LangVersion 9 (#44911)

* Introduce LanguageVersion.CSharp9
* Limit netcoreapp3.0, 3.1 and netstdard 2.1 to C# 8.0
* Map CSharp9 to language version preview
* Default to CSharp9 for netcoreapp5.0
* Add Tests

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6250,6 +6250,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_AddressOfToNonFunctionPointer" xml:space="preserve">
     <value>Cannot convert &amp;method group '{0}' to non-function pointer type '{1}'.</value>
   </data>
+  <data name="ERR_FeatureNotAvailableInVersion9" xml:space="preserve">
+    <value>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</value>
+  </data>
   <data name="ERR_UnexpectedArgumentList" xml:space="preserve">
     <value>Unexpected argument list.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1740,7 +1740,7 @@ internal enum ErrorCode
 
         #endregion diagnostics introduced for C# 8.0
 
-        #region diagnostics introduced in preview
+        #region diagnostics introduced in C# 9.0
 
         ERR_InternalError = 8751,
 
@@ -1818,6 +1818,7 @@ internal enum ErrorCode
         ERR_AddressOfMethodGroupInExpressionTree = 8810,
         ERR_CannotConvertAddressOfToDelegate = 8811,
         ERR_AddressOfToNonFunctionPointer = 8812,
+        ERR_FeatureNotAvailableInVersion9 = 8813,
 
         // Codes 8813, 8814, 8815, 8816 used by features/module-initializers
 
@@ -1846,8 +1847,7 @@ internal enum ErrorCode
         ERR_NoCopyConstructorInBaseType = 8867,
         ERR_CopyConstructorMustInvokeBaseCopyConstructor = 8868,
 
-        #endregion
-
+        #endregion diagnostics introduced for C# 9.0
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)
     }
 }

src/Compilers/CSharp/Portable/LanguageVersion.cs

@@ -128,13 +128,13 @@ public enum LanguageVersion
         /// </summary>
         CSharp8 = 800,
 
-        // When this value is released, update LanguageVersionExtensions in the IDE layer to point to it.
+        // When this value is available in the released NuGet packge, update LanguageVersionExtensions in the IDE layer to point to it.
         // https://github.com/dotnet/roslyn/issues/43348
         //
-        // /// <summary>
-        // /// C# language version 9.0
-        // /// </summary>
-        // CSharp9 = 900,
+        /// <summary>
+        /// C# language version 9.0
+        /// </summary>
+        CSharp9 = 900,
 
         /// <summary>
         /// The latest major supported version.
@@ -174,6 +174,7 @@ internal static bool IsValid(this LanguageVersion value)
                 case LanguageVersion.CSharp7_2:
                 case LanguageVersion.CSharp7_3:
                 case LanguageVersion.CSharp8:
+                case LanguageVersion.CSharp9:
                 case LanguageVersion.Preview:
                     return true;
             }
@@ -207,6 +208,8 @@ internal static ErrorCode GetErrorCode(this LanguageVersion version)
                     return ErrorCode.ERR_FeatureNotAvailableInVersion7_3;
                 case LanguageVersion.CSharp8:
                     return ErrorCode.ERR_FeatureNotAvailableInVersion8;
+                case LanguageVersion.CSharp9:
+                    return ErrorCode.ERR_FeatureNotAvailableInVersion9;
                 default:
                     throw ExceptionUtilities.UnexpectedValue(version);
             }
@@ -257,6 +260,8 @@ public static string ToDisplayString(this LanguageVersion version)
                     return "7.3";
                 case LanguageVersion.CSharp8:
                     return "8.0";
+                case LanguageVersion.CSharp9:
+                    return "9.0";
                 case LanguageVersion.Default:
                     return "default";
                 case LanguageVersion.Latest:
@@ -353,6 +358,11 @@ public static bool TryParse(string? version, out LanguageVersion result)
                     result = LanguageVersion.CSharp8;
                     return true;
 
