Extensions: extension methods cannot be entry points

Author: Julien Couvreur

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1940,7 +1940,14 @@ internal EntryPoint GetEntryPointAndDiagnostics(CancellationToken cancellationTo
                         return scriptClass.GetScriptEntryPoint();
                     }
 
-                    var mainTypeOrNamespace = globalNamespace.GetNamespaceOrTypeByQualifiedName(mainTypeName.Split('.')).OfMinimalArity();
+                    var nameParts = mainTypeName.Split('.');
+                    if (nameParts.Any(n => string.IsNullOrWhiteSpace(n)))
+                    {
+                        diagnostics.Add(ErrorCode.ERR_BadCompilationOptionValue, NoLocation.Singleton, nameof(CSharpCompilationOptions.MainTypeName), mainTypeName);
+                        return null;
+                    }
+
+                    var mainTypeOrNamespace = globalNamespace.GetNamespaceOrTypeByQualifiedName(nameParts).OfMinimalArity();
                     if (mainTypeOrNamespace is null)
                     {
                         diagnostics.Add(ErrorCode.ERR_MainClassNotFound, NoLocation.Singleton, mainTypeName);
@@ -2157,6 +2164,21 @@ private static void AddEntryPointCandidates(
             ArrayBuilder<MethodSymbol> entryPointCandidates, IEnumerable<Symbol> members)
         {
             foreach (var member in members)
+            {
+                if (member.GetIsNewExtensionMember())
+                {
+                    if (member is MethodSymbol method && method.TryGetCorrespondingExtensionImplementationMethod() is { } implementationMethod)
+                    {
+                        addIfCandidate(entryPointCandidates, implementationMethod);
+                    }
+                }
+                else
+                {
+                    addIfCandidate(entryPointCandidates, member);
+                }
+            }
+
+            static void addIfCandidate(ArrayBuilder<MethodSymbol> entryPointCandidates, Symbol member)
             {
                 if (member is MethodSymbol method &&
                     method.IsEntryPointCandidate)

src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs

@@ -710,6 +710,8 @@ internal bool IsEntryPointCandidate
         {
             get
             {
+                Debug.Assert(!this.GetIsNewExtensionMember());
+
                 if (this.IsPartialDefinition() &&
                     this.PartialImplementationPart is null)
                 {

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

@@ -35517,16 +35517,36 @@ static class E
 }
 """;
         var comp = CreateCompilation(source, options: TestOptions.DebugExe);
-        comp.VerifyEmitDiagnostics();
-        // Tracked by https://github.com/dotnet/roslyn/issues/76130
-        // FindEntryPoint/NameSymbolSearcher skip "E" since it has not declaration named "Main"
-        // CompileAndVerify(comp, expectedOutput: "ran");
+        CompileAndVerify(comp, expectedOutput: "ran").VerifyDiagnostics();
 
         comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("E"));
-        comp.VerifyEmitDiagnostics();
         CompileAndVerify(comp, expectedOutput: "ran").VerifyDiagnostics();
     }
 
+    [Fact]
+    public void Validation_EntryPoint_02()
+    {
+        string source = """
+static class E
+{
+    extension(int)
+    {
+        public static System.Action Main => throw null;
+    }
+}
+""";
+        var comp = CreateCompilation(source, options: TestOptions.DebugExe);
+        comp.VerifyEmitDiagnostics(
+            // error CS5001: Program does not contain a static 'Main' method suitable for an entry point
+            Diagnostic(ErrorCode.ERR_NoEntryPoint).WithLocation(1, 1));
+
+        comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("E"));
+        comp.VerifyDiagnostics(
+            // (1,14): error CS1558: 'E' does not have a suitable static 'Main' method
+            // static class E
+            Diagnostic(ErrorCode.ERR_NoMainInClass, "E").WithArguments("E").WithLocation(1, 14));
+    }
+
     [Fact]
     public void Validation_EntryPoint_03()
     {
@@ -35535,14 +35555,19 @@ static class E
 {
     extension(int i)
     {
-        public static void Main() { System.Console.Write("ran"); }
+        public static void Main() => throw null;
     }
 }
 """;
         var comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("E.<>E__0"));
         comp.VerifyEmitDiagnostics(
             // error CS1555: Could not find 'E.<>E__0' specified for Main method
             Diagnostic(ErrorCode.ERR_MainClassNotFound).WithArguments("E.<>E__0").WithLocation(1, 1));
+
+        Assert.Equal("<>E__0", comp.GetTypeByMetadataName("E").GetTypeMembers().Single().ExtensionName);
+
+        // Tracked by https://github.com/dotnet/roslyn/issues/76130 : we should find the unspeakable nested type
+        Assert.Null(comp.GetTypeByMetadataName("E+<>E__0"));
     }
 
     [Fact]
@@ -35557,13 +35582,48 @@ public static void Main() { }
     }
 }
 """;
-        // Tracked by https://github.com/dotnet/roslyn/issues/76130
-        // FindEntryPoint should not attempt to find types with an empty name
         var comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("E."));
         comp.VerifyEmitDiagnostics(
-            // (3,5): error CS1556: 'E.extension(int)' specified for Main method must be a non-generic class, record, struct, or interface
-            //     extension(int i)
-            Diagnostic(ErrorCode.ERR_MainClassNotClass, "extension").WithArguments("E.extension(int)").WithLocation(3, 5));
+            // error CS7088: Invalid 'MainTypeName' value: 'E.'.
+            Diagnostic(ErrorCode.ERR_BadCompilationOptionValue).WithArguments("MainTypeName", "E.").WithLocation(1, 1));
+
+        comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("E.  "));
+        comp.VerifyEmitDiagnostics(
+            // error CS7088: Invalid 'MainTypeName' value: 'E.  '.
+            Diagnostic(ErrorCode.ERR_BadCompilationOptionValue).WithArguments("MainTypeName", "E.  ").WithLocation(1, 1));
+
+        comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName(""));
+        comp.VerifyEmitDiagnostics(
+            Diagnostic(ErrorCode.ERR_BadCompilationOptionValue).WithArguments("MainTypeName", "").WithLocation(1, 1));
+
+        comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("  "));
+        comp.VerifyEmitDiagnostics(
+            // error CS7088: Invalid 'MainTypeName' value: '  '.
+            Diagnostic(ErrorCode.ERR_BadCompilationOptionValue).WithArguments("MainTypeName", "  ").WithLocation(1, 1));
+
+        comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName(".A"));
+        comp.VerifyEmitDiagnostics(
+            // error CS7088: Invalid 'MainTypeName' value: '.A'.
+            Diagnostic(ErrorCode.ERR_BadCompilationOptionValue).WithArguments("MainTypeName", ".A").WithLocation(1, 1));
+    }
+
+    [Fact]
+    public void Validation_EntryPoint_05()
+    {
+        string source = """
+static class E
+{
+    extension(int i)
+    {
+        public static async System.Threading.Tasks.Task<int> Main() { System.Console.Write("ran"); await System.Threading.Tasks.Task.Yield(); return 0; }
+    }
+}
+""";
+        var comp = CreateCompilation(source, options: TestOptions.DebugExe);
+        CompileAndVerify(comp, expectedOutput: "ran").VerifyDiagnostics();
+
+        comp = CreateCompilation(source, options: TestOptions.ReleaseExe.WithMainTypeName("E"));
+        CompileAndVerify(comp, expectedOutput: "ran").VerifyDiagnostics();
     }
 
     [Fact]