Key the date scrubber caches on DateTimeFormat

src/Verify.Tests/DateFormatLengthCalculatorTests.cs

@@ -106,6 +106,33 @@ public void Combos(string format, int max, int min)
         Assert.Equal(min + 2, length.min);
     }
 
+    // Two cultures can share a name and still render dates differently, so the cache
+    // cannot be keyed on the name: whichever was measured first would supply the bounds
+    // for the other, and rendered dates would fall outside the probed window lengths.
+    [Fact]
+    public void SameNamedCulturesWithDifferentFormatsAreNotShared()
+    {
+        var standard = new CultureInfo("en-AU");
+        var customized = new CultureInfo("en-AU")
+        {
+            DateTimeFormat =
+            {
+                // A far longer designator, so the bounds cannot coincide by accident
+                PMDesignator = "in the afternoon",
+                AMDesignator = "in the morning"
+            }
+        };
+
+        var standardLength = DateFormatLengthCalculator.GetLength("h:mm tt", standard);
+        var customizedLength = DateFormatLengthCalculator.GetLength("h:mm tt", customized);
+
+        Assert.NotEqual(standardLength, customizedLength);
+
+        var rendered = new DateTime(2020, 1, 1, 13, 30, 0).ToString("h:mm tt", customized);
+        Assert.True(rendered.Length <= customizedLength.max, $"{rendered.Length} <= {customizedLength.max}. {rendered}");
+        Assert.True(rendered.Length >= customizedLength.min, $"{rendered.Length} >= {customizedLength.min}. {rendered}");
+    }
+
     // MMMM next to a day component renders the genitive month name, which can be longer
     // than every nominative form (cs-CZ November: "listopadu" vs "listopad")
     [Fact]

src/Verify/Serialization/Scrubbers/DateFormatLengthCalculator.cs

@@ -1,6 +1,12 @@
 static class DateFormatLengthCalculator
 {
-    static ConcurrentDictionary<(string cultureName, string format), (int max, int min)> cache = new();
+    // Keyed on DateTimeFormat rather than the culture name. Two cultures can share a name
+    // and still render dates differently: CurrentCulture carrying Windows user overrides
+    // against CultureInfo.GetCultureInfo of the same name, or a clone whose DateTimeFormat
+    // was replaced. Keying on the name let whichever was measured first supply the window
+    // bounds for the other, so rendered dates fell outside the probed lengths and silently
+    // stopped scrubbing.
+    static ConcurrentDictionary<(DateTimeFormatInfo dateTimeFormat, string format), (int max, int min)> cache = new();
     const int maxSecondsFractionDigits = 7;
 
     static void ValidateSecondsFractionLength(int tokenLen)
@@ -13,11 +19,11 @@ static void ValidateSecondsFractionLength(int tokenLen)
 
     public static (int max, int min) GetLength(string format, Culture culture) =>
         cache.GetOrAdd(
-            (culture.Name, format),
+            (culture.DateTimeFormat, format),
             static (key, culture) =>
             {
-                var format = culture.DateTimeFormat.ExpandFormat(key.format);
-                return InnerGetLength(format.AsSpan(), culture);
+                var expanded = culture.DateTimeFormat.ExpandFormat(key.format);
+                return InnerGetLength(expanded.AsSpan(), culture);
             },
             culture);
 

src/Verify/Serialization/Scrubbers/DateMatchers.cs

@@ -141,17 +141,24 @@ static Scrubber ForCulture(
         // No culture was supplied, so each scrub uses the culture in effect at that
         // point. Building a scrubber reads the format lengths and expands the
         // pattern, so the result is cached per culture.
-        ConcurrentDictionary<Culture, Scrubber> cache = new();
+        //
+        // Keyed on DateTimeFormat rather than the culture, because CultureInfo equality
+        // only compares the name and the compare/text info. A CurrentCulture carrying
+        // Windows user overrides is equal to CultureInfo.GetCultureInfo of the same name
+        // while rendering dates differently, so keying on the culture let whichever was
+        // seen first answer for both.
+        ConcurrentDictionary<DateTimeFormatInfo, Scrubber> cache = new();
 
         Scrubber ForCurrentCulture()
         {
             var current = Culture.CurrentCulture;
-            if (cache.TryGetValue(current, out var existing))
+            var key = current.DateTimeFormat;
+            if (cache.TryGetValue(key, out var existing))
             {
                 return existing;
             }
 
-            return cache.GetOrAdd(current, Single(format, current, parseFactory(format, current)));
+            return cache.GetOrAdd(key, Single(format, current, parseFactory(format, current)));
         }
 
         // The registration culture instance supplies the bounds used for ordering