+                case "9":
+                case "9.0":
+                    result = LanguageVersion.CSharp9;
+                    return true;
+
                 default:
                     result = LanguageVersion.Default;
                     return false;
@@ -370,12 +380,14 @@ public static LanguageVersion MapSpecifiedToEffectiveVersion(this LanguageVersio
                 case LanguageVersion.Default:
                 case LanguageVersion.LatestMajor:
                     return LanguageVersion.CSharp8;
+                case LanguageVersion.CSharp9:
+                    return LanguageVersion.Preview;
                 default:
                     return version;
             }
         }
 
-        internal static LanguageVersion CurrentVersion => LanguageVersion.CSharp8;
+        internal static LanguageVersion CurrentVersion => LanguageVersion.CSharp9;
 
         /// <summary>Inference of tuple element names was added in C# 7.1</summary>
         internal static bool DisallowInferredTupleElementNames(this LanguageVersion self)

src/Compilers/CSharp/Portable/Microsoft.CodeAnalysis.CSharp.csproj

@@ -64,6 +64,7 @@
     <InternalsVisibleTo Include="Microsoft.CodeAnalysis.CSharp.EditorFeatures.UnitTests" />
     <InternalsVisibleTo Include="Microsoft.CodeAnalysis.EditorFeatures2.UnitTests" />
     <InternalsVisibleTo Include="CompilerBenchmarks" />
+    <InternalsVisibleTo Include="Microsoft.Build.Tasks.CodeAnalysis.UnitTests" />
   </ItemGroup>
   <Import Project="..\CSharpAnalyzerDriver\CSharpAnalyzerDriver.projitems" Label="Shared" />
 </Project>

src/Compilers/CSharp/Portable/PublicAPI.Unshipped.txt

@@ -36,6 +36,7 @@ abstract Microsoft.CodeAnalysis.CSharp.Syntax.BaseObjectCreationExpressionSyntax
 abstract Microsoft.CodeAnalysis.CSharp.Syntax.BaseObjectCreationExpressionSyntax.Initializer.get -> Microsoft.CodeAnalysis.CSharp.Syntax.InitializerExpressionSyntax
 abstract Microsoft.CodeAnalysis.CSharp.Syntax.BaseObjectCreationExpressionSyntax.NewKeyword.get -> Microsoft.CodeAnalysis.SyntaxToken
 Microsoft.CodeAnalysis.CSharp.CSharpGeneratorDriver
+Microsoft.CodeAnalysis.CSharp.LanguageVersion.CSharp9 = 900 -> Microsoft.CodeAnalysis.CSharp.LanguageVersion
 Microsoft.CodeAnalysis.CSharp.CSharpGeneratorDriver.CSharpGeneratorDriver(Microsoft.CodeAnalysis.ParseOptions parseOptions, System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.ISourceGenerator> generators, Microsoft.CodeAnalysis.Diagnostics.AnalyzerConfigOptionsProvider optionsProvider, System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.AdditionalText> additionalTexts) -> void
 Microsoft.CodeAnalysis.CSharp.Syntax.FunctionPointerTypeSyntax
 Microsoft.CodeAnalysis.CSharp.Syntax.FunctionPointerTypeSyntax.AddParameters(params Microsoft.CodeAnalysis.CSharp.Syntax.ParameterSyntax[] items) -> Microsoft.CodeAnalysis.CSharp.Syntax.FunctionPointerTypeSyntax

