[clang][C++20] P0960R3 and P1975R0: Allow initializing aggregates fro…

…m a parenthesized list of values

This patch implements P0960R3, which allows initialization of aggregates
via parentheses.

As an example:

```
struct S { int i, j; };
S s1(1, 1);

int arr1[2](1, 2);
```

This patch also implements P1975R0, which fixes the wording of P0960R3
for single-argument parenthesized lists so that statements like the
following are allowed:

```
S s2(1);
S s3 = static_cast<S>(1);
S s4 = (S)1;

int (&&arr2)[] = static_cast<int[]>(1);
int (&&arr3)[2] = static_cast<int[2]>(1);
```

This patch was originally authored by @0x59616e and completed by
@ayzhao.

Fixes #54040, Fixes #54041

Co-authored-by: Sheng <ox59616e@gmail.com>

Full write up : https://discourse.llvm.org/t/c-20-rfc-suggestion-desired-regarding-the-implementation-of-p0960r3/63744

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D129531

clang/docs/ReleaseNotes.rst

@@ -697,6 +697,10 @@ C++20 Feature Support
   (useful specially for constrained members). Fixes `GH50886 <https://github.com/llvm/llvm-project/issues/50886>`_.
 - Implemented CWG2635 as a Defect Report, which prohibits structured bindings from being constrained.
 
+- Implemented `P0960R3: <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0960r3.html>`_
+  and `P1975R0: <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1975r0.html>`_,
+  which allows parenthesized aggregate-initialization.
+
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang-c/Index.h

@@ -1531,7 +1531,13 @@ enum CXCursorKind {
    */
   CXCursor_RequiresExpr = 154,
 
-  CXCursor_LastExpr = CXCursor_RequiresExpr,
+  /**
+   * Expression that references a C++20 parenthesized list aggregate
+   * initializer.
+   */
+  CXCursor_CXXParenListInitExpr = 155,
+
+  CXCursor_LastExpr = CXCursor_CXXParenListInitExpr,
 
   /* Statements */
   CXCursor_FirstStmt = 200,

clang/include/clang/AST/ASTNodeTraverser.h

@@ -710,6 +710,12 @@ class ASTNodeTraverser
     }
   }
 
+  void VisitCXXParenListInitExpr(const CXXParenListInitExpr *PLIE) {
+    if (auto *Filler = PLIE->getArrayFiller()) {
+      Visit(Filler, "array_filler");
+    }
+  }
+
   void VisitBlockExpr(const BlockExpr *Node) { Visit(Node->getBlockDecl()); }
 
   void VisitOpaqueValueExpr(const OpaqueValueExpr *Node) {

clang/include/clang/AST/ComputeDependence.h

@@ -79,6 +79,7 @@ class CXXUnresolvedConstructExpr;
 class CXXDependentScopeMemberExpr;
 class MaterializeTemporaryExpr;
 class CXXFoldExpr;
+class CXXParenListInitExpr;
 class TypeTraitExpr;
 class ConceptSpecializationExpr;
 class SYCLUniqueStableNameExpr;
@@ -168,6 +169,7 @@ ExprDependence computeDependence(CXXUnresolvedConstructExpr *E);
 ExprDependence computeDependence(CXXDependentScopeMemberExpr *E);
 ExprDependence computeDependence(MaterializeTemporaryExpr *E);
 ExprDependence computeDependence(CXXFoldExpr *E);
+ExprDependence computeDependence(CXXParenListInitExpr *E);
 ExprDependence computeDependence(TypeTraitExpr *E);
 ExprDependence computeDependence(ConceptSpecializationExpr *E,
                                  bool ValueDependent);

clang/include/clang/AST/Decl.h

@@ -914,7 +914,10 @@ class VarDecl : public DeclaratorDecl, public Redeclarable<VarDecl> {
     CallInit,
 
     /// Direct list-initialization (C++11)
-    ListInit
+    ListInit,
+
+    /// Parenthesized list-initialization (C++20)
+    ParenListInit
   };
 
   /// Kinds of thread-local storage.

clang/include/clang/AST/ExprCXX.h

@@ -4713,6 +4713,130 @@ class CXXFoldExpr : public Expr {
   }
 };
 
