Fix(51828): `extends` field in `tsconfig.json` does nothing with empty string (#51981)

src/compiler/commandLineParser.ts

@@ -3279,7 +3279,7 @@ function parseOwnConfigOfJson(
     json.compileOnSave = convertCompileOnSaveOptionFromJson(json, basePath, errors);
     let extendedConfigPath: string | string[] | undefined;
 
-    if (json.extends) {
+    if (json.extends || json.extends === "") {
         if (!isCompilerOptionsValue(extendsOptionDeclaration, json.extends)) {
             errors.push(createCompilerDiagnostic(Diagnostics.Compiler_option_0_requires_a_value_of_type_1, "extends", getCompilerOptionValueTypeString(extendsOptionDeclaration)));
         }
@@ -3293,7 +3293,7 @@ function parseOwnConfigOfJson(
                 for (const fileName of json.extends as unknown[]) {
                     if (isString(fileName)) {
                         extendedConfigPath = append(extendedConfigPath, getExtendsConfigPath(fileName, host, newBase, errors, createCompilerDiagnostic));
-                }
+                    }
                     else {
                         errors.push(createCompilerDiagnostic(Diagnostics.Compiler_option_0_requires_a_value_of_type_1, "extends", getCompilerOptionValueTypeString(extendsOptionDeclaration.element)));
                     }
@@ -3428,7 +3428,12 @@ function getExtendsConfigPath(
     if (resolved.resolvedModule) {
         return resolved.resolvedModule.resolvedFileName;
     }
-    errors.push(createDiagnostic(Diagnostics.File_0_not_found, extendedConfig));
+    if (extendedConfig === "") {
+        errors.push(createDiagnostic(Diagnostics.Compiler_option_0_cannot_be_given_an_empty_string, "extends"));
+    }
+    else {
+        errors.push(createDiagnostic(Diagnostics.File_0_not_found, extendedConfig));
+    }
     return undefined;
 }
 

src/compiler/diagnosticMessages.json

@@ -7636,5 +7636,9 @@
     "The value '{0}' cannot be used here.": {
         "category": "Error",
         "code": 18050
+    },
+    "Compiler option '{0}' cannot be given an empty string.": {
+        "category": "Error",
+        "code": 18051
     }
 }

src/testRunner/unittests/config/configurationExtension.ts

@@ -199,6 +199,8 @@ function createFileSystem(ignoreCase: boolean, cwd: string, root: string) {
                 }),
                 "dev/extends.json": JSON.stringify({ extends: 42 }),
                 "dev/extends2.json": JSON.stringify({ extends: "configs/base" }),
+                "dev/extends3.json": JSON.stringify({ extends: "" }),
+                "dev/extends4.json": JSON.stringify({ extends: [""] }),
                 "dev/main.ts": "",
                 "dev/supplemental.ts": "",
                 "dev/tests/unit/spec.ts": "",
@@ -358,6 +360,16 @@ describe("unittests:: config:: configurationExtension", () => {
                 messageText: `Compiler option 'extends' requires a value of type string or Array.`
             }]);
 
+            testFailure("can error when 'extends' is given an empty string", "extends3.json", [{
+                code: 18051,
+                messageText: `Compiler option 'extends' cannot be given an empty string.`
+            }]);
+
+            testFailure("can error when 'extends' is given an empty string in an array", "extends4.json", [{
+                code: 18051,
+                messageText: `Compiler option 'extends' cannot be given an empty string.`
+            }]);
+
             testSuccess("can overwrite compiler options using extended 'null'", "configs/third.json", {
                 allowJs: true,
                 noImplicitAny: true,

tests/baselines/reference/tsserver/plugins/getSupportedCodeFixes-can-be-proxied.js

@@ -1470,6 +1470,7 @@ Info 32   [00:01:13.000] response:
         "18048",
         "18049",
         "18050",
+        "18051",
         "80005",
         "80003",
         "80008",
@@ -2804,6 +2805,7 @@ Info 38   [00:01:19.000] response:
         "18048",
         "18049",
         "18050",
+        "18051",
         "80005",
         "80003",
         "80008",
@@ -4050,6 +4052,7 @@ Info 40   [00:01:21.000] response:
         "18048",
         "18049",
         "18050",
+        "18051",
         "80005",
         "80003",
         "80008",