Fold an inline (?i) culture-invariantly too [patch]

TextFilter.Test/TextFilterTests.cs

@@ -682,6 +682,35 @@ public void ARegexCompiledUnderOneCultureAnswersTheSameUnderAnother()
 		}
 	}
 
+	[TestMethod]
+	public void AnInlineIgnoreCaseFoldsTheSameWhateverCultureCompiledIt()
+	{
+		// Under CaseSensitive the pattern can still ask for a fold itself with (?i), and that fold is
+		// culture-dependent unless CultureInvariant is set. Compiled under tr-TR first, "(?i)img" would
+		// then miss "IMG_1234.JPG" for every later caller, because the cache keeps the first compile.
+		CultureInfo original = CultureInfo.CurrentCulture;
+		try
+		{
+			CultureInfo.CurrentCulture = new CultureInfo("tr-TR");
+			if (CultureInfo.CurrentCulture.TextInfo.ToUpper("i") == "I")
+			{
+				Assert.Inconclusive("This runtime does not apply Turkish case mapping, so the regression cannot be provoked here.");
+			}
+
+			bool underTurkish = TextFilter.IsMatch("IMG_1234.JPG", "(?i)img", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString);
+
+			CultureInfo.CurrentCulture = new CultureInfo("en-US");
+			bool underEnglish = TextFilter.IsMatch("IMG_1234.JPG", "(?i)img", TextFilterType.Regex, TextFilterMatchOptions.ByWholeString);
+
+			Assert.IsTrue(underTurkish, "An inline (?i) should fold i and I whatever locale compiled the pattern.");
+			Assert.IsTrue(underEnglish, "The first caller's culture should not decide the answer for later callers.");
+		}
+		finally
+		{
+			CultureInfo.CurrentCulture = original;
+		}
+	}
+
 	[TestMethod]
 	public void TheTwoSensitivitiesDoNotCollideInTheRegexCache()
 	{

TextFilter/TextFilter.cs

@@ -124,7 +124,7 @@
 	private static readonly TimeSpan RegexMatchTimeout = TimeSpan.FromSeconds(1);
 
 	[System.Diagnostics.CodeAnalysis.SuppressMessage("Performance", "SYSLIB1045:Convert to 'GeneratedRegexAttribute'.", Justification = "Not available in older frameworks")]
 	private static Regex RegexMatchAnything() => new(".*", RegexOptions.Compiled);
 
 	/// <summary>
 	/// Gets a hint for the specified filter type.
@@ -276,7 +276,7 @@
 
 			return ExcludedTokenPrefixes.Contains(prefix)
 				? TextFilterTokenType.Excluded
 				: RequiredTokenPrefixes.Contains(prefix)
 				? TextFilterTokenType.Required
 				: TextFilterTokenType.Optional;
 		})
@@ -458,10 +458,12 @@
 			// matches "IMG_1234.JPG" -- and the cache below is keyed by pattern and sensitivity only,
 			// so whichever culture happened to compile the pattern first decides the answer for every
 			// later caller on any thread. A filter pattern is machine text, not prose, and the glob
-			// path already folds invariantly, so the two paths now agree.
+			// path already folds invariantly, so the two paths now agree. It applies under CaseSensitive
+			// too, because the pattern itself can ask for a fold with an inline (?i), and that fold is
+			// just as culture-dependent and just as cached as the one the option asks for.
 			RegexOptions regexOptions = caseSensitivity is TextFilterCaseSensitivity.CaseInsensitive
 				? RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.CultureInvariant
-				: RegexOptions.Compiled;
+				: RegexOptions.Compiled | RegexOptions.CultureInvariant;
 
 			try
 			{