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -342,6 +342,11 @@
         <target state="translated">Funkce {0} není v C# 8.0 dostupná. Použijte prosím jazykovou verzi {1} nebo větší.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">Událost podobná poli {0} nemůže mít modifikátor readonly.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -342,6 +342,11 @@
         <target state="translated">Das Feature "{0}" ist in C# 8.0 nicht verfügbar. Verwenden Sie Sprachversion {1} oder höher.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">Ein feldähnliches Ereignis "{0}" darf nicht "readonly" sein.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -342,6 +342,11 @@
         <target state="translated">La característica "{0}" no está disponible en C# 8.0. Use la versión {1} del lenguaje o una posterior.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">El evento de tipo campo "{0}" no puede ser "readonly".</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -342,6 +342,11 @@
         <target state="translated">La fonctionnalité '{0}' n'est pas disponible en C# 8.0. Utilisez la version de langage {1} ou une version ultérieure.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">L'événement de type champ '{0}' ne peut pas être 'readonly'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -342,6 +342,11 @@
         <target state="translated">La funzionalità '{0}' non è disponibile in C# 8.0. Usare la versione {1} o versioni successive del linguaggio.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">L'evento simile a campo '{0}' non può essere 'readonly'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -342,6 +342,11 @@
         <target state="translated">機能 '{0}' は C# 8.0 では使用できません。言語バージョン {1} 以上を使用してください。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">フィールドに類似したイベント '{0}' を 'readonly' にすることはできません。</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -342,6 +342,11 @@
         <target state="translated">'{0}' 기능은 C# 8.0에서 사용할 수 없습니다. 언어 버전 {1} 이상을 사용하세요.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">필드와 유사한 이벤트 '{0}'이(가) 'readonly'일 수 없습니다.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -342,6 +342,11 @@
         <target state="translated">Funkcja „{0}” nie jest dostępna w języku C# 8.0. Użyj języka w wersji {1} lub nowszej.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">Zdarzenie-pole „{0}” nie może być zadeklarowane jako „readonly”.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -342,6 +342,11 @@
         <target state="translated">O recurso '{0}' não está disponível em C# 8.0. Use a versão de linguagem {1} ou superior.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">O evento '{0}' semelhante ao de campo não pode ser 'readonly'.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -342,6 +342,11 @@
         <target state="translated">Функция "{0}" недоступна в C# 8.0. Используйте версию языка {1} или более позднюю.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">Подобное полю событие "{0}" не может быть readonly.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -342,6 +342,11 @@
         <target state="translated">'{0}' özelliği C# 8.0'da kullanılamaz. Lütfen {1} veya daha yüksek bir dil sürümü kullanın.</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">Alan benzeri '{0}' olayı 'readonly' olamaz.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -342,6 +342,11 @@
         <target state="translated">功能“{0}”在 C# 8.0 中不可用。请使用语言版本 {1} 或更高版本。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">类似字段的事件 "{0}" 不能为 "readonly"。</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -342,6 +342,11 @@
         <target state="translated">C# 8.0 中無法使用功能 '{0}'。請使用 {1} 或更新的語言版本。</target>
         <note />
       </trans-unit>
+      <trans-unit id="ERR_FeatureNotAvailableInVersion9">
+        <source>Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</source>
+        <target state="new">Feature '{0}' is not available in C# 9.0. Please use language version {1} or greater.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="ERR_FieldLikeEventCantBeReadOnly">
         <source>Field-like event '{0}' cannot be 'readonly'.</source>
         <target state="translated">類似欄位的事件 '{0}' 不能是 'readonly'。</target>

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -1599,7 +1599,7 @@ public void LangVersion_NoValueSpecified()
         [InlineData("iso-3")]
         [InlineData("iso1")]
         [InlineData("8.1")]
-        [InlineData("9")]
+        [InlineData("10")]
         [InlineData("1000")]
         public void LangVersion_BadVersion(string value)
         {
@@ -1614,6 +1614,8 @@ public void LangVersion_BadVersion(string value)
         [InlineData("05")]
         [InlineData("07")]
         [InlineData("07.1")]
+        [InlineData("08")]
+        [InlineData("09")]
         public void LangVersion_LeadingZeroes(string value)
         {
             DefaultParse(new[] { $"/langversion:{value}", "a.cs" }, WorkingDirectory).Errors.Verify(
@@ -1652,7 +1654,7 @@ public void LanguageVersionAdded_Canary()
             // - update the "UpgradeProject" codefixer
             // - update the IDE drop-down for selecting Language Version (in project-systems repo)
             // - update all the tests that call this canary
-            AssertEx.SetEqual(new[] { "default", "1", "2", "3", "4", "5", "6", "7.0", "7.1", "7.2", "7.3", "8.0", "latest", "latestmajor", "preview" },
+            AssertEx.SetEqual(new[] { "default", "1", "2", "3", "4", "5", "6", "7.0", "7.1", "7.2", "7.3", "8.0", "9.0", "latest", "latestmajor", "preview" },
                 Enum.GetValues(typeof(LanguageVersion)).Cast<LanguageVersion>().Select(v => v.ToDisplayString()));
             // For minor versions and new major versions, the format should be "x.y", such as "7.1"
         }
@@ -1683,6 +1685,7 @@ public void LanguageVersion_GetErrorCode()
                 ErrorCode.ERR_FeatureNotAvailableInVersion7_2,
                 ErrorCode.ERR_FeatureNotAvailableInVersion7_3,
                 ErrorCode.ERR_FeatureNotAvailableInVersion8,
+                ErrorCode.ERR_FeatureNotAvailableInVersion9,
             };
 
