go/printer: do not treat '\f' as a newline in (*printer).writeString

src/go/printer/printer.go

@@ -311,7 +311,7 @@ func (p *printer) writeString(pos token.Position, s string, isLit bool) {
 	var li int // index of last newline; valid if nlines > 0
 	for i := 0; i < len(s); i++ {
 		// Raw string literals may contain any character except back quote (`).
-		if ch := s[i]; ch == '\n' || ch == '\f' {
+		if ch := s[i]; ch == '\n' {
 			// account for line break
 			nlines++
 			li = i

src/go/printer/printer_test.go

@@ -16,6 +16,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
@@ -863,3 +864,28 @@ func TestEmptyDecl(t *testing.T) { // issue 63566
 		}
 	}
 }
+
+func TestIssue69858(t *testing.T) {
+	cases := []string{
+		"package A\nimport(\"a\"/*\f*/\n\"bb\")",
+		"package A\nfunc test() {\"a\"/*\f*/\n\"bb\"}",
+	}
+	for _, src := range cases {
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, "test.go", src, parser.ParseComments|parser.SkipObjectResolution)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var out strings.Builder
+		if err := Fprint(&out, fset, f); err != nil {
+			t.Fatal(err)
+		}
+
+		_, err = parser.ParseFile(fset, "test.go", out.String(), parser.ParseComments|parser.SkipObjectResolution)
+		if err != nil {
+			t.Logf("source:\n%s\nformatted as:\n%s", src, out.String())
+			t.Error(err)
+		}
+	}
+}