scanner: fix unknown escape sequence in ident_char (fix #19363) (#19369)

vlib/v/scanner/scanner.v

@@ -1262,8 +1262,7 @@ fn (mut s Scanner) ident_string() string {
 				u_escapes_pos << s.pos - 1
 			}
 			// Unknown escape sequence
-			if c !in [`x`, `u`, `e`, `n`, `r`, `t`, `v`, `a`, `f`, `b`, `\\`, `\``, `$`, `@`, `?`, `{`, `}`, `'`, `"`]
-				&& !c.is_digit() {
+			if !is_escape_sequence(c) && !c.is_digit() {
 				s.error('`${c.ascii_str()}` unknown escape sequence')
 			}
 		}
@@ -1448,6 +1447,12 @@ fn trim_slash_line_break(s string) string {
 	return ret_str
 }
 
+[inline]
+fn is_escape_sequence(c u8) bool {
+	return c in [`x`, `u`, `e`, `n`, `r`, `t`, `v`, `a`, `f`, `b`, `\\`, `\``, `$`, `@`, `?`, `{`,
+		`}`, `'`, `"`]
+}
+
 /// ident_char is called when a backtick "single-char" is parsed from the code
 /// it is needed because some runes (chars) are written with escape sequences
 /// the string it returns should be a standardized, simplified version of the character
@@ -1541,10 +1546,15 @@ fn (mut s Scanner) ident_char() string {
 					lspos)
 			}
 		}
-	} else if c == '\n' {
+	} else if c.ends_with('\n') {
 		s.add_error_detail_with_pos('use quotes for strings, backticks for characters',
 			lspos)
 		s.error_with_pos('invalid character literal, use \`\\n\` instead', lspos)
+	} else if c.len > len {
+		ch := c[c.len - 1]
+		if !is_escape_sequence(ch) && !ch.is_digit() {
+			s.error('`${ch.ascii_str()}` unknown escape sequence')
+		}
 	}
 	// Escapes a `'` character
 	if c == "'" {

vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_1_err.out

@@ -0,0 +1,13 @@
+vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_1_err.vv:2:6: error: invalid character literal, use `\n` instead
+    1 | fn main() {
+    2 |     _ = `\
+      |         ^
+    3 | `
+    4 | }
+Details: 
+vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_1_err.vv:2:6: details: use quotes for strings, backticks for characters
+    1 | fn main() {
+    2 |     _ = `\
+      |         ^
+    3 | `
+    4 | }

vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_1_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+	_ = `\
+`
+}

vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_2_err.out

@@ -0,0 +1,5 @@
+vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_2_err.vv:2:9: error: `j` unknown escape sequence
+    1 | fn main() {
+    2 |     _ = `\j`
+      |            ^
+    3 | }

vlib/v/scanner/tests/unknown_escape_sequence_in_ident_char_2_err.vv

@@ -0,0 +1,3 @@
+fn main() {
+	_ = `\j`
+}