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From: "gcc-bugs at marehr dot dialup.fu-berlin.de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/99433] [11 Regression] custom friend pipe-operator|
 conflicts with range adaptor?
Date: Fri, 09 Apr 2021 08:57:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99433

--- Comment #7 from gcc-bugs at marehr dot dialup.fu-berlin.de ---
Thank you for the quick analysis!

> views=E2=80=8B::=E2=80=8Bdrop(E, F) is specified to be expression-equival=
ent to the braced init ranges=E2=80=8B::=E2=80=8Bdrop_=C2=ADview{E, F}

Is not completely true, right? As the narrowing warning shows:

```
libstdc++-v3/include/std/ranges:2101:24: warning: narrowing conversion of
=E2=80=98std::declval<long unsigned int>()=E2=80=99 from =E2=80=98long unsi=
gned int=E2=80=99 to
=E2=80=98std::ranges::range_difference_t<std::ranges::ref_view<std::__cxx11=
::list<char>
> >=E2=80=99 {aka =E2=80=98long int=E2=80=99} [-Wnarrowing]
```

There is some `std::views::all` involved.

But the following expressions

```
#include <list>
#include <ranges>

int main()
{
  std::list<char> list;
  // std::views::drop(list, 0ull); // does not compile
  std::ranges::drop_view{list, 0ull}; // does compile without warnings
  std::ranges::drop_view{std::views::all(list), 0ull}; // does compile with=
out
warnings
}
```

do compile without any warnings when using `g++-11 -std=3Dc++2a -pedantic -=
Wall
-Wextra`!

Even when adding `-Wsystem-headers` there is no "narrowing" warning found in
those expressions.

Thank you for your incredible help!=