Just make it a const, env var controlled for later use

Herebyfile.mjs

@@ -175,7 +175,6 @@ async function runDtsBundler(entrypoint, output) {
  * @property {boolean} [exportIsTsObject]
  * @property {boolean} [treeShaking]
  * @property {boolean} [usePublicAPI]
- * @property {boolean} [tsgoCompat]
  * @property {() => void} [onWatchRebuild]
  */
 function createBundler(entrypoint, outfile, taskOptions = {}) {
@@ -197,9 +196,6 @@ function createBundler(entrypoint, outfile, taskOptions = {}) {
             treeShaking: taskOptions.treeShaking,
             packages: "external",
             logLevel: "warning",
-            define: {
-                __TSGO_COMPAT__: taskOptions.tsgoCompat ? "true" : "false",
-            },
             // legalComments: "none", // If we add copyright headers to the source files, uncomment.
         };
 
@@ -236,8 +232,7 @@ function createBundler(entrypoint, outfile, taskOptions = {}) {
             const require = "require";
             const fakeName = "Q".repeat(require.length);
             const fakeNameRegExp = new RegExp(fakeName, "g");
-            options.define = options.define || {};
-            options.define[require] = fakeName;
+            options.define = { [require]: fakeName };
 
             // For historical reasons, TypeScript does not set __esModule. Hack esbuild's __toCommonJS to be a noop.
             // We reference `__copyProps` to ensure the final bundle doesn't have any unreferenced code.
@@ -546,7 +541,6 @@ const { main: tests, watch: watchTests } = entrypointBuildTask({
     output: testRunner,
     mainDeps: [generateLibs],
     bundlerOptions: {
-        tsgoCompat: true,
         // Ensure we never drop any dead code, which might be helpful while debugging.
         treeShaking: false,
         onWatchRebuild() {

src/compiler/checker.ts

@@ -1,5 +1,6 @@
 import {
     __String,
+    __TSGO_COMPAT__,
     AccessExpression,
     AccessFlags,
     AccessorDeclaration,

src/compiler/corePublic.ts

@@ -5,11 +5,10 @@ export const versionMajorMinor = "5.7";
 /** The version of the TypeScript compiler release */
 export const version: string = `${versionMajorMinor}.0-dev`;
 
-declare global {
-    var __TSGO_COMPAT__: boolean | undefined; // eslint-disable-line no-var, @typescript-eslint/naming-convention
-}
+const tsgoCompatEnv = typeof process !== "undefined" && process.env ? process.env.TSGO_COMPAT : undefined;
 
-globalThis.__TSGO_COMPAT__ ??= false;
+/** @internal */
+export const __TSGO_COMPAT__: boolean = tsgoCompatEnv ? tsgoCompatEnv === "true" : true;
 
 /**
  * Type of objects whose values are all of the same type.