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Re: [PATCH] Define std::to_chars and std::from_chars for C++17 (P0067R5, partial)

On 07/04/17 13:29 +0100, Jonathan Wakely wrote:
+	      constexpr char __abc[] = "abcdefghijklmnopqrstuvwxyz";
+	      unsigned char __lc = std::tolower(__c);
+	      constexpr bool __consecutive = __consecutive_chars(__abc, 26);
+	      if _GLIBCXX17_CONSTEXPR (__consecutive)
+		{
+		  // Characters 'a'..'z' are consecutive
+		  if (std::isalpha(__c) && (__lc - 'a') < __b)
+		    __c = __lc - 'a' + 10;

Is that alright? I have my eyes set on the following line in particular:

+	      unsigned char __lc = std::tolower(__c);

Correct me if I'm wrong but that's locale sensitive. As it turns out in
a Turkish locale the lowercase for <I> is <ı> "U+0131 LATIN SMALL LETTER

I installed a Turkish locale on an old system of mine (Linux, UTF-8) and
sketched out the lines to quickly test the logic. As best as I can tell
the following happens when the global locale is set to Turkish:

 - __lc is 'I' unchanged because <ı> U+0131 would encode to two UTF-8
   codepoints aka can't fit in a char
 - the test succeeds
 - the computed value that ends up being written to __c is decimal -14

Keep in mind I didn't run the actual code from the patch, so I may have
got something wrong. Still, the Turkish i/İ and ı/I pairs are just one
example in one locale. Are std::tolower and std::isalpha the right tools
for this job? (These are the only two locale sensitive parts I
noticed in the code.)

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