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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/91706] [8/9/10/11 Regression] ICE: tree check: expected
 class 'type', have 'exceptional' (error_mark) in equate_type_number_to_die,
 at dwarf2out.c:5782
Date: Fri, 16 Apr 2021 16:07:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D91706

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|ice-on-invalid-code         |ice-on-valid-code

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> ---
Valid variant:

template <bool> struct A;

struct B { static constexpr bool g =3D false; };

struct C {
  template <typename> static B c ();
};

template <class T> struct D : C {
  using c =3D decltype (c<T>());
  using E =3D A<c::g>;
};

D<int> g;

This is a complex interaction of issues.  Looking up c<T> in the definition=
 of
D::c finds C::c, OK.  Looking up c in the definition of E finds D::c, which=
 is
depedent, OK.  Since the alias is not dependent, we strip it from the templ=
ate
argument, leaving

using E =3D A<decltype(c<T>())>;

where 'c' still refers to C::c.  But instantiating E looks up 'c' again and
finds D::c, which isn't a function, so things go wrong.

I think the bug here is looking up 'c' in D at instantiation time; the
declaration we found before is not dependent.=