Removed '\1' (and other) invalid escape sequences from chars. Fixes #4309

Author: Ary Borenszweig

spec/compiler/lexer/lexer_spec.cr

@@ -456,6 +456,8 @@ describe "Lexer" do
   assert_syntax_error "'\\u{110000}'", "invalid unicode codepoint (too large)"
   assert_syntax_error ":+1", "unexpected token"
 
+  assert_syntax_error "'\\1'", "invalid char escape sequence"
+
   it_lexes_string %("\\1"), String.new(Bytes[1])
   it_lexes_string %("\\4"), String.new(Bytes[4])
   it_lexes_string %("\\10"), String.new(Bytes[8])

src/compiler/crystal/syntax/lexer.cr

@@ -587,6 +587,10 @@ module Crystal
         case char1 = next_char
         when '\\'
           case char2 = next_char
+          when '\\'
+            @token.value = '\\'
+          when '\''
+            @token.value = '\''
           when 'b'
             @token.value = '\b'
           when 'e'
@@ -609,7 +613,7 @@ module Crystal
           when '\0'
             raise "unterminated char literal", line, column
           else
-            @token.value = char2
+            raise "invalid char escape sequence", line, column
           end
         when '\''
           raise "invalid empty char literal (did you mean '\\\''?)", line, column