Extract `clang::DecompositionDecl` for structured bindings

[Ukilele]

A structured binding declaration can declare variables at global/namespace scope. But it is not part of the clang::VarDecl, but the clang::DecompositionDecl instead, which itself contains multiple clang::BindingDecls.

int a[2]{};
const auto& [b, c] = a;

TranslationUnitDecl
|-VarDecl <line:1:1, col:10> col:5 used a 'int[2]' listinit
| `-InitListExpr <col:9, col:10> 'int[2]'
|   `-array_filler: ImplicitValueInitExpr 0x55da82684710 'int'
|-BindingDecl <line:2:14> col:14 b 'const int'
| `-ArraySubscriptExpr <col:14> 'const int' lvalue
|   |-ImplicitCastExpr <col:14> 'const int *' <ArrayToPointerDecay>
|   | `-DeclRefExpr <col:14> 'const int[2]':'const int[2]' lvalue Decomposition 0x55da826848d8 '' 'const int &[2]' non_odr_use_constant
|   `-IntegerLiteral <col:14> 'int' 0
|-BindingDecl <col:17> col:17 c 'const int'
| `-ArraySubscriptExpr <col:17> 'const int' lvalue
|   |-ImplicitCastExpr <col:17> 'const int *' <ArrayToPointerDecay>
|   | `-DeclRefExpr <col:17> 'const int[2]':'const int[2]' lvalue Decomposition 0x55da826848d8 '' 'const int &[2]' non_odr_use_constant
|   `-IntegerLiteral <col:17> 'int' 1
`-DecompositionDecl <col:1, col:22> col:13 referenced 'const int &[2]' cinit
  |-ImplicitCastExpr <col:22> 'const int[2]':'const int[2]' lvalue <NoOp>
  | `-DeclRefExpr <col:22> 'int[2]' lvalue Var 0x55da826845d8 'a' 'int[2]'
  |-BindingDecl <col:14> col:14 b 'const int'
  | `-ArraySubscriptExpr <col:14> 'const int' lvalue
  |   |-ImplicitCastExpr <col:14> 'const int *' <ArrayToPointerDecay>
  |   | `-DeclRefExpr <col:14> 'const int[2]':'const int[2]' lvalue Decomposition 0x55da826848d8 '' 'const int &[2]' non_odr_use_constant
  |   `-IntegerLiteral <col:14> 'int' 0
  `-BindingDecl <col:17> col:17 c 'const int'
    `-ArraySubscriptExpr <col:17> 'const int' lvalue
      |-ImplicitCastExpr <col:17> 'const int *' <ArrayToPointerDecay>
      | `-DeclRefExpr <col:17> 'const int[2]':'const int[2]' lvalue Decomposition 0x55da826848d8 '' 'const int &[2]' non_odr_use_constant
      `-IntegerLiteral <col:17> 'int' 1

Docu: clang::DecompositionDecl

Docu: clang::BindingDecl

Example: https://structbind.compiler-explorer.com/z/4sa3vaKb7

[vinniefalco]

I mean.. that's going deep into the weeds :) Adding a new top level object is not a trivial exercise. I would like to polish up the top level objects we already have before we go creating new ones. But this is helpful :)

[Ukilele]

I am not sure if I will ever pick up this issue. But I at least wanted to report my finding :)

[vinniefalco]

Yeah its good