[clang] [Clang][RFC] Intrododuce a builtin to determine the structure binding size (PR #131515)

[Erich Keane via cfe-commits]

================
@@ -434,6 +434,37 @@ __datasizeof
 ``__datasizeof`` behaves like ``sizeof``, except that it returns the size of the
 type ignoring tail padding.
 
+.. _builtin_structured_binding_size-doc:
+
+__builtin_structured_binding_size (C++)
+---------------------------------------
+``__builtin_structured_binding_size`` returns the *structured binding size*
+([dcl.struct.bind]) of the type ``T`` (or unevaluated expression ``arg``)
+passed as argument.
+
+This is equivalent to the size of the pack ``p`` in ``auto&& [...p] = arg;``.
+If the argument is not destructurable (ie not a builtin array, builtin SIMD vector,
+builtin complex, *tuple-like* type or destructurable class type),
+``__builtin_structured_binding_size(T)`` is not a valid expression
+(``__builtin_structured_binding_size`` is SFINEA-friendly).
----------------
erichkeane wrote:

```suggestion
(``__builtin_structured_binding_size`` is SFINAE-friendly).
```

https://github.com/llvm/llvm-project/pull/131515