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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/104594] narrowing of -1 to unsigned char not detected with
 requires concepts
Date: Wed, 06 Apr 2022 22:42:19 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104594

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=3D67898
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
I wonder if this is ultimately a manifestation of PR67898.  After substitut=
ing
into an NTTP, if the type of the NTTP or the type of its argument is still
dependent then we can't yet check the implicit conversion from the argument
type to the parameter type.  And we don't encode this implicit conversion
within the substituted argument (as e.g. an IMPLICIT_CONV_EXPR) because we'=
ll
check the conversion again when we call coerce_template_parms the next time
around anyway.

However, as PR67898 illustrates, this causes problems if a subsequent templ=
ate
parameter refers to this NTTP, e.g:

  template<class T, T V, class =3D decltype(V)> struct A;
  template<class U, int W> using B =3D A<U, W>;

Here A<U, W> should resolve to A<U, W, U> but it instead resolves to A<U, W,
int> because we don't encode the implicit conversion (from int to U) within=
 the
substituted argument for V, and hence substitution into the default argument
yields decltype(W), which is just int, rather than yielding decltype(U(W)).

Seems we can run into the same issue during normalization as illustrated in
this current PR.  The normal form for E in commment #2 is just (X !=3D 0) (=
with
mapping X -> X) but we really should be encoding the implicit conversions i=
nto
the mapping (yielding something like X -> unsigned(int(X))).

So I wonder if the best way to tackle this PR is to tackle PR67898 more
generally?=