_Alignas on a struct declaration gives a low-quality diagnostic

[AaronBallman]

_Alignas(int) struct T {
  int i;
};

gives the diagnostic: warning: attribute '_Alignas' is ignored, place it after "struct" to apply attribute to type declaration which seems like a really helpful message at first glance. However, C does not allow you to put an _Alignas specifier on the struct type itself. So following that advice:

struct _Alignas(int) T {
  int i;
};

gives: error: declaration of anonymous struct must be a definition and warning: declaration does not declare anything.

The original diagnostic should say the attribute is ignored without the helpful message about placement (which is correct for C++).

[llvmbot]

@llvm/issue-subscribers-c11

[llvmbot]

@llvm/issue-subscribers-good-first-issue

[theo-lw]

Here's an attempt at a fix:

https://reviews.llvm.org/D141177

[Unique-Usman]

@AaronBallman , I will like work on this issue, @theo-lw are you still working on this?

[jerinphilip]

Hi, I'm trying to take a shot at this building on top of D141177 and its existing review. My active branch is here. I'm looking for some clarification w.r.t the solution before bringing in any review. Changes in bd41371 has D141177 outdated. I was expecting to use the replacements - something like isCXX11Attribute() && getLangOpts().C11 to identify the exception case and add code fixing the diagnostic.

Looks like I'm running into a few problems:

sample.c

#include <stdalign.h>

_Alignas(int) struct TLeft {
  int i;
};

#ifdef ENABLE_PLACE_AFTER
struct _Alignas(int) TRight { int i; };
#endif

int main() {
  struct TLeft t1 = {3};
#ifdef ENABLE_PLACE_AFTER
  TRight t1 = {4};
#endif
  return 0;
}

./bin/clang ../user-scripts/sample.c

--- <EmitAttributeDiagnostic> 
ParsedAttr.isCXX11Attribute() = 0
ParsedAttr.isInvalid() = 0
ParsedAttr.isTypeAttr() = 0
ParsedAttr.getForm().isAlignas() = 0
getLangOpts().C11 = 1
getLangOpts().C99 = 1
getLangOpts().C23 = 0
Firing normal /home/jerin/code/llvm-project/clang/lib/Sema/SemaDecl.cpp: 5366
../user-scripts/sample.c:3:1: warning: attribute '_Alignas' is ignored, place it after "struct" to apply attribute to type declaration [-Wignored-attributes]
    3 | _Alignas(int) struct TLeft {
      | ^
--- </EmitAttributeDiagnostic> 

./bin/clang++ ../user-scripts/sample.c -DENABLE_PLACE_AFTER=1

clang++: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated [-Wdeprecated]
--- <EmitAttributeDiagnostic> 
ParsedAttr.isCXX11Attribute() = 0
ParsedAttr.isInvalid() = 0
ParsedAttr.isTypeAttr() = 0
ParsedAttr.getForm().isAlignas() = 0
getLangOpts().C11 = 0
getLangOpts().C99 = 0
getLangOpts().C23 = 0
Firing normal /home/jerin/code/llvm-project/clang/lib/Sema/SemaDecl.cpp: 5366
../user-scripts/sample.c:3:1: warning: attribute '_Alignas' is ignored, place it after "struct" to apply attribute to type declaration [-Wignored-attributes]
    3 | _Alignas(int) struct TLeft {
      | ^
--- </EmitAttributeDiagnostic> 
../user-scripts/sample.c:8:1: error: declaration of anonymous struct must be a definition
    8 | struct _Alignas(int) TRight { int i; };
      | ^
../user-scripts/sample.c:8:1: warning: declaration does not declare anything [-Wmissing-declarations]
../user-scripts/sample.c:14:3: error: unknown type name 'TRight'
   14 |   TRight t1 = {4};
      |   ^
2 warnings and 2 errors generated.

Not sure what's going on above, but isCXX11Attribute() and isAlignAs() are not returning true as I expect to make the solution work. Is this expected? Excuse my debugging like a caveman with printf, but I hope this suffice to explain.