Fix RegionInfo creation when using LOCALE_CUSTOM_UNSPECIFIED LCID

src/libraries/System.Globalization/tests/System/Globalization/RegionInfoTests.cs

@@ -254,5 +254,18 @@ public void EqualsTest(RegionInfo regionInfo1, object obj, bool expected)
                 Assert.Equal(expected, regionInfo1.GetHashCode().Equals(obj.GetHashCode()));
             }
         }
+
+        [Fact]
+        public void ValidateThrowingWhenUsingCustomUnspecifiedLcid()
+        {
+            // When trying to create RegionInfo using Lcid, we call LcidToLocaleName to map the Lcid to the culture name.
+            // LOCALE_CUSTOM_UNSPECIFIED (0x1000) is considered invalid Lcid to create cultures with. Sometimes Windows can
+            // map which is wrong. This happen if we enumerate the cultures before trying to create the RegionInfo object.
+            // This test is to ensure we'll not allow this creation in .NET.
+
+            CultureInfo [] cultures = CultureInfo.GetCultures(CultureTypes.SpecificCultures);
+
+            AssertExtensions.Throws<ArgumentException>("culture", () => new RegionInfo(4096));
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/System/Globalization/CultureData.Windows.cs

@@ -186,6 +186,13 @@ internal static unsafe CultureData GetCurrentRegionData()
         {
             Debug.Assert(!GlobalizationMode.Invariant);
 
+            // LOCALE_CUSTOM_UNSPECIFIED ia an unspecified custom locale, used to identify all supplemental locales.
+            // Supplemental locales cannot be distinguished from one another by their locale identifiers, but can be distinguished by their locale names.
+            if (culture == CultureInfo.LOCALE_CUSTOM_UNSPECIFIED)
+            {
+                return null;
+            }
+
             char* pBuffer = stackalloc char[Interop.Kernel32.LOCALE_NAME_MAX_LENGTH + 1]; // +1 for the null termination
             int length = Interop.Kernel32.LCIDToLocaleName(culture, pBuffer, Interop.Kernel32.LOCALE_NAME_MAX_LENGTH + 1, Interop.Kernel32.LOCALE_ALLOW_NEUTRAL_NAMES);
 

src/libraries/System.Private.CoreLib/src/System/Globalization/RegionInfo.cs

@@ -63,7 +63,7 @@ public RegionInfo(int culture)
                 throw new ArgumentException(SR.Format(SR.Argument_CultureIsNeutral, culture), nameof(culture));
             }
 
-            if (culture == CultureInfo.LOCALE_CUSTOM_DEFAULT)
+            if (culture == CultureInfo.LOCALE_CUSTOM_DEFAULT || culture == CultureInfo.LOCALE_CUSTOM_UNSPECIFIED)
             {
                 // Not supposed to be neutral
                 throw new ArgumentException(SR.Format(SR.Argument_CustomCultureCannotBePassedByNumber, culture), nameof(culture));