[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

Author: cor3ntin

clang/docs/LanguageExtensions.rst

@@ -1912,6 +1912,40 @@ A simplistic usage example as might be seen in standard C++ headers follows:
   // Emulate type trait for compatibility with other compilers.
   #endif
 
+
+.. _builtin_structured_binding_size-doc:
+
+__builtin_structured_binding_size (C++)
+---------------------------------------
+
+The ``__builtin_structured_binding_size(T)`` type trait returns
+the *structured binding size* ([dcl.struct.bind]) of type ``T``
+
+This is equivalent to the size of the pack ``p`` in ``auto&& [...p] = declval<T&>();``.
+If the argument cannot be decomposed, ``__builtin_structured_binding_size(T)``
+is not a valid expression (``__builtin_structured_binding_size`` is SFINAE-friendly).
+
+builtin arrays, builtin SIMD vectors,
+builtin complex types, *tuple-like* types, and decomposable class types
+are decomposable types.
+
+A type is considered a valid *tuple-like* 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.
+
+.. code-block:: c++
+
+  template<std::size_t Idx, typename T>
+  requires (Idx < __builtin_structured_binding_size(T))
+  decltype(auto) constexpr get_binding(T&& obj) {
+      auto && [...p] = std::forward<T>(obj);
+      return p...[Idx];
+  }
+  struct S { int a = 0, b = 42; };
+  static_assert(__builtin_structured_binding_size(S) == 2);
+  static_assert(get_binding<1>(S{}) == 42);
+
+
 Blocks
 ======
 

clang/docs/ReleaseNotes.rst

@@ -74,6 +74,9 @@ What's New in Clang |release|?
 C++ Language Changes
 --------------------
 
+- Added a :ref:`__builtin_structured_binding_size <builtin_structured_binding_size-doc>` (T)
+  builtin that returns the number of structured bindings that would be produced by destructuring ``T``.
+
 - Similarly to GCC, Clang now supports constant expressions in
   the strings of a GNU ``asm`` statement.
 

clang/include/clang/AST/ExprCXX.h

@@ -51,6 +51,7 @@
 #include <cstdint>
 #include <memory>
 #include <optional>
+#include <variant>
 
 namespace clang {
 
@@ -2765,27 +2766,27 @@ class CXXPseudoDestructorExpr : public Expr {
 /// \endcode
 class TypeTraitExpr final
     : public Expr,
-      private llvm::TrailingObjects<TypeTraitExpr, TypeSourceInfo *> {
+      private llvm::TrailingObjects<TypeTraitExpr, APValue, TypeSourceInfo *> {
   /// The location of the type trait keyword.
   SourceLocation Loc;
 
   ///  The location of the closing parenthesis.
   SourceLocation RParenLoc;
 
-  // Note: The TypeSourceInfos for the arguments are allocated after the
-  // TypeTraitExpr.
-
   TypeTraitExpr(QualType T, SourceLocation Loc, TypeTrait Kind,
-                ArrayRef<TypeSourceInfo *> Args,
-                SourceLocation RParenLoc,
-                bool Value);
+                ArrayRef<TypeSourceInfo *> Args, SourceLocation RParenLoc,
+                std::variant<bool, APValue> Value);
 
   TypeTraitExpr(EmptyShell Empty) : Expr(TypeTraitExprClass, Empty) {}
 
   size_t numTrailingObjects(OverloadToken<TypeSourceInfo *>) const {
     return getNumArgs();
   }
 
+  size_t numTrailingObjects(OverloadToken<APValue>) const {
+    return TypeTraitExprBits.IsBooleanTypeTrait ? 0 : 1;
+  }
+
 public:
   friend class ASTStmtReader;
   friend class ASTStmtWriter;
@@ -2798,19 +2799,34 @@ class TypeTraitExpr final
                                SourceLocation RParenLoc,
                                bool Value);
 
+  static TypeTraitExpr *Create(const ASTContext &C, QualType T,
+                               SourceLocation Loc, TypeTrait Kind,
+                               ArrayRef<TypeSourceInfo *> Args,
+                               SourceLocation RParenLoc, APValue Value);
+
   static TypeTraitExpr *CreateDeserialized(const ASTContext &C,
+                                           bool IsStoredAsBool,
                                            unsigned NumArgs);
 
   /// Determine which type trait this expression uses.
   TypeTrait getTrait() const {
     return static_cast<TypeTrait>(TypeTraitExprBits.Kind);
   }
 
