C#: Handle some broken types in BMN.

Author: michaelnebel

csharp/extractor/Semmle.Extraction.CSharp/Entities/Types/NamedType.cs

@@ -28,11 +28,32 @@ public static NamedType Create(Context cx, INamedTypeSymbol type) =>
         public static NamedType CreateNamedTypeFromTupleType(Context cx, INamedTypeSymbol type) =>
             UnderlyingTupleTypeFactory.Instance.CreateEntity(cx, (new SymbolEqualityWrapper(type), typeof(TupleType)), type);
 
-        public override bool NeedsPopulation => base.NeedsPopulation || Symbol.TypeKind == TypeKind.Error;
+        public override bool NeedsPopulation => base.NeedsPopulation || IsBrokenType(Symbol);
+
+        /// <summary>
+        /// Returns true in case we suspect this is broken type.
+        /// </summary>
+        /// <param name="symbol">Type symbol</param>
+        private bool IsBrokenType(ITypeSymbol symbol)
+        {
+            if (symbol.TypeKind == TypeKind.Error)
+            {
+                return true;
+            }
+
+            if (!Context.ExtractionContext.IsStandalone)
+            {
+                return false;
+            }
+            // (1) public class { ... } is a broken type and doesn't have a name.
+            // (2) public class var { ... } is a an allowed type, but it overrides the var keyword for all uses.
+            //     It is probably a better heuristic to treat it as a broken type.
+            return string.IsNullOrEmpty(symbol.Name) || symbol.Name == "var";
+        }
 
         public override void Populate(TextWriter trapFile)
         {
-            if (Symbol.TypeKind == TypeKind.Error)
+            if (IsBrokenType(Symbol))
             {
                 UnknownType.Create(Context); // make sure this exists so we can use it in `TypeRef::getReferencedType`
                 Context.ExtractionContext.MissingType(Symbol.ToString()!, Context.FromSource);
@@ -166,7 +187,9 @@ private class UnderlyingTupleTypeFactory : CachedEntityFactory<INamedTypeSymbol,
         // Create typerefs for constructed error types in case they are fully defined elsewhere.
         // We cannot use `!this.NeedsPopulation` because this would not be stable as it would depend on
         // the assembly that was being extracted at the time.
-        private bool UsesTypeRef => Symbol.TypeKind == TypeKind.Error || SymbolEqualityComparer.Default.Equals(Symbol.OriginalDefinition, Symbol);
+        private bool UsesTypeRef =>
+            IsBrokenType(Symbol) ||
+            SymbolEqualityComparer.Default.Equals(Symbol.OriginalDefinition, Symbol);
 
         public override Type TypeRef => UsesTypeRef ? (Type)NamedTypeRef.Create(Context, Symbol) : this;
     }