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From: "brycelelbach at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/100594] New: Lambdas in unevaluated contexts interact oddly
 with alias templates
Date: Fri, 14 May 2021 01:25:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100594

            Bug ID: 100594
           Summary: Lambdas in unevaluated contexts interact oddly with
                    alias templates
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: brycelelbach at gmail dot com
  Target Milestone: ---

This code:

```
template <typename T> struct identity {};

template <int N>
using A =3D decltype([] () constexpr { return int{}; }());

template <int N>
using B =3D identity<A<N>>;
```

Is incorrectly rejected by GCC in C++20 mode:

```
<source>:7:23: error: template argument 1 is invalid
    7 | using B =3D identity<A<N>>;
      |                       ^~
ASM generation compiler returned: 1
<source>:7:23: error: template argument 1 is invalid
    7 | using B =3D identity<A<N>>;
      |                       ^~
Execution build compiler returned: 1
```


https://godbolt.org/z/hGjfvjGzc

For context, I ran into this while writing this code:

https://godbolt.org/z/36xz8zGMd

I discussed this with Ville and Jason via email:

Ville wrote:
> Specifically looks like there's an interaction with an alias template
> here, because
>=20
> decltype([] () constexpr { return int{}; }())* oink;
> static_assert(std::is_same_v<decltype(oink), int*>);
>=20
> is just fine according to GCC.

Jason wrote:
> Yeah, how (unevaluated) lambdas should interact with alias templates is
> unclear, connected to various existing alias template issues around exact=
ly
> how transparent (or not) they are.
>=20
> You could work around this with a class template or variable template
> that forces the lambda instantiation to be delayed until we have a full
> set of template arguments.=