https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101566
Bug ID: 101566
Summary: gcc miscompiles lambda used as tuple-like object
applied to function for call
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: weilercdale at gmail dot com
Target Milestone: ---
Created attachment 51191
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51191&action=edit
minimized test case
The following code uses a compiler generated structure via a lambda and `[=]`
capture-list to create a "tuple"-like object, `get<N>` is implemented in terms
of the gcc provided intrinsic `__integer_pack(N)` to emulate
`std::index_sequence` for peeling off the value from the "tuple"-like object.
`apply` calls any callable by applying `get<>` to each value in the
"tuple"-like object. `PackCount` exposes the parameter pack count of the
"tuple"-like object created with `make`, A `decltype` of the lambda is used to
create a new struct `Lambda` that inherits both, making
`PackCount<Ts...>::ELEMENTS` available as a compile-time constant to each
created "tuple"-like object. A `Tuple` type is provided by taking the
`decltype` of `make` synthesizing values for the call with `declval`. The code
compiles in gcc, msvc, and clang but only works in the latter two compilers,
gcc appears to miscompile the code under -O3. When compiling in gcc with
-fsanitize=undefined, the code isn't miscompiled, and it runs fine. Under other
optimization levels the code isn't miscompiled either. This appears to affect
multiple versions of gcc including 10.1, 10.2, 10.3, 11.1.0, 11.1.1, and
current trunk as can be verified on compiler explorer here
https://godbolt.org/z/3sYnrj6nW
