From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
	id 024F43858418; Sat,  3 Sep 2022 05:05:47 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 024F43858418
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1662181548;
	bh=GFbkF3CL+rbbBh20i8sZo4E1+PD1FJmjtNNSX7jLEx4=;
	h=From:To:Subject:Date:From;
	b=iLrToI660653NtQTZrXRiXXDhlrKrj6khTcfQjDGcz5TKMDFgLtieSHtCOeKalCv/
	 ypf7JKjD5ZtW4VQ0qSYPuMWbwI3qRMRG0jK0BUS2GhQch8S0ZJcvux1GCOcBaZJaK3
	 JfBnwHnoTFttsjxTT+EQC3DJmmDFN/g2l2VDvr2M=
From: "Dylan.Ferris at amd dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/106821] New: Incorrect behavior when using type alias
 template containing unevaluated lambda expression in a template context
Date: Sat, 03 Sep 2022 05:05:47 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.2.1
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: Dylan.Ferris at amd dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-106821-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106821

            Bug ID: 106821
           Summary: Incorrect behavior when using type alias template
                    containing unevaluated lambda expression in a template
                    context
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Dylan.Ferris at amd dot com
  Target Milestone: ---

I have prepared three different examples for when this fails:

https://godbolt.org/z/G8Eq5fo5G
https://godbolt.org/z/fajvnfo91
https://godbolt.org/z/e9s9Ws5o7

In the first one, `z.obj.a()` should be valid because `obj` is simply an
instantiation of the `A` template.

In the second one, `z.a()` should be valid because `bar` is inheriting from=
 an
instantiation of the `A` template.

In the third one, `obj.a()` should be valid because `obj` is simply an
instantiation of the `A` template.


All three of these have the following in common:

1. `alias` contains an unevaluated lambda expression, ie. `decltype([]{})`
2. The instantiation of `alias` as the inherited type uses a template param=
eter


So, this means that the three examples correctly compile if we apply any of=
 the
following workarounds:

1. "inline" the alias by replacing all instances of it with `A<decltype([]{=
})>`
(Requiring this workaround violates [temp.alias.2])
2. Remove the unevaluated lambda expression from `alias` (such as replacing=
 it
with `int`)
3. Remove the template parameter of `alias`
4. Instantiate `alias` with a fixed type such as `int`.


The bug persists even under a variety of transformations. Such as:

1. Removing the template on `A` and instead making an expression such as:
    template<typename T>
    using alias =3D std::conditional_t<true, A, decltype([]{})>;
2. Providing `bar`'s template parameter a default value of `int` and
subsequently using it


I believe this is related to PR 92707, but here the type of the unevaluated
lambda is used as a template parameter in each case, and it holds true in w=
hat
seems to be all template contexts.=