syntax: \p{Lc} should map to \p{Cased_Letter}

This is more similar to the \p{Cf} bug than the \p{Sc} bug, but
basically, 'lc' is an abbreviation for both 'Cased_Letter' and
'Lowercase_Mapping'. Since we don't support the latter (currently), we
make 'lc' map to 'Cased_Letter'.

If we do ever add 'Lowercase_Mapping' in the future, then we will just
require users to type out its full form.

Fixes #965

regex-syntax/src/unicode.rs

@@ -248,7 +248,12 @@ impl<'a> ClassQuery<'a> {
         // also the abbreviation for the 'Script' property. So we avoid calling
         // 'canonical_prop' for it too, which would erroneously normalize it
         // to 'Script'.
-        if norm != "cf" && norm != "sc" {
+        //
+        // Another case: 'lc' is an abbreviation for the 'Cased_Letter'
+        // general category, but is also an abbreviation for the 'Lowercase_Mapping'
+        // property. We don't currently support the latter, so as with 'cf'
+        // above, we treat 'lc' as 'Cased_Letter'.
+        if norm != "cf" && norm != "sc" && norm != "lc" {
             if let Some(canon) = canonical_prop(&norm)? {
                 return Ok(CanonicalClassQuery::Binary(canon));
             }

tests/unicode.rs

@@ -35,6 +35,8 @@ mat!(uni_not_boundary_ogham, r"\d\B", "6 ", None);
 // We should test more, but there's a lot. Write a script to generate more of
 // these tests.
 mat!(uni_class_gencat_cased_letter, r"\p{Cased_Letter}", "Ａ", Some((0, 3)));
+mat!(uni_class_gencat_cased_letter2, r"\p{gc=LC}", "Ａ", Some((0, 3)));
+mat!(uni_class_gencat_cased_letter3, r"\p{LC}", "Ａ", Some((0, 3)));
 mat!(
     uni_class_gencat_close_punctuation,
     r"\p{Close_Punctuation}",