Do not fold dotless i to ASCII i in reference labels

lib/inlines.js

@@ -99,12 +99,22 @@ var text = function(s) {
 // collapse internal space, unicode case fold.
 // See commonmark/commonmark.js#168.
 var normalizeReference = function(string) {
-    return string
+    var trimmed = string
         .slice(1, string.length - 1)
         .trim()
-        .replace(/[ \t\r\n]+/g, " ")
-        .toLowerCase()
-        .toUpperCase();
+        .replace(/[ \t\r\n]+/g, " ");
+    // Dotless i (U+0131) has no default CaseFolding.txt entry, so it folds to
+    // itself (the I to dotless i mapping is Turkic-only), but it uppercases to
+    // ASCII I, so toLowerCase/toUpperCase would merge it with i. Fold the runs
+    // between occurrences instead. A run boundary can only affect final sigma,
+    // whose two lowercase forms uppercase alike; every other mapping these
+    // locale-independent methods apply is context-free.
+    if (trimmed.indexOf("\u0131") === -1) {
+        return trimmed.toLowerCase().toUpperCase();
+    }
+    return trimmed.replace(/[^\u0131]+/g, function(part) {
+        return part.toLowerCase().toUpperCase();
+    });
 };
 
 // INLINE PARSER

test/regression.txt

@@ -591,3 +591,37 @@ Issue #212 - Non-empty blank lines in fenced code ignored when in lists
 <pre><code>  
 </code></pre>
 ````````````````````````````````
+
+Dotless i (U+0131) is not folded to ASCII i
+
+```````````````````````````````` example
+[I]: /a
+
+[ı]: /b
+
+[a][i] [b][ı]
+.
+<p><a href="/a">a</a> <a href="/b">b</a></p>
+````````````````````````````````
+
+Labels containing dotless i still fold their other characters
+
+```````````````````````````````` example
+[FIQ]: /a
+
+[Fıq]: /b
+
+[a][fiq] [b][fıQ]
+.
+<p><a href="/a">a</a> <a href="/b">b</a></p>
+````````````````````````````````
+
+A dotless i reference does not resolve against an ASCII I definition
+
+```````````````````````````````` example
+[I]: /a
+
+[ı]
+.
+<p>[ı]</p>
+````````````````````````````````