Revert "Revert "Support GetCultureInfo with predefinedOnly flag""

src/coreclr/src/System.Private.CoreLib/Resources/Strings.resx

@@ -1129,6 +1129,9 @@
   <data name="Argument_InvalidCultureName" xml:space="preserve">
     <value>Culture name '{0}' is not supported.</value>
   </data>
+  <data name="Argument_InvalidPredefinedCultureName" xml:space="preserve">
+    <value>Culture name '{0}' is not a predefined culture.</value>
+  </data>
   <data name="Argument_InvalidDateTimeKind" xml:space="preserve">
     <value>Invalid DateTimeKind value.</value>
   </data>

src/libraries/Native/Unix/System.Globalization.Native/pal_locale.c

@@ -233,3 +233,24 @@ int32_t GlobalizationNative_GetDefaultLocaleName(UChar* value, int32_t valueLeng
 
     return UErrorCodeToBool(status);
 }
+
+// GlobalizationNative_IsPredefinedLocale returns TRUE if ICU has a real data for the locale.
+// Otherwise it returns FALSE;
+
+int32_t GlobalizationNative_IsPredefinedLocale(const UChar* localeName)
+{
+    UErrorCode err = U_ZERO_ERROR;
+    char locale[ULOC_FULLNAME_CAPACITY];
+    GetLocale(localeName, locale, ULOC_FULLNAME_CAPACITY, FALSE, &err);
+
+    if (U_FAILURE(err))
+        return FALSE;
+
+    // ures_open returns err = U_ZERO_ERROR when ICU has data for localeName.
+    // If it is fake locale, it will return err = U_USING_FALLBACK_WARNING || err = U_USING_DEFAULT_WARNING.
+    // Other err values would be just a failure.
+    UResourceBundle* uresb = ures_open(NULL, locale, &err);
+    ures_close(uresb);
+
+    return err == U_ZERO_ERROR;
+}

src/libraries/Native/Unix/System.Globalization.Native/pal_locale.h

@@ -56,3 +56,5 @@ DLLEXPORT int32_t GlobalizationNative_GetLocales(UChar *value, int32_t valueLeng
 DLLEXPORT int32_t GlobalizationNative_GetLocaleName(const UChar* localeName, UChar* value, int32_t valueLength);
 
 DLLEXPORT int32_t GlobalizationNative_GetDefaultLocaleName(UChar* value, int32_t valueLength);
+
+DLLEXPORT int32_t GlobalizationNative_IsPredefinedLocale(const UChar* localeName);

src/libraries/System.Globalization/tests/CultureInfo/GetCultureInfo.cs

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+using Xunit;
+
+namespace System.Globalization.Tests
+{
+    public class GetCultureInfoTests
+    {
+        public static bool PlatformSupportsFakeCulture => !PlatformDetection.IsWindows || (PlatformDetection.WindowsVersion >= 10 && !PlatformDetection.IsFullFramework);
+
+        [ConditionalTheory(nameof(PlatformSupportsFakeCulture))]
+        [InlineData("en")]
+        [InlineData("en-US")]
+        [InlineData("ja-JP")]
+        [InlineData("ar-SA")]
+        [InlineData("xx-XX")]
+        public void GetCultureInfo(string name)
+        {
+            Assert.Equal(name, CultureInfo.GetCultureInfo(name).Name);
+            Assert.Equal(name, CultureInfo.GetCultureInfo(name, predefinedOnly: false).Name);
+        }
+
+        [ConditionalTheory(nameof(PlatformSupportsFakeCulture))]
+        [InlineData("en@US")]
+        [InlineData("\uFFFF")]
+        public void TestInvalidCultureNames(string name)
+        {
+            Assert.Throws<CultureNotFoundException>(() => CultureInfo.GetCultureInfo(name));
+            Assert.Throws<CultureNotFoundException>(() => CultureInfo.GetCultureInfo(name, predefinedOnly: false));
+            Assert.Throws<CultureNotFoundException>(() => CultureInfo.GetCultureInfo(name, predefinedOnly: true));
+        }
+
+        [ConditionalTheory(nameof(PlatformSupportsFakeCulture))]
+        [InlineData("xx")]
+        [InlineData("xx-XX")]
+        [InlineData("xx-YY")]
+        public void TestFakeCultureNames(string name)
+        {
+            Assert.Equal(name, CultureInfo.GetCultureInfo(name).Name);
+            Assert.Equal(name, CultureInfo.GetCultureInfo(name, predefinedOnly: false).Name);
+            Assert.Throws<CultureNotFoundException>(() => CultureInfo.GetCultureInfo(name, predefinedOnly: true));
+        }
+    }
+}

