatus*/ () => {},
-    );
+    ) as WatchCompilerHostOfConfigFile<ts.SemanticDiagnosticsBuilderProgram>;
 
     // ensure readFile reads the code being linted instead of the copy on disk
     const oldReadFile = watchCompilerHost.readFile;
@@ -144,8 +147,7 @@ export function calculateProjectParserOptions(
     };
 
     // register callbacks to trigger program updates without using fileWatchers
-    // eslint-disable-next-line @typescript-eslint/explicit-function-return-type
-    watchCompilerHost.watchFile = (fileName, callback) => {
+    watchCompilerHost.watchFile = (fileName, callback): ts.FileWatcher => {
       const normalizedFileName = path.normalize(fileName);
       watchCallbackTrackingMap.set(normalizedFileName, callback);
       return {
@@ -156,26 +158,20 @@ export function calculateProjectParserOptions(
     };
 
     // ensure fileWatchers aren't created for directories
-    watchCompilerHost.watchDirectory = (): typeof noopFileWatcher =>
-      noopFileWatcher;
+    watchCompilerHost.watchDirectory = (): ts.FileWatcher => noopFileWatcher;
 
-    // we're using internal typescript APIs which aren't on the types
-    /* eslint-disable @typescript-eslint/no-explicit-any */
     // allow files with custom extensions to be included in program (uses internal ts api)
-    const oldOnDirectoryStructureHostCreate = (watchCompilerHost as any)
-      .onCachedDirectoryStructureHostCreate;
-    (watchCompilerHost as any).onCachedDirectoryStructureHostCreate = (
-      host: any,
-    ): void => {
+    const oldOnDirectoryStructureHostCreate =
+      watchCompilerHost.onCachedDirectoryStructureHostCreate;
+    watchCompilerHost.onCachedDirectoryStructureHostCreate = (host): void => {
       const oldReadDirectory = host.readDirectory;
-      // eslint-disable-next-line @typescript-eslint/explicit-function-return-type
       host.readDirectory = (
-        path: string,
-        extensions?: readonly string[],
-        exclude?: readonly string[],
-        include?: readonly string[],
-        depth?: number,
-      ) =>
+        path,
+        extensions,
+        exclude,
+        include,
+        depth,
+      ): string[] =>
         oldReadDirectory(
           path,
           !extensions
@@ -187,7 +183,6 @@ export function calculateProjectParserOptions(
         );
       oldOnDirectoryStructureHostCreate(host);
     };
-    /* eslint-enable @typescript-eslint/no-explicit-any */
 
     // create program
     const programWatch = ts.createWatchProgram(watchCompilerHost);

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