https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83470
Bug ID: 83470
Summary: Type morphing nested lambda capture
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: patrick.a.moran at gmail dot com
Target Milestone: ---
Created attachment 42908
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42908&action=edit
The minimal reproduction as a separate file
I found this compiling with gcc 7.2.0 with
--std=c++1z -Wall -Wextra
Including my very-minimal repro inline here because I don't know how else to
explain the issue.
--------------
#include <type_traits>
// This has to be templated, but the argument doesn't need to be
// used for anything
template <class T> double& g();
template <class T>
int f() {
auto& v = g<T>();
static_assert(std::is_same<decltype(v), T&>::value);
[](auto inner) {
inner(true);
}([&](auto) {
static_assert(std::is_same<decltype(v), bool&>::value);
});
return 0;
}
int dummy = f<double>();
--------------
As you can see, in this very specific circumstance, the type of v morphs from
double& to bool& in the inner lambda. It isn't that bool is special - whatever
type we pass as the ignored argument to the inner lambda, v will become that
type (lvalue-ref'ed). Changing any of the following will cause the bug to go
away:
* Making f() not a template
* Making g() not a template, or giving g() a template argument that isn't the
template argument to f().
* Changing the declaration of v to explicitly spell out double& rather than
letting auto& deduce it.
* Eliminating the lambda-wrapping. If you directly call the inner lambda, it
won't reproduce.