-  bool getValue() const {
-    assert(!isValueDependent());
+  bool isStoredAsBoolean() const {
+    return TypeTraitExprBits.IsBooleanTypeTrait;
+  }
+
+  bool getBoolValue() const {
+    assert(!isValueDependent() && TypeTraitExprBits.IsBooleanTypeTrait);
     return TypeTraitExprBits.Value;
   }
 
+  const APValue &getAPValue() const {
+    assert(!isValueDependent() && !TypeTraitExprBits.IsBooleanTypeTrait);
+    return *getTrailingObjects<APValue>();
+  }
+
   /// Determine the number of arguments to this type trait.
   unsigned getNumArgs() const { return TypeTraitExprBits.NumArgs; }
 

clang/include/clang/AST/Stmt.h

@@ -954,11 +954,13 @@ class alignas(void *) Stmt {
     LLVM_PREFERRED_TYPE(TypeTrait)
     unsigned Kind : 8;
 
-    /// If this expression is not value-dependent, this indicates whether
-    /// the trait evaluated true or false.
     LLVM_PREFERRED_TYPE(bool)
-    unsigned Value : 1;
+    unsigned IsBooleanTypeTrait : 1;
 
+    /// If this expression is a non value-dependent boolean trait,
+    /// this indicates whether the trait evaluated true or false.
+    LLVM_PREFERRED_TYPE(bool)
+    unsigned Value : 1;
     /// The number of arguments to this type trait. According to [implimits]
     /// 8 bits would be enough, but we require (and test for) at least 16 bits
     /// to mirror FunctionType.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -591,6 +591,8 @@ def err_decomp_decl_std_tuple_size_not_constant : Error<
   "is not a valid integral constant expression">;
 def note_in_binding_decl_init : Note<
   "in implicit initialization of binding declaration %0">;
+def err_arg_is_not_destructurable : Error<
+  "type %0 cannot be decomposed">;
 
 def err_std_type_trait_not_class_template : Error<
   "unsupported standard library implementation: "

clang/include/clang/Basic/TokenKinds.def

@@ -553,8 +553,8 @@ TYPE_TRAIT_2(__reference_converts_from_temporary, ReferenceConvertsFromTemporary
 // IsDeducible is only used internally by clang for CTAD implementation and
 // is not exposed to users.
 TYPE_TRAIT_2(/*EmptySpellingName*/, IsDeducible, KEYCXX)
-
 TYPE_TRAIT_1(__is_bitwise_cloneable, IsBitwiseCloneable, KEYALL)
+TYPE_TRAIT_1(__builtin_structured_binding_size, StructuredBindingSize, KEYCXX)
 
 // Embarcadero Expression Traits
 EXPRESSION_TRAIT(__is_lvalue_expr, IsLValueExpr, KEYCXX)

clang/include/clang/Sema/Sema.h

@@ -6116,7 +6116,8 @@ class Sema final : public SemaBase {
                                          RecordDecl *ClassDecl,
                                          const IdentifierInfo *Name);
 
-  unsigned GetDecompositionElementCount(QualType DecompType);
+  std::optional<unsigned int> GetDecompositionElementCount(QualType DecompType,
+                                                           SourceLocation Loc);
   void CheckCompleteDecompositionDeclaration(DecompositionDecl *DD);
 
   /// Stack containing information needed when in C++2a an 'auto' is encountered

clang/lib/AST/ASTImporter.cpp

@@ -8955,13 +8955,16 @@ ExpectedStmt ASTNodeImporter::VisitTypeTraitExpr(TypeTraitExpr *E) {
   if (Error Err = ImportContainerChecked(E->getArgs(), ToArgs))
     return std::move(Err);
 
-  // According to Sema::BuildTypeTrait(), if E is value-dependent,
-  // Value is always false.
-  bool ToValue = (E->isValueDependent() ? false : E->getValue());
-
-  return TypeTraitExpr::Create(
-      Importer.getToContext(), ToType, ToBeginLoc, E->getTrait(), ToArgs,
-      ToEndLoc, ToValue);
+  if (E->isStoredAsBoolean()) {
+    // According to Sema::BuildTypeTrait(), if E is value-dependent,
+    // Value is always false.
+    bool ToValue = (E->isValueDependent() ? false : E->getBoolValue());
+    return TypeTraitExpr::Create(Importer.getToContext(), ToType, ToBeginLoc,
+                                 E->getTrait(), ToArgs, ToEndLoc, ToValue);
+  }
+  return TypeTraitExpr::Create(Importer.getToContext(), ToType, ToBeginLoc,
+                               E->getTrait(), ToArgs, ToEndLoc,
+                               E->getAPValue());
 }
 