+/// Represents a list-initialization with parenthesis.
+///
+/// As per P0960R3, this is a C++20 feature that allows aggregate to
+/// be initialized with a parenthesized list of values:
+/// ```
+/// struct A {
+///   int a;
+///   double b;
+/// };
+///
+/// void foo() {
+///   A a1(0);        // Well-formed in C++20
+///   A a2(1.5, 1.0); // Well-formed in C++20
+/// }
+/// ```
+/// It has some sort of similiarity to braced
+/// list-initialization, with some differences such as
+/// it allows narrowing conversion whilst braced
+/// list-initialization doesn't.
+/// ```
+/// struct A {
+///   char a;
+/// };
+/// void foo() {
+///   A a(1.5); // Well-formed in C++20
+///   A b{1.5}; // Ill-formed !
+/// }
+/// ```
+class CXXParenListInitExpr final
+    : public Expr,
+      private llvm::TrailingObjects<CXXParenListInitExpr, Expr *> {
+  friend class TrailingObjects;
+  friend class ASTStmtReader;
+  friend class ASTStmtWriter;
+
+  unsigned NumExprs;
+  unsigned NumUserSpecifiedExprs;
+  SourceLocation InitLoc, LParenLoc, RParenLoc;
+  Expr *ArrayFiller = nullptr;
+
+  CXXParenListInitExpr(ArrayRef<Expr *> Args, QualType T,
+                       unsigned NumUserSpecifiedExprs, SourceLocation InitLoc,
+                       SourceLocation LParenLoc, SourceLocation RParenLoc)
+      : Expr(CXXParenListInitExprClass, T,
+             T->isLValueReferenceType()   ? VK_LValue
+             : T->isRValueReferenceType() ? VK_XValue
+                                          : VK_PRValue,
+             OK_Ordinary),
+        NumExprs(Args.size()), NumUserSpecifiedExprs(NumUserSpecifiedExprs),
+        InitLoc(InitLoc), LParenLoc(LParenLoc), RParenLoc(RParenLoc) {
+    std::copy(Args.begin(), Args.end(), getTrailingObjects<Expr *>());
+    assert(NumExprs >= NumUserSpecifiedExprs &&
+           "number of user specified inits is greater than the number of "
+           "passed inits");
+    setDependence(computeDependence(this));
+  }
+
+  size_t numTrailingObjects(OverloadToken<Expr *>) const { return NumExprs; }
+
+public:
+  static CXXParenListInitExpr *
+  Create(ASTContext &C, ArrayRef<Expr *> Args, QualType T,
+         unsigned NumUserSpecifiedExprs, SourceLocation InitLoc,
+         SourceLocation LParenLoc, SourceLocation RParenLoc);
+
+  static CXXParenListInitExpr *CreateEmpty(ASTContext &C, unsigned numExprs,
+                                           EmptyShell Empty);
+
+  explicit CXXParenListInitExpr(EmptyShell Empty, unsigned NumExprs)
+      : Expr(CXXParenListInitExprClass, Empty), NumExprs(NumExprs),
+        NumUserSpecifiedExprs(0) {}
+
+  void updateDependence() { setDependence(computeDependence(this)); }
+
+  ArrayRef<Expr *> getInitExprs() {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(), NumExprs);
+  }
+
+  const ArrayRef<Expr *> getInitExprs() const {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(), NumExprs);
+  }
+
+  ArrayRef<Expr *> getUserSpecifiedInitExprs() {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(),
+                              NumUserSpecifiedExprs);
+  }
+
+  const ArrayRef<Expr *> getUserSpecifiedInitExprs() const {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(),
+                              NumUserSpecifiedExprs);
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY { return LParenLoc; }
+
+  SourceLocation getEndLoc() const LLVM_READONLY { return RParenLoc; }
+
+  SourceLocation getInitLoc() const LLVM_READONLY { return InitLoc; }
+
+  SourceRange getSourceRange() const LLVM_READONLY {
+    return SourceRange(getBeginLoc(), getEndLoc());
+  }
+
+  void setArrayFiller(Expr *E) { ArrayFiller = E; }
+
+  Expr *getArrayFiller() { return ArrayFiller; }
+
+  const Expr *getArrayFiller() const { return ArrayFiller; }
+
+  child_range children() {
+    Stmt **Begin = reinterpret_cast<Stmt **>(getTrailingObjects<Expr *>());
+    return child_range(Begin, Begin + NumExprs);
+  }
+
+  const_child_range children() const {
+    Stmt *const *Begin =
+        reinterpret_cast<Stmt *const *>(getTrailingObjects<Expr *>());
+    return const_child_range(Begin, Begin + NumExprs);
+  }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == CXXParenListInitExprClass;
+  }
+};
+
 /// Represents an expression that might suspend coroutine execution;
 /// either a co_await or co_yield expression.
 ///

