[Clang] Introduce a trait to determine the structure binding size (#131515)

Introduce a trait to determine the number of bindings that would be
produced by

```cpp

   auto [...p] = expr;

```

This is necessary to implement P2300
(https://eel.is/c++draft/exec#snd.concepts-5), but can also be used to
implement a general get<N> function that supports aggregates

`__builtin_structured_binding_size` is a unary type trait that evaluates
to the number of bindings in a decomposition

If the argument cannot be decomposed, a sfinae-friendly error is
produced.

A type is considered a valid tuple if `std::tuple_size_v<T>` is a valid
expression, even if there is no valid `std::tuple_element`
specialization or suitable `get` function for that type.


Fixes #46049

1 files changed, 6 insertions, 1 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 7803b10..022a201 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -12102,7 +12102,12 @@ public:
   }
 
   bool VisitTypeTraitExpr(const TypeTraitExpr *E) {
-    return Success(E->getValue(), E);
+    if (E->isStoredAsBoolean())
+      return Success(E->getBoolValue(), E);
+    if (E->getAPValue().isAbsent())
+      return false;
+    assert(E->getAPValue().isInt() && "APValue type not supported");
+    return Success(E->getAPValue().getInt(), E);
   }
 
   bool VisitArrayTypeTraitExpr(const ArrayTypeTraitExpr *E) {