"Use of a supplier's language extensions and non-standard-conforming features limits the portability of your code and can prevent you from choosing a new implementation supplier."
A sibling thread claims the same syntax is supported by clang, and allowed by gcc using -fpermissive, so it seems like that's not a practical concern in this instance.
Quite a valid observation from Stroustrup. But in this case the programmer has chosen to use a non-standard, vendor specific feature. So it is not clear how it applies here? (Especially given the cost of breaking existing code)
The programmer inadvertently used a vendor-specific feature. This is the key difference. Nobody writes perfect, standards-compliant code all the time, so the compiler needs to tell you when you are doing this.
Right, but it's a trivial fix, in this case at least. I'm happy as long as standard code works in their compiler. Considering how much trouble MS are having with getting their compiler standard-conformant, I'd rather they work on the more important features.
"Use of a supplier's language extensions and non-standard-conforming features limits the portability of your code and can prevent you from choosing a new implementation supplier."
http://www.stroustrup.com/compilers.html