[Bug c++/112926] New: issues with nested lambdas and decltype of uncaptured local variable

[Bug c++/112926] New: issues with nested lambdas and decltype of uncaptured local variable

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112926

            Bug ID: 112926
           Summary: issues with nested lambdas and decltype of uncaptured
                    local variable
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

Here since the outer lambda has no capture-default, using x outside of an
unevaluated operand wouldn't capture it (despite the by-copy capture-default of
the inner lambda), so the special case in
https://eel.is/c++draft/expr.prim.id.unqual#3 doesn't apply and decltype((x)
should be int& not const int&.

int main() {
  int x;

  [] {
    [=] {
      using type = decltype((x)); // should be int& not const int&
      using type = int&;
    };
  };
}

[Bug c++/112926] issues with nested lambdas and decltype of uncaptured local variable

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112926

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=83167,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=86697
           Keywords|                            |accepts-invalid,
                   |                            |rejects-valid

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Complete rejects-valid testcase:

  int main() {
    int x;
    [] {
      [=] {
        using ty1 = decltype((x)); // refers to local variable despite
                                   // innermost by-copy capture-default
        using ty1 = int&;
      };
    };
    [=] {
      [] {
        using ty1 = decltype((x)); // same
        using ty1 = int&;
      };
    };
    [=] {
      [&] {
        using ty1 = decltype((x)); // refers to hypothetical capture proxy
        using ty1 = const int&;
      };
    };
    [&] {
      [=] {
        using ty1 = decltype((x)); // same
        using ty1 = const int&;
      };
    };
    [x] {
       [x] {
         using ty1 = decltype((x)); // refers to actual capture proxy,
                                    // found by HIDDEN_LAMBDA name lookup
         using ty1 = const int&;
       };
    };
    [x] {
       [] {
         using ty1 = decltype((x)); // refers to local variable,
                                    // HIDDEN_LAMBDA name lookup not performed
         using ty1 = int&;
       };
    };
}

Discussion: https://gcc.gnu.org/pipermail/gcc-patches/2023-December/638976.html

[Bug c++/112926] issues with nested lambdas and decltype of uncaptured local variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112926

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-04-13
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.