Reintroduce case sensitive comparison optimization for FrozenDictionary in some cases

src/libraries/System.Collections.Immutable/src/System/Collections/Frozen/String/KeyAnalyzer.cs

@@ -129,7 +129,7 @@ private static AnalysisResults CreateAnalysisResults(
                 // actually perform the comparison as case-sensitive even if case-insensitive
                 // was requested, as there's nothing that would compare equally to the substring
                 // other than the substring itself.
-                bool canSwitchIgnoreCaseHashToCaseSensitive = !isSubstring;
+                bool canSwitchIgnoreCaseHashToCaseSensitive = true;
 
                 foreach (string s in uniqueStrings)
                 {
@@ -144,11 +144,21 @@ private static AnalysisResults CreateAnalysisResults(
                         break;
                     }
 
-                    // All substrings so far are still ASCII only.  If this one contains any ASCII
-                    // letters, mark that we can't switch to case-sensitive.
-                    if (canSwitchIgnoreCaseHashToCaseSensitive && ContainsAnyLetters(substring))
+                    // All substrings so far are still ASCII only.  Now we check whether
+                    // the entire string either is not ASCII or contains ASCII letters,
+                    // in which case we disable the switch to case sensitive.
+                    if (canSwitchIgnoreCaseHashToCaseSensitive)
                     {
-                        canSwitchIgnoreCaseHashToCaseSensitive = false;
+                        // If we are testing the entire string then s == substring
+                        // and we have already checked IsAllAscii(s).
+                        if (isSubstring && !IsAllAscii(s.AsSpan()))
+                        {
+                            canSwitchIgnoreCaseHashToCaseSensitive = false;
+                        }
+                        else if (ContainsAnyLetters(s.AsSpan()))
+                        {
+                            canSwitchIgnoreCaseHashToCaseSensitive = false;
+                        }
                     }
                 }
 

src/libraries/System.Collections.Immutable/tests/Frozen/KeyAnalyzerTests.cs

@@ -107,6 +107,13 @@ public static void LeftHandCaseInsensitive()
             Assert.Equal(0, r.HashIndex);
             Assert.Equal(1, r.HashCount);
 
+            r = RunAnalysis(new[] { "0001", "0002", "0003", "0004", "0005", "0006" }, true);
+            Assert.False(r.RightJustifiedSubstring);
+            Assert.False(r.IgnoreCase);
+            Assert.True(r.AllAsciiIfIgnoreCase);
+            Assert.Equal(3, r.HashIndex);
+            Assert.Equal(1, r.HashCount);
+
         }
 
         [Fact]