CopyFrom parses strings to encode into binary format

https://github.com/jackc/pgx/issues/1277
https://github.com/jackc/pgx/issues/1267

copy_from_test.go

@@ -615,3 +615,32 @@ func TestConnCopyFromCopyFromSourceErrorEnd(t *testing.T) {
 
 	ensureConnValid(t, conn)
 }
+
+func TestConnCopyFromAutomaticStringConversion(t *testing.T) {
+	t.Parallel()
+
+	conn := mustConnectString(t, os.Getenv("PGX_TEST_DATABASE"))
+	defer closeConn(t, conn)
+
+	mustExec(t, conn, `create temporary table foo(
+		a int8
+	)`)
+
+	inputRows := [][]interface{}{
+		{"42"},
+		{"7"},
+		{8},
+	}
+
+	copyCount, err := conn.CopyFrom(context.Background(), pgx.Identifier{"foo"}, []string{"a"}, pgx.CopyFromRows(inputRows))
+	require.NoError(t, err)
+	require.EqualValues(t, len(inputRows), copyCount)
+
+	rows, _ := conn.Query(context.Background(), "select * from foo")
+	nums, err := pgx.CollectRows(rows, pgx.RowTo[int64])
+	require.NoError(t, err)
+
+	require.Equal(t, []int64{42, 7, 8}, nums)
+
+	ensureConnValid(t, conn)
+}

values.go

@@ -1,6 +1,8 @@
 package pgx
 
 import (
+	"errors"
+
 	"github.com/jackc/pgx/v5/internal/anynil"
 	"github.com/jackc/pgx/v5/internal/pgio"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -36,11 +38,31 @@ func encodeCopyValue(m *pgtype.Map, buf []byte, oid uint32, arg any) ([]byte, er
 	buf = pgio.AppendInt32(buf, -1)
 	argBuf, err := m.Encode(oid, BinaryFormatCode, arg, buf)
 	if err != nil {
-		return nil, err
+		if argBuf2, err2 := tryScanStringCopyValueThenEncode(m, buf, oid, arg); err2 == nil {
+			argBuf = argBuf2
+		} else {
+			return nil, err
+		}
 	}
+
 	if argBuf != nil {
 		buf = argBuf
 		pgio.SetInt32(buf[sp:], int32(len(buf[sp:])-4))
 	}
 	return buf, nil
 }
+
+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")
+	}
+
+	var v any
+	err := m.Scan(oid, TextFormatCode, []byte(s), &v)
+	if err != nil {
+		return nil, err
+	}
+
+	return m.Encode(oid, BinaryFormatCode, v, buf)
+}