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SFINAEing p.x on a pointer wrongly tries to complete the pointee's type #52970

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Quuxplusone opened this issue Jan 3, 2022 · 3 comments
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SFINAEing p.x on a pointer wrongly tries to complete the pointee's type #52970

Quuxplusone opened this issue Jan 3, 2022 · 3 comments
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@Quuxplusone
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c++ clang:frontend Language frontend issues, e.g. anything involving "Sema" rejects-valid

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@Quuxplusone
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This program hard-errors trying to complete Holder<Incomplete>; but it shouldn't.

struct Incomplete;
template<class T> struct Holder { T t; int x; };
template<class T> auto f(T t) -> decltype(t.x);
void f(...);
void test() {
    ::f((Holder<Incomplete>*)nullptr);
}

It appears to me that the problem is here, in LookupMemberExpr:

  // Recover from dot accesses to pointers, e.g.:
  //   type *foo;
  //   foo.bar
  [...]
    if (!IsArrow && Ptr->getPointeeType()->isRecordType() &&
        MemberName.getNameKind() != DeclarationName::CXXDestructorName) {
      S.Diag(OpLoc, diag::err_typecheck_member_reference_suggestion)
          << BaseType << int(IsArrow) << BaseExpr.get()->getSourceRange()
          << FixItHint::CreateReplacement(OpLoc, "->");

      // Recurse as an -> access.
      IsArrow = true;
      return LookupMemberExpr(S, R, BaseExpr, IsArrow, OpLoc, SS,
                              ObjCImpDecl, HasTemplateArgs, TemplateKWLoc);
    }
  }

That second recursive call to LookupMemberExpr triggers completion of the pointed-to type, which is not OK if we're in a SFINAE context.
@Quuxplusone
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I've managed to fix this locally by adding

  if (S.isUnevaluatedContext())
    return ExprError();

after the S.Diag; but I don't know if there might be a less blunt instrument available.

@Quuxplusone Quuxplusone self-assigned this Jan 18, 2022
@Quuxplusone
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This is now https://reviews.llvm.org/D117603

@Quuxplusone Quuxplusone reopened this Jan 30, 2022
@EugeneZelenko EugeneZelenko added the clang:frontend Language frontend issues, e.g. anything involving "Sema" label Feb 1, 2022
@llvmbot
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llvmbot commented Feb 1, 2022

@llvm/issue-subscribers-clang-frontend

@xiaoyang-sde xiaoyang-sde mentioned this issue Mar 18, 2024
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mordante pushed a commit that referenced this issue Mar 20, 2024
@xiaoyang-sde
[libc++][ranges] remove __workaround_52970 (#85683)
211eebf 
## Abstract

This pull request removes the `__workaround_52970` concept. This concept
is a workaround for a bug described in #52970, which causes the compiler
to trigger ADL on a pointer to an incomplete type in an SFINAE context.
This bug is fixed in Clang 14.

## Reference

- [[clang] Don't typo-fix an expression in a SFINAE
context](https://reviews.llvm.org/D117603)
- [[libc++] [ranges] ADL-proof the [range.access]
CPOs.](https://reviews.llvm.org/D116239)
chencha3 pushed a commit to chencha3/llvm-project that referenced this issue Mar 23, 2024
@xiaoyang-sde @chencha3
[libc++][ranges] remove __workaround_52970 (llvm#85683)
e83cc9d 
## Abstract

This pull request removes the `__workaround_52970` concept. This concept
is a workaround for a bug described in llvm#52970, which causes the compiler
to trigger ADL on a pointer to an incomplete type in an SFINAE context.
This bug is fixed in Clang 14.

## Reference

- [[clang] Don't typo-fix an expression in a SFINAE
context](https://reviews.llvm.org/D117603)
- [[libc++] [ranges] ADL-proof the [range.access]
CPOs.](https://reviews.llvm.org/D116239)
devnexen pushed a commit to devnexen/llvm-project that referenced this issue Mar 23, 2024
@xiaoyang-sde @devnexen
[libc++][ranges] remove __workaround_52970 (llvm#85683)
ed7f672 
## Abstract

This pull request removes the `__workaround_52970` concept. This concept
is a workaround for a bug described in llvm#52970, which causes the compiler
to trigger ADL on a pointer to an incomplete type in an SFINAE context.
This bug is fixed in Clang 14.

## Reference

- [[clang] Don't typo-fix an expression in a SFINAE
context](https://reviews.llvm.org/D117603)
- [[libc++] [ranges] ADL-proof the [range.access]
CPOs.](https://reviews.llvm.org/D116239)
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