https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127290
Bug ID: 127290
Summary: this is accepted in the trailing return type of an
explicit object member function
Product: gcc
Version: 16.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: berne at notadragon dot com
Target Milestone: ---
Created attachment 65536
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65536&action=edit
The accepted explicit-object case, with the rejected static control and an
implicit-object case that must compile
[expr.prim.this]/3, as amended by P0847R7 (deducing this):
"It shall not appear within the declaration of either a static member
function or an explicit object member function of the current class
(although its type and value category are defined within such member
functions as they are within an implicit object member function)."
A trailing return type is within the declaration, so `this` is ill-formed
in it for both static member functions and explicit object member
functions. GCC rejects the static half and accepts the explicit-object
half.
```
struct Bug {
int x;
// Accepted, and should not be.
auto f(this Bug& self) -> decltype(this->x);
};
struct Control {
int x;
// The same shape on a static member function, governed by the same
// sentence. Rejected -- so the rule is implemented, and applied to
// only half of what it says.
static auto g() -> decltype(this->x);
};
struct MustCompile {
int x;
// An implicit object member function may of course name `this' here.
auto h() -> decltype(this->x);
};
```
```
$ ./gcc-16.2.0/bin/g++ -std=c++23 -fsyntax-only \
this-in-xobj-trailing-return.cpp
this-in-xobj-trailing-return.cpp:33:33: error: invalid use of 'this' at top
level
33 | static auto g() -> decltype(this->x);
| ^~~~
this-in-xobj-trailing-return.cpp:33:33: error: invalid use of 'this' at top
level
```
The only diagnostic is for Control::g. Bug::f -- the same shape, governed
by the same sentence -- is accepted.
DISCOVERY
Found while implementing C++26 contract predicates on explicit object member
functions, where the analogous question -- may a predicate name `this`? --
had to be settled. Answering it meant reading [expr.prim.this]/3 closely
enough to notice that its static and explicit-object halves are enforced
differently outside contracts too. Nothing here involves contracts.
VERSIONS -- all on x86_64-linux-gnu
source version accepts Bug::f
compiler-explorer 14.4.0 yes
compiler-explorer 15.3.0 yes
compiler-explorer 16.1.0 yes
compiler-explorer 16.2.0 yes
compiler-explorer 17.0.0 20260909, 919c0d16c91 yes
local build -g 17.0.0 20260909, 7dab38c9d71 yes
```
$ ./gcc-16.2.0/bin/g++ -v
Using built-in specs.
COLLECT_GCC=./gcc-16.2.0/bin/g++
COLLECT_LTO_WRAPPER=/home/jberne4/repos/compilers/gcc-16.2.0/bin/../libexec/gcc/x86_64-linux-gnu/16.2.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../gcc-16.2.0/configure
--prefix=/opt/compiler-explorer/gcc-build/staging --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu
--enable-languages=c,c++,fortran,ada,objc,obj-c++,go,d,m2,rust,cobol,algol68
--enable-ld=yes --enable-gold=yes --enable-libstdcxx-time=yes
--enable-linker-build-id --enable-lto --enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build-gcc--binutils-2.44
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 16.2.0 (Compiler-Explorer-Build-gcc--binutils-2.44)
```
