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Incorrect mention of 'alignof' in a diagnostic about 'alignas' #55175

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int6 opened this issue Apr 28, 2022 · 3 comments
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Incorrect mention of 'alignof' in a diagnostic about 'alignas' #55175

int6 opened this issue Apr 28, 2022 · 3 comments
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@yronglin
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clang:diagnostics New/improved warning or error message in Clang, but not in clang-tidy or static analyzer confirmed Verified by a second party

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@int6
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int6 commented Apr 28, 2022 •
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% cat main.cpp
class alignas(void) Bar {};

➜  test ../build/debug/bin/clang++ -std=c++11 ./alignas.cpp
./alignas.cpp:1:15: error: invalid application of 'alignof' to an incomplete type 'void'
class alignas(void) A {};
             ~^~~~~
1 error generated.

Should 'alignof' in the first diagnostic message be replaced with 'alignas'? Although alignas equivalent to alignas(alignof(type)).
@EugeneZelenko EugeneZelenko added clang:diagnostics New/improved warning or error message in Clang, but not in clang-tidy or static analyzer and removed new issue labels Apr 28, 2022
@AaronBallman AaronBallman changed the title Does Clang's diagnostic message for alignas need to be improved? Incorrect mention of 'alignof' in a diagnostic about 'alignas' May 3, 2022
@AaronBallman AaronBallman added the confirmed Verified by a second party label May 3, 2022
@AaronBallman
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The trouble is this bit: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDecl.cpp#L2882

When parsing the alignment argument for alignas and given a parenthesized type, we process it how the standard specifies by parsing it as an alignof expression (http://eel.is/c++draft/dcl.attr#dcl.align-3), but we have no way to tell Sema that the type trait we're actually after is alignas instead. Thus, we complain about the invalid alignof incorrectly.

Making alignas into an actual type trait would be possible but a bit novel (it's not a unary expression or a type trait, it's an attribute). Threading through "hey, you're building an alignas expression" also doesn't seem particularly clean. I think it might be that the correct approach here is to lift the logic out of ActOnUnaryExprOrTypeTraitExpr() into a new ActOnAlignasTypeArgument() function or somesuch.

@int6
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int6 commented May 4, 2022

The trouble is this bit: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDecl.cpp#L2882

When parsing the alignment argument for alignas and given a parenthesized type, we process it how the standard specifies by parsing it as an alignof expression (http://eel.is/c++draft/dcl.attr#dcl.align-3), but we have no way to tell Sema that the type trait we're actually after is alignas instead. Thus, we complain about the invalid alignof incorrectly.

Making alignas into an actual type trait would be possible but a bit novel (it's not a unary expression or a type trait, it's an attribute). Threading through "hey, you're building an alignas expression" also doesn't seem particularly clean. I think it might be that the correct approach here is to lift the logic out of ActOnUnaryExprOrTypeTraitExpr() into a new ActOnAlignasTypeArgument() function or somesuch.

Thanks for your reply and suggestion aaron, I'll try to understand this part and I'd be happy to do something about it

@yronglin
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Fixed by f75b735

@yronglin yronglin self-assigned this May 25, 2023
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