typeparam test file does not need to be separate

The typeparam test was in a separate file with a go1.18 build
constraint. However the test doesn't necessarily have to be in a
separate file; we can instead make the typeparam testdata package, that
is used by the test, build with all go versions.

exhaustive.go

@@ -194,7 +194,7 @@ To ignore specific constants in exhaustiveness checks, use the
 -ignore-enum-members flag. Similarly, to ignore specific types, use the
 -ignore-enum-types flag. For example:
 
-	exhaustive -ignore-enum-types '^time\.Duration$|^example.org/measure\.Unit$'
+	exhaustive -ignore-enum-types '^time\.Duration$|^example\.org/measure\.Unit$'
 
 [language spec]: https://golang.org/ref/spec
 */

exhaustive_test.go

@@ -7,26 +7,26 @@ import (
 	"golang.org/x/tools/go/analysis/analysistest"
 )
 
-func runTest(t *testing.T, pattern string, setup ...func()) {
-	t.Helper()
-	t.Run(pattern, func(t *testing.T) {
-		resetFlags()
-		// default to checking switch and map for test.
-		fCheck = stringsFlag{
-			[]string{
-				string(elementSwitch),
-				string(elementMap),
-			},
-			nil,
-		}
-		for _, f := range setup {
-			f()
-		}
-		analysistest.Run(t, analysistest.TestData(), Analyzer, pattern)
-	})
-}
-
 func TestExhaustive(t *testing.T) {
+	runTest := func(t *testing.T, pattern string, setup ...func()) {
+		t.Helper()
+		t.Run(pattern, func(t *testing.T) {
+			resetFlags()
+			// default to checking switch and map for test.
+			fCheck = stringsFlag{
+				[]string{
+					string(elementSwitch),
+					string(elementMap),
+				},
+				nil,
+			}
+			for _, f := range setup {
+				f()
+			}
+			analysistest.Run(t, analysistest.TestData(), Analyzer, pattern)
+		})
+	}
+
 	if !testing.Short() {
 		// Analysis of code that uses complex packages, such as package os and
 		// package reflect, should not fail.
@@ -76,6 +76,7 @@ func TestExhaustive(t *testing.T) {
 	runTest(t, "duplicate-enum-value/...")
 
 	runTest(t, "typealias/...")
+	runTest(t, "typeparam/...")
 
 	// mixture of general tests.
 	runTest(t, "general/...")

testdata/src/complexpkg/complexpkg.go

@@ -14,13 +14,14 @@ import (
 func useComplexPackages() {
 	// see issue 25: https://github.com/nishanths/exhaustive/issues/25
 	var (
-		_ http.Server
+		_ elliptic.Curve
 		_ tls.Conn
-		_ reflect.ChanDir
 		_ json.Encoder
-		_ elliptic.Curve
-		_ time.Ticker
+		_ fmt.Formatter
+		_ http.Server
 		_ os.File
+		_ reflect.ChanDir
+		_ time.Ticker
 	)
-	fmt.Println(os.LookupEnv(""))
+	fmt.Println(os.Getgid())
 }

testdata/src/typeparam/build.go

@@ -0,0 +1,4 @@
+package typeparam
+
+// This file exists so that the package (which, otherwise, consists only of
+// go1.18 files) can build with all go versions.