[[clang::lifetimebound]] is ignored without warning if the function parameter doesn't have a name.

[hokein]

See the example:

int* good(const int& abc[[clang::lifetimebound]]);

int* bad(const int& [[clang::lifetimebound]]);

The AST looks like:

-FunctionDecl  ... good 'int *(const int &)'
| `-ParmVarDecl ... abc 'const int &'
|   `-LifetimeBoundAttr 
`-FunctionDecl ... bad 'int *(const int &)'
  `-ParmVarDecl ... 'const int &'

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

See the example:

int* good(const int& abc[[clang::lifetimebound]]);

int* bad(const int& [[clang::lifetimebound]]);

The AST looks like:

-FunctionDecl  ... good 'int *(const int &)'
| `-ParmVarDecl ... abc 'const int &'
|   `-LifetimeBoundAttr 
`-FunctionDecl ... bad 'int *(const int &)'
  `-ParmVarDecl ... 'const int &'

[MitalAshok]

It looks like the attribute is just ignored without warning at that position: https://godbolt.org/z/nM56PbTbe

int* good(const int& abc[[clang::lifetimebound]]);
int* bad(const int& [[clang::lifetimebound]]);
int* good2([[clang::lifetimebound]] const int&);
int* good3([[clang::lifetimebound]] const int& abc);
int* bad2(const int& [[clang::lifetimebound]] abc);

int main() {
    int* a = good(0);
    int* b = bad(0); // no -Wdangling
    int* c = good2(0);
    int* d = good3(0);
    int* e = bad2(0); // no -Wdangling
}

The fact that it doesn't work makes sense as [[clang::lifetimebound]] is supposed to apply to the parameter, not the type of the parameter, which is what it would do at that position. It looks like it is supposed to error for bad and bad2, but just doesn't.

Compare that to the similar [[gnu::nonnull]]: https://godbolt.org/z/1jKrEq7MG

int* good(const int* abc[[gnu::nonnull]]);
int* bad(const int* [[gnu::nonnull]]);  // warning: attribute ignored
int* good2([[gnu::nonnull]] const int*);
int* good3([[gnu::nonnull]] const int* abc);
int* bad2(const int* [[gnu::nonnull]] abc);  // warning: attribute ignored

int main() {
    int* a = good(0);
    int* b = bad(0); // no warning
    int* c = good2(0);
    int* d = good3(0);
    int* e = bad2(0); // no warning
}

llvm-project/clang/include/clang/Basic/Attr.td

Line 1793 in f991ebb

let Subjects = SubjectList<[ParmVar, ImplicitObjectParameter], ErrorDiag>;

llvm-project/clang/include/clang/Basic/Attr.td

Line 2190 in f991ebb

let Subjects = SubjectList<[ObjCMethod, HasFunctionProto, ParmVar], WarnDiag,

[hokein]

Thanks for the clarification.

The fact that it doesn't work makes sense as [[clang::lifetimebound]] is supposed to apply to the parameter, not the type of the parameter, which is what it would do at that position. It looks like it is supposed to error for bad and bad2, but just doesn't.

Yeah, this makes sense. For the bad case, it would be great if Clang emitted an attribute-ignored warning. I was confused by this when experimenting with the lifetimebound feature.

[zygoloid]

I think we're not diagnosing this as a non-type attribute applied to a type because it is a type attribute -- specifically, when applied to ah implicit object parameter, what it actually appertains to is a function type. We'll need some better checking for the ImplicitObjectParameter subject that diagnoses uses as a type attribute in a position other than on the (innermost) function declarator of a member function.

[emaxx-google]

I've published #118281 which adds warnings to diagnose this and similar cases of a silent [[clang::lifetimebound]] ignore.