https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106821
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 parameter
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 = 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 what
seems to be all template contexts.
