Do not mutate headers when initialising tableprinter

internal/tableprinter/table_printer.go

@@ -63,9 +63,10 @@ func NewWithWriter(w io.Writer, isTTY bool, maxWidth int, cs *iostreams.ColorSch
 	}
 
 	if isTTY && len(headers.columns) > 0 {
-		// Make sure all headers are uppercase.
+		// Make sure all headers are uppercase, taking a copy of the headers to avoid modifying the original slice.
+		upperCasedHeaders := make([]string, len(headers.columns))
 		for i := range headers.columns {
-			headers.columns[i] = strings.ToUpper(headers.columns[i])
+			upperCasedHeaders[i] = strings.ToUpper(headers.columns[i])
 		}
 
 		// Make sure all header columns are padded - even the last one. Previously, the last header column
@@ -77,7 +78,7 @@ func NewWithWriter(w io.Writer, isTTY bool, maxWidth int, cs *iostreams.ColorSch
 		}
 
 		tp.AddHeader(
-			headers.columns,
+			upperCasedHeaders,
 			WithPadding(paddingFunc),
 			WithColor(cs.LightGrayUnderline),
 		)

internal/tableprinter/table_printer_test.go

@@ -0,0 +1,22 @@
+package tableprinter_test
+
+import (
+	"testing"
+
+	"github.com/cli/cli/v2/internal/tableprinter"
+	"github.com/cli/cli/v2/pkg/iostreams"
+	"github.com/stretchr/testify/require"
+)
+
+func TestHeadersAreNotMutated(t *testing.T) {
+	// Given a TTY environment so that headers are included in the table
+	ios, _, _, _ := iostreams.Test()
+	ios.SetStdoutTTY(true)
+
+	// When creating a new table printer
+	headers := []string{"one", "two", "three"}
+	_ = tableprinter.New(ios, tableprinter.WithHeader(headers...))
+
+	// The provided headers should not be mutated
+	require.Equal(t, []string{"one", "two", "three"}, headers)
+}