src/libraries/System.Globalization/tests/System.Globalization.Tests.csproj

@@ -52,6 +52,7 @@
     <Compile Include="CultureInfo\CultureInfoReadOnly.cs" />
     <Compile Include="CultureInfo\CultureInfoTwoLetterISOLanguageName.cs" />
     <Compile Include="CultureInfo\CultureInfoCurrentCulture.cs" />
+    <Compile Include="CultureInfo\GetCultureInfo.cs" />
     <Compile Include="DateTimeFormatInfo\DateTimeFormatInfoAbbreviatedDayNames.cs" />
     <Compile Include="DateTimeFormatInfo\DateTimeFormatInfoAbbreviatedMonthGenitiveNames.cs" />
     <Compile Include="DateTimeFormatInfo\DateTimeFormatInfoAbbreviatedMonthNames.cs" />

src/libraries/System.Private.CoreLib/src/Interop/Unix/System.Globalization.Native/Interop.Locale.cs

@@ -21,6 +21,10 @@ internal static partial class Globalization
         [return: MarshalAs(UnmanagedType.Bool)]
         internal static extern unsafe bool GetDefaultLocaleName(char* value, int valueLength);
 
+        [DllImport(Libraries.GlobalizationNative, CharSet = CharSet.Unicode, EntryPoint = "GlobalizationNative_IsPredefinedLocale")]
+        [return: MarshalAs(UnmanagedType.Bool)]
+        internal static extern unsafe bool IsPredefinedLocale(string localeName);
+
         [DllImport(Libraries.GlobalizationNative, CharSet = CharSet.Unicode, EntryPoint = "GlobalizationNative_GetLocaleTimeFormat")]
         [return: MarshalAs(UnmanagedType.Bool)]
         internal static extern unsafe bool GetLocaleTimeFormat(string localeName, bool shortFormat, char* value, int valueLength);

src/libraries/System.Private.CoreLib/src/System/Globalization/CultureInfo.Unix.cs

@@ -28,6 +28,16 @@ internal static CultureInfo GetUserDefaultCulture()
             return cultureInfo;
         }
 
+        private static CultureInfo GetPredefinedCultureInfo(string name)
+        {
+            if (!Interop.Globalization.IsPredefinedLocale(name))
+            {
+                throw new CultureNotFoundException(nameof(name), SR.Format(SR.Argument_InvalidPredefinedCultureName, name));
+            }
+
+            return GetCultureInfo(name);
+        }
+
         private static CultureInfo GetUserDefaultUICulture()
         {
             return InitializeUserDefaultCulture();

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

@@ -26,6 +26,24 @@ internal static CultureInfo GetUserDefaultCulture()
             return GetCultureByName(strDefault);
         }
 
+        private static CultureInfo GetPredefinedCultureInfo(string name)
+        {
+            CultureInfo culture = GetCultureInfo(name);
+            string englishName = culture.EnglishName;
+
+            // Check if the English Name starts with "Unknown Locale" or "Unknown Language" terms.
+            const int SecondTermIndex = 8;
+
+            if (englishName.StartsWith("Unknown ", StringComparison.Ordinal) && englishName.Length > SecondTermIndex &&
+                (englishName.IndexOf("Locale", SecondTermIndex, StringComparison.Ordinal) == SecondTermIndex ||
+                 englishName.IndexOf("Language", SecondTermIndex, StringComparison.Ordinal) == SecondTermIndex))
+            {
+                throw new CultureNotFoundException(nameof(name), SR.Format(SR.Argument_InvalidPredefinedCultureName, name));
+            }
+
+            return culture;
+        }
+
         private static unsafe CultureInfo GetUserDefaultUICulture()
         {
             if (GlobalizationMode.Invariant)

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

@@ -1114,6 +1114,21 @@ public static CultureInfo GetCultureInfo(string name, string altName)
             return result;
         }
 
+        public static CultureInfo GetCultureInfo(string name, bool predefinedOnly)
+        {
+            if (name is null)
+            {
+                throw new ArgumentNullException(nameof(name));
+            }
+
+            if (predefinedOnly)
+            {
+                return GetPredefinedCultureInfo(name);
+            }
+
+            return GetCultureInfo(name);
+        }
+
         private static Dictionary<string, CultureInfo> CachedCulturesByName
         {
             get

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -4432,6 +4432,7 @@ public void ClearCachedData() { }
         public System.Globalization.CultureInfo GetConsoleFallbackUICulture() { throw null; }
         public static System.Globalization.CultureInfo GetCultureInfo(int culture) { throw null; }
         public static System.Globalization.CultureInfo GetCultureInfo(string name) { throw null; }
+        public static System.Globalization.CultureInfo GetCultureInfo(string name, bool predefinedOnly) { throw null; }
         public static System.Globalization.CultureInfo GetCultureInfo(string name, string altName) { throw null; }
         public static System.Globalization.CultureInfo GetCultureInfoByIetfLanguageTag(string name) { throw null; }
         public static System.Globalization.CultureInfo[] GetCultures(System.Globalization.CultureTypes types) { throw null; }