[Bug c++/101213] New: Improve support for decltype(std)

[Bug c++/101213] New: Improve support for decltype(std)

[llvm at rifkin dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101213

            Bug ID: 101213
           Summary: Improve support for decltype(std)
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

Gcc treats decltype(std) as an integer. It only works in some situations
though:

// OK
decltype(std) a;
static_assert(std::is_same<int, decltype(a)>::value);

// Not OK
static_assert(std::is_same<int, decltype(std)>::value);

// OK
decltype(std) a;
using b = decltype(a);
using c = b;

// Not OK
using a = decltype(std);
using b = a;

decltype(std) foo(int a) {} // OK
auto foo() -> decltype(std) {} // OK
void foo(decltype(std) a) {} // Not OK

// Not OK
std::vector<decltype(std)> v;

This bug is hilarious please improve support for it and make it a feature :)
It's invaluable for code bowling and nerd sniping.

[Bug c++/101213] Improve support for decltype(std)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101213

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is just how GCC recovers from a bad parse, so that it can attempt to
continue and give diagnostics for the rest of the code. An invalid type in
certain declaration contexts is replaced with 'int'. Often it does more harm
than good, because you get errors about int::foo being invalid because it's not
a class type (which is true, but then it wasn't int in the original code!)

I've sometimes wondered whether it would be better to use an invented,
incomplete class type as the default instead of int. On balance, most
un-parseable types are probably classes, not fundamental types.

[Bug c++/101213] Improve support for decltype(std)

[llvm at rifkin dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101213

--- Comment #2 from Jeremy R. <llvm at rifkin dot dev> ---
(In reply to Jonathan Wakely from comment #1)
> This is just how GCC recovers from a bad parse, so that it can attempt to
> continue and give diagnostics for the rest of the code. An invalid type in
> certain declaration contexts is replaced with 'int'. Often it does more harm
> than good, because you get errors about int::foo being invalid because it's
> not a class type (which is true, but then it wasn't int in the original
> code!)
> 
> I've sometimes wondered whether it would be better to use an invented,
> incomplete class type as the default instead of int. On balance, most
> un-parseable types are probably classes, not fundamental types.

That's super cool, thanks for the insight into why decltype(std) is replaced
with int in the first place. Is the substitution but lack of diagnostic
intentional in a case like this https://godbolt.org/z/K91j35rTs? The lack of
diagnostic is of course appreciated for code bowling :)

What's the reason why this substitution is only made in certain contexts?