[release/9.0-staging] Fix case-insensitive JSON deserialization of enum member names

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/EnumConverter.cs

@@ -601,9 +601,9 @@ public void AppendConflictingField(EnumFieldInfo other)
             {
                 Debug.Assert(JsonName.Equals(other.JsonName, StringComparison.OrdinalIgnoreCase), "The conflicting entry must be equal up to case insensitivity.");
 
-                if (Kind is EnumFieldNameKind.Default || JsonName.Equals(other.JsonName, StringComparison.Ordinal))
+                if (ConflictsWith(this, other))
                 {
-                    // Silently discard if the preceding entry is the default or has identical name.
+                    // Silently discard if the new entry conflicts with the preceding entry
                     return;
                 }
 
@@ -612,13 +612,34 @@ public void AppendConflictingField(EnumFieldInfo other)
                 // Walk the existing list to ensure we do not add duplicates.
                 foreach (EnumFieldInfo conflictingField in conflictingFields)
                 {
-                    if (conflictingField.Kind is EnumFieldNameKind.Default || conflictingField.JsonName.Equals(other.JsonName, StringComparison.Ordinal))
+                    if (ConflictsWith(conflictingField, other))
                     {
                         return;
                     }
                 }
 
                 conflictingFields.Add(other);
+
+                // Determines whether the first field info matches everything that the second field info matches,
+                // in which case the second field info is redundant and doesn't need to be added to the list.
+                static bool ConflictsWith(EnumFieldInfo current, EnumFieldInfo other)
+                {
+                    // The default name matches everything case-insensitively.
+                    if (current.Kind is EnumFieldNameKind.Default)
+                    {
+                        return true;
+                    }
+
+                    // current matches case-sensitively since it's not the default name.
+                    // other matches case-insensitively, so it matches more than current.
+                    if (other.Kind is EnumFieldNameKind.Default)
+                    {
+                        return false;
+                    }
+
+                    // Both are case-sensitive so they need to be identical.
+                    return current.JsonName.Equals(other.JsonName, StringComparison.Ordinal);
+                }
             }
 
             public EnumFieldInfo? GetMatchingField(ReadOnlySpan<char> input)

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/EnumConverterTests.cs

@@ -1207,5 +1207,55 @@ public enum EnumWithInvalidMemberName6
             [JsonStringEnumMemberName("Comma separators not allowed, in flags enums")]
             Value
         }
+
+        [Theory]
+        [InlineData("\"cAmElCaSe\"", EnumWithVaryingNamingPolicies.camelCase, JsonKnownNamingPolicy.SnakeCaseUpper)]
+        [InlineData("\"cAmElCaSe\"", EnumWithVaryingNamingPolicies.camelCase, JsonKnownNamingPolicy.SnakeCaseLower)]
+        [InlineData("\"cAmElCaSe\"", EnumWithVaryingNamingPolicies.camelCase, JsonKnownNamingPolicy.KebabCaseUpper)]
+        [InlineData("\"cAmElCaSe\"", EnumWithVaryingNamingPolicies.camelCase, JsonKnownNamingPolicy.KebabCaseLower)]
+        [InlineData("\"pAsCaLcAsE\"", EnumWithVaryingNamingPolicies.PascalCase, JsonKnownNamingPolicy.SnakeCaseUpper)]
+        [InlineData("\"pAsCaLcAsE\"", EnumWithVaryingNamingPolicies.PascalCase, JsonKnownNamingPolicy.SnakeCaseLower)]
+        [InlineData("\"pAsCaLcAsE\"", EnumWithVaryingNamingPolicies.PascalCase, JsonKnownNamingPolicy.KebabCaseUpper)]
+        [InlineData("\"pAsCaLcAsE\"", EnumWithVaryingNamingPolicies.PascalCase, JsonKnownNamingPolicy.KebabCaseLower)]
+        [InlineData("\"sNaKe_CaSe_UpPeR\"", EnumWithVaryingNamingPolicies.SNAKE_CASE_UPPER, JsonKnownNamingPolicy.SnakeCaseUpper)]
+        [InlineData("\"sNaKe_CaSe_LoWeR\"", EnumWithVaryingNamingPolicies.snake_case_lower, JsonKnownNamingPolicy.SnakeCaseLower)]
+        [InlineData("\"cAmElCaSe\"", EnumWithVaryingNamingPolicies.camelCase, JsonKnownNamingPolicy.CamelCase)]
+        [InlineData("\"a\"", EnumWithVaryingNamingPolicies.A, JsonKnownNamingPolicy.CamelCase)]
+        [InlineData("\"a\"", EnumWithVaryingNamingPolicies.A, JsonKnownNamingPolicy.SnakeCaseUpper)]
+        [InlineData("\"a\"", EnumWithVaryingNamingPolicies.A, JsonKnownNamingPolicy.SnakeCaseLower)]
+        [InlineData("\"a\"", EnumWithVaryingNamingPolicies.A, JsonKnownNamingPolicy.KebabCaseUpper)]
+        [InlineData("\"a\"", EnumWithVaryingNamingPolicies.A, JsonKnownNamingPolicy.KebabCaseLower)]
+        [InlineData("\"B\"", EnumWithVaryingNamingPolicies.b, JsonKnownNamingPolicy.CamelCase)]
+        [InlineData("\"B\"", EnumWithVaryingNamingPolicies.b, JsonKnownNamingPolicy.SnakeCaseUpper)]
+        [InlineData("\"B\"", EnumWithVaryingNamingPolicies.b, JsonKnownNamingPolicy.SnakeCaseLower)]
+        [InlineData("\"B\"", EnumWithVaryingNamingPolicies.b, JsonKnownNamingPolicy.KebabCaseUpper)]
+        [InlineData("\"B\"", EnumWithVaryingNamingPolicies.b, JsonKnownNamingPolicy.KebabCaseLower)]
+        public static void StringConverterWithNamingPolicyIsCaseInsensitive(string json, EnumWithVaryingNamingPolicies expectedValue, JsonKnownNamingPolicy namingPolicy)
+        {
+            JsonNamingPolicy policy = namingPolicy switch
+            {
+                JsonKnownNamingPolicy.CamelCase => JsonNamingPolicy.CamelCase,
+                JsonKnownNamingPolicy.SnakeCaseLower => JsonNamingPolicy.SnakeCaseLower,
+                JsonKnownNamingPolicy.SnakeCaseUpper => JsonNamingPolicy.SnakeCaseUpper,
+                JsonKnownNamingPolicy.KebabCaseLower => JsonNamingPolicy.KebabCaseLower,
+                JsonKnownNamingPolicy.KebabCaseUpper => JsonNamingPolicy.KebabCaseUpper,
+                _ => throw new ArgumentOutOfRangeException(nameof(namingPolicy)),
+            };
+
+            JsonSerializerOptions options = new() { Converters = { new JsonStringEnumConverter(policy) } };
+
+            EnumWithVaryingNamingPolicies value = JsonSerializer.Deserialize<EnumWithVaryingNamingPolicies>(json, options);
+            Assert.Equal(expectedValue, value);
+        }
+
+        public enum EnumWithVaryingNamingPolicies
+        {
+            SNAKE_CASE_UPPER,
+            snake_case_lower,
+            camelCase,
+            PascalCase,
+            A,
+            b,
+        }
     }
 }