foo1.cs

      var argumentType = typeofExpression.ArgumentType;
      if (!(argumentType is IDeclaredType declaredType)
          || declaredType.IsVoid()
          || declaredType is IDynamicType) return null;

      var typeElement = declaredType.GetTypeElement();
      if (typeElement is ITypeParameter) return null;

      return typeElement?.TypeParameters.Count == 0 ? typeElement : null;

foo2.cs

      var argumentType = typeofExpression.ArgumentType as IDeclaredType;
      if (argumentType == null
          || argumentType.IsVoid()
          || argumentType is IDynamicType) return null;

      var typeElement = argumentType.GetTypeElement();
      if (typeElement == null
          || typeElement is ITypeParameter
          || typeElement.TypeParameters.Count > 0) return null;

      return typeElement; // bingo!