Fix an off-by-one error in getBaseDirectoriesFromRootDirs

src/services/stringCompletions.ts

@@ -650,7 +650,10 @@ function getBaseDirectoriesFromRootDirs(rootDirs: string[], basePath: string, sc
     rootDirs = rootDirs.map(rootDirectory => normalizePath(isRootedDiskPath(rootDirectory) ? rootDirectory : combinePaths(basePath, rootDirectory)));
 
     // Determine the path to the directory containing the script relative to the root directory it is contained within
-    const relativeDirectory = firstDefined(rootDirs, rootDirectory => containsPath(rootDirectory, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectory.length) : undefined)!; // TODO: GH#18217
+    const relativeDirectory = firstDefined(rootDirs, rootDirectory => {
+        const rootDirectoryWithSlash = rootDirectory.endsWith("/") ? rootDirectory : rootDirectory + "/";
+        return containsPath(rootDirectoryWithSlash, scriptDirectory, basePath, ignoreCase) ? scriptDirectory.substr(rootDirectoryWithSlash.length) : undefined;
+    })!; // TODO: GH#18217
 
     // Now find a path for each potential directory that is to be merged with the one containing the script
     return deduplicate<string>(

tests/cases/fourslash/completionForStringLiteralRelativeImport5.ts

@@ -0,0 +1,37 @@
+/// <reference path='fourslash.ts' />
+
+// Should give correct completions for directories when invoked from within nested folders
+
+// @rootDirs: /repo/src1,/repo/src2/,/repo/generated1,/repo/generated2/
+
+// @Filename: /dir/secret_file.ts
+//// /*secret_file*/
+
+// @Filename: /repo/src1/dir/test1.ts
+//// import * as foo1 from ".//*import_as1*/
+//// import foo2 = require(".//*import_equals1*/
+//// var foo3 = require(".//*require1*/
+
+// @Filename: /repo/src2/dir/test2.ts
+//// import * as foo1 from ".//*import_as2*/
+//// import foo2 = require(".//*import_equals2*/
+//// var foo3 = require(".//*require2*/
+
+// @Filename: /repo/generated1/dir/f1.ts
+//// /*f1*/
+
+// @Filename: /repo/generated2/dir/f2.ts
+//// /*f2*/
+
+verify.completions(
+    {
+        marker: ["import_as1", "import_equals1", "require1"],
+        unsorted: ["f1", "f2", "test2"],
+        isNewIdentifierLocation: true,
+    },
+    {
+        marker: ["import_as2", "import_equals2", "require2"],
+        unsorted: ["f1", "f2", "test1"],
+        isNewIdentifierLocation: true,
+    }
+);

tests/cases/fourslash/completionForStringLiteralRelativeImport6.ts

@@ -0,0 +1,34 @@
+/// <reference path='fourslash.ts' />
+
+// Should give correct completions for rootDirs regardless of the slash at the end
+
+// @rootDirs: /repo/src1,/repo/src2/,/repo/generated1,/repo/generated2/
+
+// @Filename: /repo/src1/test1.ts
+//// import * as foo1 from "./dir//*import_as1*/
+//// import foo2 = require("./dir//*import_equals1*/
+//// var foo3 = require("./dir//*require1*/
+
+// @Filename: /repo/src2/test2.ts
+//// import * as foo1 from "./dir//*import_as2*/
+//// import foo2 = require("./dir//*import_equals2*/
+//// var foo3 = require("./dir//*require2*/
+
+// @Filename: /repo/generated1/dir/f1.ts
+//// /*f1*/
+
+// @Filename: /repo/generated2/dir/f2.ts
+//// /*f2*/
+
+verify.completions(
+    {
+        marker: ["import_as1", "import_equals1", "require1"],
+        exact: ["f1", "f2"],
+        isNewIdentifierLocation: true,
+    },
+    {
+        marker: ["import_as2", "import_equals2", "require2"],
+        exact: ["f1", "f2"],
+        isNewIdentifierLocation: true,
+    }
+);