Determine Turkish casing from the locale name instead of collation

src/libraries/System.Private.CoreLib/src/System/Globalization/TextInfo.Icu.cs

@@ -14,7 +14,20 @@ private static bool NeedsTurkishCasing(string localeName)
         {
             Debug.Assert(localeName != null);
 
-            return CultureInfo.GetCultureInfo(localeName).CompareInfo.Compare("\u0131", "I", CompareOptions.IgnoreCase) == 0;
+            // ICU applies the Turkish dotted/dotless "i" casing rules to the "tr" and "az"
+            // languages. This is determined from the locale name rather than by probing the
+            // collation tailoring, because some platforms (notably Android, which uses the
+            // system ICU) do not ship the collation data that probe relies on, which would
+            // silently fall back to non-Turkish casing.
+            ReadOnlySpan<char> language = localeName.AsSpan();
+            int separatorIndex = language.IndexOfAny('-', '_');
+            if (separatorIndex >= 0)
+            {
+                language = language.Slice(0, separatorIndex);
+            }
+
+            return language.Equals("tr", StringComparison.OrdinalIgnoreCase) ||
+                   language.Equals("az", StringComparison.OrdinalIgnoreCase);
         }
 
         internal unsafe void IcuChangeCase(char* src, int srcLen, char* dstBuffer, int dstBufferCapacity, bool bToUpper)

src/libraries/System.Runtime/tests/System.Globalization.Tests/System/Globalization/TextInfoTests.cs

@@ -280,21 +280,17 @@ public static IEnumerable<object[]> ToLower_TestData()
 
             foreach (string cultureName in GetTestLocales())
             {
-                // Android has its own ICU, which doesn't work well with tr
-                if (!PlatformDetection.IsAndroid && !PlatformDetection.IsLinuxBionic)
-                {
-                    yield return new object[] { cultureName, "I", "\u0131" };
-                    yield return new object[] { cultureName, "HI!", "h\u0131!" };
-                    yield return new object[] { cultureName, "HI\n\0H\u0130\t!", "h\u0131\n\0hi\u0009!" };
-                }
+                yield return new object[] { cultureName, "I", "\u0131" };
+                yield return new object[] { cultureName, "HI!", "h\u0131!" };
+                yield return new object[] { cultureName, "HI\n\0H\u0130\t!", "h\u0131\n\0hi\u0009!" };
                 yield return new object[] { cultureName, "\u0130", "i" };
                 yield return new object[] { cultureName, "i", "i" };
 
             }
 
             // ICU has special tailoring for the en-US-POSIX locale which treats "i" and "I" as different letters
             // instead of two letters with a case difference during collation.  Make sure this doesn't confuse our
-            // casing implementation, which uses collation to understand if we need to do Turkish casing or not.
+            // casing implementation, which must apply Turkish casing only for the tr and az languages.
             if (!PlatformDetection.IsWindows && PlatformDetection.IsNotBrowser)
             {
                 yield return new object[] { "en-US-POSIX", "I", "i" };
@@ -431,20 +427,16 @@ public static IEnumerable<object[]> ToUpper_TestData()
             // Turkish i
             foreach (string cultureName in GetTestLocales())
             {
-                // Android has its own ICU, which doesn't work well with tr
-                if (!PlatformDetection.IsAndroid && !PlatformDetection.IsLinuxBionic)
-                {
-                    yield return new object[] { cultureName, "i", "\u0130" };
-                    yield return new object[] { cultureName, "H\u0131\n\0Hi\u0009!", "HI\n\0H\u0130\t!" };
-                }
+                yield return new object[] { cultureName, "i", "\u0130" };
+                yield return new object[] { cultureName, "H\u0131\n\0Hi\u0009!", "HI\n\0H\u0130\t!" };
                 yield return new object[] { cultureName, "\u0130", "\u0130" };
                 yield return new object[] { cultureName, "\u0131", "I" };
                 yield return new object[] { cultureName, "I", "I" };
             }
 
             // ICU has special tailoring for the en-US-POSIX locale which treats "i" and "I" as different letters
             // instead of two letters with a case difference during collation.  Make sure this doesn't confuse our
-            // casing implementation, which uses collation to understand if we need to do Turkish casing or not.
+            // casing implementation, which must apply Turkish casing only for the tr and az languages.
             if (!PlatformDetection.IsWindows && PlatformDetection.IsNotBrowser)
             {
                 yield return new object[] { "en-US-POSIX", "i", "I" };