From 9906b43ffa5263b97758947d54d722ff6164d2a2 Mon Sep 17 00:00:00 2001
From: Daniel Rosenwasser <DanielRosenwasser@users.noreply.github.com>
Date: Wed, 12 Jan 2022 21:04:36 +0000
Subject: [PATCH] Guard against undefined relative path.

---
 src/compiler/moduleSpecifiers.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/compiler/moduleSpecifiers.ts b/src/compiler/moduleSpecifiers.ts
index c17dfeabcf1..db0b49f4436 100644
--- a/src/compiler/moduleSpecifiers.ts
+++ b/src/compiler/moduleSpecifiers.ts
@@ -854,8 +854,8 @@ namespace ts.moduleSpecifiers {
 
     function getPathRelativeToRootDirs(path: string, rootDirs: readonly string[], getCanonicalFileName: GetCanonicalFileName): string | undefined {
         return firstDefined(rootDirs, rootDir => {
-            const relativePath = getRelativePathIfInDirectory(path, rootDir, getCanonicalFileName)!; // TODO: GH#18217
-            return isPathRelativeToParent(relativePath) ? undefined : relativePath;
+            const relativePath = getRelativePathIfInDirectory(path, rootDir, getCanonicalFileName);
+            return relativePath !== undefined && isPathRelativeToParent(relativePath) ? undefined : relativePath;
         });
     }