[PATCH] D148505: Allow `__attribute__((warn_unused))` on individual constructors

[Stephan Bergmann via Phabricator via cfe-commits]

sberg added inline comments.


================
Comment at: clang/include/clang/Basic/Attr.td:2996
   let Spellings = [GCC<"warn_unused">];
-  let Subjects = SubjectList<[Record]>;
+  let Subjects = SubjectList<[Record, CXXConstructor]>;
   let Documentation = [Undocumented];
----------------
aaron.ballman wrote:
> I'm confused -- if you want this to behave like `nodiscard`, why aren't these changes for `warn_unused_result` (which is what implements `nodiscard`)? I don't think this should go on `warn_unused` because that's for the declaration of the type as a unit and not written on functions.
> 
> I think you don't need any changes to Attr.td for this functionality.
`[[nodiscard]]` and `__attribute__((warn_unused))` already have different semantics when applied to a class:  While both warn about unused expression results (`-Wunused-value`),  only the latter warns about unused variables (`-Wunused-variable`).  This patch keeps that difference, just extends it to individual constructors:
```
$ cat test.cc
struct [[nodiscard]] S1 {
  S1(int) {}
  S1(double) {}
};
struct S2 {
  [[nodiscard]] S2(int) {}
  S2(double) {}
};
struct __attribute__((warn_unused)) S3 {
  S3(int) {}
  S3(double) {}
};
struct S4 {
  __attribute__((warn_unused)) S4(int) {}
  S4(double) {}
};
int main() {
  S1(0); // expected-warning {{ignoring temporary}}
  S1(0.0); // expected-warning {{ignoring temporary}}
  S2(0); // expected-warning {{ignoring temporary}}
  S2(0.0);
  S3(0); // expected-warning {{expression result unused}}
  S3(0.0); // expected-warning {{expression result unused}}
  S4(0); // expected-warning {{expression result unused}}
  S4(0.0);
  S1 s1a(0);
  S1 s1b(0.0);
  S2 s2a(0);
  S2 s2b(0.0);
  S3 s3a(0); // expected-warning {{unused variable}}
  S3 s3b(0.0); // expected-warning {{unused variable}}
  S4 s4a(0); // expected-warning {{unused variable}}
  S4 s4b(0.0);
}
```
```
$ clang++ -Wunused-value -Wunused-variable -fsyntax-only -Xclang -verify test.cc
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D148505/new/

https://reviews.llvm.org/D148505