Handle extension blocks in CLS compliance checker (#80251)

Author: jjonescz

src/Compilers/CSharp/Portable/Compiler/ClsComplianceChecker.cs

@@ -529,7 +529,7 @@ private void CheckBaseTypeCompliance(NamedTypeSymbol symbol)
             else
             {
                 NamedTypeSymbol baseType = symbol.EnumUnderlyingType ?? symbol.BaseTypeNoUseSiteDiagnostics; // null for interfaces
-                System.Diagnostics.Debug.Assert((object)baseType != null || symbol.SpecialType == SpecialType.System_Object, "Only object has no base.");
+                System.Diagnostics.Debug.Assert((object)baseType != null || symbol.SpecialType == SpecialType.System_Object || symbol.IsExtension, "Only object or extension has no base.");
                 if ((object)baseType != null && !IsCompliantType(baseType, symbol))
                 {
                     // TODO: it would be nice to report this on the base type clause.

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs

@@ -769,6 +769,19 @@ static void local<U>(U u, System.Linq.Expressions.Expression<System.Func<U, obje
         CompileAndVerify(comp, expectedOutput: ExpectedOutput("{ A = 43 }"), verify: Verification.Skipped).VerifyDiagnostics();
     }
 
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/80250")]
+    public void Cls()
+    {
+        var src = """
+            [assembly: System.CLSCompliant(true)]
+            public static class Extensions
+            {
+                extension(object) { }
+            }
+            """;
+        CreateCompilation(src).VerifyEmitDiagnostics();
+    }
+
     [Fact]
     public void Attribute_01()
     {