cl: remove Underlying() call to reject string type aliases in rewrites

Type aliases like `type T string` are no longer supported for
-ldflags -X rewrites. Only direct *string types are now allowed.

- Removed Underlying() call from isStringPtrType
- Added TestRewriteIgnoresStringAlias test case

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <8459+cpunion@users.noreply.github.com>

cl/compile.go

@@ -151,7 +151,7 @@ func (p *context) isStringPtrType(typ types.Type) bool {
 	if !ok {
 		return false
 	}
-	basic, ok := ptr.Elem().Underlying().(*types.Basic)
+	basic, ok := ptr.Elem().(*types.Basic)
 	return ok && basic.Kind() == types.String
 }
 

cl/rewrite_internal_test.go

@@ -153,3 +153,17 @@ var VarStruct = wrapper{v: 1}
 		t.Fatalf("non-string variables must not be rewritten:\n%s", ir)
 	}
 }
+
+func TestRewriteIgnoresStringAlias(t *testing.T) {
+	const src = `package rewritepkg
+type T string
+var VarAlias T = "original_value"
+`
+	ir := compileWithRewrites(t, src, map[string]string{"VarAlias": "rewrite_alias"})
+	if strings.Contains(ir, `c"rewrite_alias"`) {
+		t.Fatalf("string alias types must not be rewritten:\n%s", ir)
+	}
+	if !strings.Contains(ir, `c"original_value"`) {
+		t.Fatalf("original value should remain for alias type:\n%s", ir)
+	}
+}