Fix bug: normalize path after combining (#21100)

src/services/pathCompletions.ts

@@ -216,7 +216,8 @@ namespace ts.Completions.PathCompletions {
         const expandedPrefixDirectory = fragmentHasPath ? combinePaths(normalizedPrefixDirectory, normalizedPrefixBase + getDirectoryPath(fragment)) : normalizedPrefixDirectory;
 
         const normalizedSuffix = normalizePath(parsed.suffix);
-        const baseDirectory = combinePaths(baseUrl, expandedPrefixDirectory);
+        // Need to normalize after combining: If we combinePaths("a", "../b"), we want "b" and not "a/../b".
+        const baseDirectory = normalizePath(combinePaths(baseUrl, expandedPrefixDirectory));
         const completePrefix = fragmentHasPath ? baseDirectory : ensureTrailingDirectorySeparator(baseDirectory) + normalizedPrefixBase;
 
         // If we have a suffix, then we need to read the directory all the way down. We could create a glob

tests/cases/fourslash/completionsPaths_pathMapping_parentDirectory.ts

@@ -0,0 +1,20 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: /src/a.ts
+////import { } from "foo/[|/**/|]";
+
+// @Filename: /oof/x.ts
+////export const x = 0;
+
+// @Filename: /tsconfig.json
+////{
+////    "compilerOptions": {
+////        "baseUrl": "src",
+////        "paths": {
+////            "foo/*": ["../oof/*"]
+////        }
+////    }
+////}
+
+const [replacementSpan] = test.ranges();
+verify.completionsAt("", [{ name: "x", replacementSpan }]);