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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112288] [11/12/13/14 Regression] ICE - internal compiler
 error: in instantiate_decl, at cp/pt.cc:26861 since r6-6830-g20a0c6f9bdbd78
Date: Fri, 03 Nov 2023 17:27:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112288

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
The issue was probably latent before r6-6830.  The testcase is kind of stra=
nge,
e.g. 'slot_allocated' is defined within 'allocate_slot' instead of within
'slot', which would arguably be more natural given that its 'slot<T>' funct=
ion
parameter.  And consecutive equivalent calls to 'next' apparently resolve to
different overloads due to 'slot_allocated' being defined only after we
instantiate the second 'next' overload.

Reduced:

template<class T>
struct slot {
  template<class U>
  friend constexpr bool slot_allocated(slot<T>, U);
};

template<class T>
struct allocate_slot {
  template<class U>
  friend constexpr bool slot_allocated(slot<T>, U) { return true; }
};

template<class T, bool =3D slot_allocated(slot<T>{}, 0)>
constexpr int next(int) { return 0; }

template<class T>
constexpr int next(...) { allocate_slot<T> s; return 42; }

// slot_allocated<slot<int>, int>() not defined yet
static_assert(next<int>(0) =3D=3D 42);
// now it's defined
static_assert(next<int>(0) =3D=3D 0);=