             AssertEx.SetEqual(versions, errorCodes);
@@ -1706,7 +1709,7 @@ public void LanguageVersion_GetErrorCode()
             InlineData(LanguageVersion.CSharp8, LanguageVersion.LatestMajor),
             InlineData(LanguageVersion.CSharp8, LanguageVersion.Latest),
             InlineData(LanguageVersion.CSharp8, LanguageVersion.Default),
-            InlineData(LanguageVersion.Preview, LanguageVersion.Preview),
+            InlineData(LanguageVersion.Preview, LanguageVersion.CSharp9),
             ]
         public void LanguageVersion_MapSpecifiedToEffectiveVersion(LanguageVersion expectedMappedVersion, LanguageVersion input)
         {
@@ -1742,6 +1745,8 @@ public void LanguageVersion_MapSpecifiedToEffectiveVersion(LanguageVersion expec
             InlineData("7.3", true, LanguageVersion.CSharp7_3),
             InlineData("8", true, LanguageVersion.CSharp8),
             InlineData("8.0", true, LanguageVersion.CSharp8),
+            InlineData("9", true, LanguageVersion.CSharp9),
+            InlineData("9.0", true, LanguageVersion.CSharp9),
             InlineData("08", false, LanguageVersion.Default),
             InlineData("07.1", false, LanguageVersion.Default),
             InlineData("default", true, LanguageVersion.Default),

src/Compilers/Core/MSBuildTask/Microsoft.CSharp.Core.targets

@@ -3,10 +3,21 @@
 <Project ToolsVersion="14.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="Microsoft.Managed.Core.targets"/>
 
-  <PropertyGroup Condition="('$(TargetFrameworkIdentifier)' != '.NETCoreApp' OR '$(_TargetFrameworkVersionWithoutV)' &lt; '3.0') AND
-                            ('$(TargetFrameworkIdentifier)' != '.NETStandard' OR '$(_TargetFrameworkVersionWithoutV)' &lt; '2.1')">
-    <MaxSupportedLangVersion Condition="'$(MaxSupportedLangVersion)' == ''">7.3</MaxSupportedLangVersion>
-    <LangVersion Condition="'$(LangVersion)' == ''">$(MaxSupportedLangVersion)</LangVersion>
+  <PropertyGroup>
+    <!-- .NETCoreApp < 3.0, .NETStandard < 2.1, or any other target framework -->
+    <_MaxSupportedLangVersion Condition="('$(TargetFrameworkIdentifier)' != '.NETCoreApp' OR '$(_TargetFrameworkVersionWithoutV)' &lt; '3.0') AND
+                                         ('$(TargetFrameworkIdentifier)' != '.NETStandard' OR '$(_TargetFrameworkVersionWithoutV)' &lt; '2.1')">7.3</_MaxSupportedLangVersion>
+    
+    <!-- .NETCoreApp < 5.0, .NETStandard == 2.1 -->
+    <_MaxSupportedLangVersion Condition="(('$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND '$(_TargetFrameworkVersionWithoutV)' &lt; '5.0') OR
+                                          ('$(TargetFrameworkIdentifier)' == '.NETStandard' AND '$(_TargetFrameworkVersionWithoutV)' == '2.1')) AND
+                                          '$(_MaxSupportedLangVersion)' == ''">8.0</_MaxSupportedLangVersion>
+
+    <!-- .NETCoreApp == 5.0 -->
+    <_MaxSupportedLangVersion Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND '$(_TargetFrameworkVersionWithoutV)' == '5.0' AND
+                                         '$(_MaxSupportedLangVersion)' == ''">9.0</_MaxSupportedLangVersion>
+    
+    <LangVersion Condition="'$(LangVersion)' == '' AND '$(_MaxSupportedLangVersion)' != ''">$(_MaxSupportedLangVersion)</LangVersion>
   </PropertyGroup>
 