 ExpectedStmt ASTNodeImporter::VisitCXXTypeidExpr(CXXTypeidExpr *E) {

clang/lib/AST/ByteCode/Compiler.cpp

@@ -2844,9 +2844,13 @@ template <class Emitter>
 bool Compiler<Emitter>::VisitTypeTraitExpr(const TypeTraitExpr *E) {
   if (DiscardResult)
     return true;
-  if (E->getType()->isBooleanType())
-    return this->emitConstBool(E->getValue(), E);
-  return this->emitConst(E->getValue(), E);
+  if (E->isStoredAsBoolean()) {
+    if (E->getType()->isBooleanType())
+      return this->emitConstBool(E->getBoolValue(), E);
+    return this->emitConst(E->getBoolValue(), E);
+  }
+  PrimType T = classifyPrim(E->getType());
+  return this->visitAPValue(E->getAPValue(), T, E);
 }
 
 template <class Emitter>

clang/lib/AST/ExprCXX.cpp

@@ -1854,23 +1854,34 @@ bool MaterializeTemporaryExpr::isUsableInConstantExpressions(
 
 TypeTraitExpr::TypeTraitExpr(QualType T, SourceLocation Loc, TypeTrait Kind,
                              ArrayRef<TypeSourceInfo *> Args,
-                             SourceLocation RParenLoc, bool Value)
+                             SourceLocation RParenLoc,
+                             std::variant<bool, APValue> Value)
     : Expr(TypeTraitExprClass, T, VK_PRValue, OK_Ordinary), Loc(Loc),
       RParenLoc(RParenLoc) {
   assert(Kind <= TT_Last && "invalid enum value!");
+
   TypeTraitExprBits.Kind = Kind;
   assert(static_cast<unsigned>(Kind) == TypeTraitExprBits.Kind &&
          "TypeTraitExprBits.Kind overflow!");
-  TypeTraitExprBits.Value = Value;
+
+  TypeTraitExprBits.IsBooleanTypeTrait = std::holds_alternative<bool>(Value);
+  if (TypeTraitExprBits.IsBooleanTypeTrait)
+    TypeTraitExprBits.Value = std::get<bool>(Value);
+  else
+    *getTrailingObjects<APValue>() = std::get<APValue>(std::move(Value));
+
   TypeTraitExprBits.NumArgs = Args.size();
   assert(Args.size() == TypeTraitExprBits.NumArgs &&
          "TypeTraitExprBits.NumArgs overflow!");
-
   auto **ToArgs = getTrailingObjects<TypeSourceInfo *>();
   for (unsigned I = 0, N = Args.size(); I != N; ++I)
     ToArgs[I] = Args[I];
 
   setDependence(computeDependence(this));
+
+  assert((TypeTraitExprBits.IsBooleanTypeTrait || isValueDependent() ||
+          getAPValue().isInt() || getAPValue().isAbsent()) &&
+         "Only int values are supported by clang");
 }
 
 TypeTraitExpr *TypeTraitExpr::Create(const ASTContext &C, QualType T,
@@ -1879,13 +1890,25 @@ TypeTraitExpr *TypeTraitExpr::Create(const ASTContext &C, QualType T,
                                      ArrayRef<TypeSourceInfo *> Args,
                                      SourceLocation RParenLoc,
                                      bool Value) {
-  void *Mem = C.Allocate(totalSizeToAlloc<TypeSourceInfo *>(Args.size()));
+  void *Mem =
+      C.Allocate(totalSizeToAlloc<APValue, TypeSourceInfo *>(0, Args.size()));
+  return new (Mem) TypeTraitExpr(T, Loc, Kind, Args, RParenLoc, Value);
+}
+
+TypeTraitExpr *TypeTraitExpr::Create(const ASTContext &C, QualType T,
+                                     SourceLocation Loc, TypeTrait Kind,
+                                     ArrayRef<TypeSourceInfo *> Args,
+                                     SourceLocation RParenLoc, APValue Value) {
+  void *Mem =
+      C.Allocate(totalSizeToAlloc<APValue, TypeSourceInfo *>(1, Args.size()));
   return new (Mem) TypeTraitExpr(T, Loc, Kind, Args, RParenLoc, Value);
 }
 
 TypeTraitExpr *TypeTraitExpr::CreateDeserialized(const ASTContext &C,
+                                                 bool IsStoredAsBool,
                                                  unsigned NumArgs) {
-  void *Mem = C.Allocate(totalSizeToAlloc<TypeSourceInfo *>(NumArgs));
+  void *Mem = C.Allocate(totalSizeToAlloc<APValue, TypeSourceInfo *>(
+      IsStoredAsBool ? 0 : 1, NumArgs));
   return new (Mem) TypeTraitExpr(EmptyShell());
 }