Cherry-pick PR #32982 into release-3.6 (#32985)

Component commits:
c4c6df9 Fix default behavior for transpileModule when fileName not provided

Author: TypeScript Bot

src/services/transpile.ts

@@ -66,7 +66,7 @@ namespace ts {
         options.noResolve = true;
 
         // if jsx is specified then treat file as .tsx
-        const inputFileName = transpileOptions.fileName || (options.jsx ? "module.tsx" : "module.ts");
+        const inputFileName = transpileOptions.fileName || (transpileOptions.compilerOptions && transpileOptions.compilerOptions.jsx ? "module.tsx" : "module.ts");
         const sourceFile = createSourceFile(inputFileName, input, options.target!); // TODO: GH#18217
         if (transpileOptions.moduleName) {
             sourceFile.moduleName = transpileOptions.moduleName;

src/testRunner/unittests/services/transpile.ts

@@ -3,6 +3,7 @@ namespace ts {
 
         interface TranspileTestSettings {
             options?: TranspileOptions;
+            noSetFileName?: boolean;
         }
 
         function transpilesCorrectly(name: string, input: string, testSettings: TranspileTestSettings) {
@@ -30,15 +31,19 @@ namespace ts {
 
                 transpileOptions.compilerOptions.sourceMap = true;
 
-                if (!transpileOptions.fileName) {
-                    transpileOptions.fileName = transpileOptions.compilerOptions.jsx ? "file.tsx" : "file.ts";
+                let unitName = transpileOptions.fileName;
+                if (!unitName) {
+                    unitName = transpileOptions.compilerOptions.jsx ? "file.tsx" : "file.ts";
+                    if (!testSettings.noSetFileName) {
+                        transpileOptions.fileName = unitName;
+                    }
                 }
 
                 transpileOptions.reportDiagnostics = true;
 
                 justName = "transpile/" + name.replace(/[^a-z0-9\-. ]/ig, "") + (transpileOptions.compilerOptions.jsx ? Extension.Tsx : Extension.Ts);
                 toBeCompiled = [{
-                    unitName: transpileOptions.fileName,
+                    unitName,
                     content: input
                 }];
 
@@ -456,5 +461,9 @@ var x = 0;`, {
         transpilesCorrectly("Supports 'as const' arrays", `([] as const).forEach(k => console.log(k));`, {
             options: { compilerOptions: { module: ModuleKind.CommonJS } }
         });
+
+        transpilesCorrectly("Infer correct file extension", `const fn = <T>(a: T) => a`, {
+            noSetFileName: true
+        });
     });
 }