   <Target Name="CoreCompile"

src/Compilers/Core/MSBuildTaskTests/TargetTests.cs

@@ -390,6 +390,76 @@ public void GenerateEditorConfigCoreHandlesMalformedCompilerVisibleItemMetadata(
             Assert.Equal("", metaName.EvaluatedValue);
         }
 
+        [Theory]
+        [InlineData(".NETFramework", "4.5", "7.3")]
+        [InlineData(".NETFramework", "4.7.2", "7.3")]
+        [InlineData(".NETFramework", "4.8", "7.3")]
+
+        [InlineData(".NETCoreApp", "1.0", "7.3")]
+        [InlineData(".NETCoreApp", "2.0", "7.3")]
+        [InlineData(".NETCoreApp", "2.1", "7.3")]
+        [InlineData(".NETCoreApp", "3.0", "8.0")]
+        [InlineData(".NETCoreApp", "3.1", "8.0")]
+        [InlineData(".NETCoreApp", "5.0", "9.0")]
+        [InlineData(".NETCoreApp", "6.0", "")]
+
+        [InlineData(".NETStandard", "1.0", "7.3")]
+        [InlineData(".NETStandard", "1.5", "7.3")]
+        [InlineData(".NETStandard", "2.0", "7.3")]
+        [InlineData(".NETStandard", "2.1", "8.0")]
+
+        [InlineData("UnknownTFM", "0.0", "7.3")]
+        [InlineData("UnknownTFM", "5.0", "7.3")]
+        public void LanguageVersionGivenTargetFramework(string tfi, string tfv, string expectedVersion)
+        {
+            XmlReader xmlReader = XmlReader.Create(new StringReader($@"
+<Project>
+    <PropertyGroup>
+        <TargetFrameworkIdentifier>{tfi}</TargetFrameworkIdentifier>
+        <_TargetFrameworkVersionWithoutV>{tfv}</_TargetFrameworkVersionWithoutV>
+    </PropertyGroup>
+    <Import Project=""Microsoft.CSharp.Core.targets"" />
+</Project>
+"));
+
+            var instance = CreateProjectInstance(xmlReader);
+            instance.Build(GetTestLoggers());
+
+            var langVersion = instance.GetPropertyValue("LangVersion");
+            var maxLangVersion = instance.GetPropertyValue("_MaxSupportedLangVersion");
+
+            Assert.Equal(expectedVersion, langVersion);
+            Assert.Equal(expectedVersion, maxLangVersion);
+
+            // This will fail whenever the current language version is updated.
+            // Ensure you update the target files to select the correct CSharp version for the newest target framework
+            // and add to the theory data above to cover it, before changing this version to make the test pass again.
+            Assert.Equal(CSharp.LanguageVersion.CSharp9, CSharp.LanguageVersionFacts.CurrentVersion);
+        }
+
+        [Fact]
+        public void ExplicitLangVersion()
+        {
+            XmlReader xmlReader = XmlReader.Create(new StringReader($@"
+<Project>
+    <PropertyGroup>
+        <TargetFrameworkIdentifier>.NETCoreApp</TargetFrameworkIdentifier>
+        <_TargetFrameworkVersionWithoutV>2.0</_TargetFrameworkVersionWithoutV>
+        <LangVersion>55.0</LangVersion>
+    </PropertyGroup>
+    <Import Project=""Microsoft.CSharp.Core.targets"" />
+</Project>
+"));
+
+            var instance = CreateProjectInstance(xmlReader);
+            instance.Build(GetTestLoggers());
+
+            var langVersion = instance.GetPropertyValue("LangVersion");
+            var maxLangVersion = instance.GetPropertyValue("_MaxSupportedLangVersion");
+
+            Assert.Equal("55.0", langVersion);
+            Assert.Equal("7.3", maxLangVersion);
+        }
         [Fact]
         public void GenerateEditorConfigIsPassedToTheCompiler()
         {