Improve CopyFrom auto-conversion of text-ish values

CopyFrom requires that all values are encoded in the binary format. It
already tried to parse strings to values that can then be encoded into
the binary format. But it didn't handle types that can be encoded as
text and then parsed and converted to binary. It now does.

copy_from_test.go

@@ -803,6 +803,40 @@ func TestConnCopyFromAutomaticStringConversion(t *testing.T) {
 	ensureConnValid(t, conn)
 }
 
+// https://github.com/jackc/pgx/discussions/1891
+func TestConnCopyFromAutomaticStringConversionArray(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	conn := mustConnectString(t, os.Getenv("PGX_TEST_DATABASE"))
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, `create temporary table foo(
+		a numeric[]
+	)`)
+
+	inputRows := [][]interface{}{
+		{[]string{"42"}},
+		{[]string{"7"}},
+		{[]string{"8", "9"}},
+		{[][]string{{"10", "11"}, {"12", "13"}}},
+	}
+
+	copyCount, err := conn.CopyFrom(ctx, pgx.Identifier{"foo"}, []string{"a"}, pgx.CopyFromRows(inputRows))
+	require.NoError(t, err)
+	require.EqualValues(t, len(inputRows), copyCount)
+
+	// Test reads as int64 and flattened array for simplicity.
+	rows, _ := conn.Query(ctx, "select * from foo")
+	nums, err := pgx.CollectRows(rows, pgx.RowTo[[]int64])
+	require.NoError(t, err)
+	require.Equal(t, [][]int64{{42}, {7}, {8, 9}, {10, 11, 12, 13}}, nums)
+
+	ensureConnValid(t, conn)
+}
+
 func TestCopyFromFunc(t *testing.T) {
 	t.Parallel()
 

values.go

@@ -55,7 +55,11 @@ func encodeCopyValue(m *pgtype.Map, buf []byte, oid uint32, arg any) ([]byte, er
 func tryScanStringCopyValueThenEncode(m *pgtype.Map, buf []byte, oid uint32, arg any) ([]byte, error) {
 	s, ok := arg.(string)
 	if !ok {
-		return nil, errors.New("not a string")
+		textBuf, err := m.Encode(oid, TextFormatCode, arg, nil)
+		if err != nil {
+			return nil, errors.New("not a string and cannot be encoded as text")
+		}
+		s = string(textBuf)
 	}
 
 	var v any