clang/include/clang/AST/RecursiveASTVisitor.h

@@ -2852,6 +2852,7 @@ DEF_TRAVERSE_STMT(SubstNonTypeTemplateParmExpr, {})
 DEF_TRAVERSE_STMT(FunctionParmPackExpr, {})
 DEF_TRAVERSE_STMT(CXXFoldExpr, {})
 DEF_TRAVERSE_STMT(AtomicExpr, {})
+DEF_TRAVERSE_STMT(CXXParenListInitExpr, {})
 
 DEF_TRAVERSE_STMT(MaterializeTemporaryExpr, {
   if (S->getLifetimeExtendedTemporaryDecl()) {

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2164,6 +2164,9 @@ def err_init_list_bad_dest_type : Error<
 def warn_cxx20_compat_aggregate_init_with_ctors : Warning<
   "aggregate initialization of type %0 with user-declared constructors "
   "is incompatible with C++20">, DefaultIgnore, InGroup<CXX20Compat>;
+def warn_cxx17_compat_aggregate_init_paren_list : Warning<
+  "aggregate initialization of type %0 from a parenthesized list of values "
+  "is a C++20 extension">, DefaultIgnore, InGroup<CXX20>;
 
 def err_reference_bind_to_bitfield : Error<
   "%select{non-const|volatile}0 reference cannot bind to "

clang/include/clang/Basic/StmtNodes.td

@@ -160,6 +160,7 @@ def FunctionParmPackExpr : StmtNode<Expr>;
 def MaterializeTemporaryExpr : StmtNode<Expr>;
 def LambdaExpr : StmtNode<Expr>;
 def CXXFoldExpr : StmtNode<Expr>;
+def CXXParenListInitExpr: StmtNode<Expr>;
 
 // C++ Coroutines TS expressions
 def CoroutineSuspendExpr : StmtNode<Expr, 1>;

clang/include/clang/Sema/Initialization.h

@@ -923,7 +923,11 @@ class InitializationSequence {
     SK_OCLSamplerInit,
 
     /// Initialize an opaque OpenCL type (event_t, queue_t, etc.) with zero
-    SK_OCLZeroOpaqueType
+    SK_OCLZeroOpaqueType,
+
+    /// Initialize an aggreagate with parenthesized list of values.
+    /// This is a C++20 feature.
+    SK_ParenthesizedListInit
   };
 
   /// A single step in the initialization sequence.
@@ -1099,6 +1103,10 @@ class InitializationSequence {
 
     /// List-copy-initialization chose an explicit constructor.
     FK_ExplicitConstructor,
+
+    /// Parenthesized list initialization failed at some point.
+    /// This is a C++20 feature.
+    FK_ParenthesizedListInitFailed,
   };
 
 private:
@@ -1357,6 +1365,8 @@ class InitializationSequence {
   /// from a zero constant.
   void AddOCLZeroOpaqueTypeStep(QualType T);
 
+  void AddParenthesizedListInitStep(QualType T);
+
   /// Add steps to unwrap a initializer list for a reference around a
   /// single element and rewrap it at the end.
   void RewrapReferenceInitList(QualType T, InitListExpr *Syntactic);

clang/include/clang/Serialization/ASTBitCodes.h

@@ -1861,6 +1861,9 @@ enum StmtCode {
   /// A CXXBoolLiteralExpr record.
   EXPR_CXX_BOOL_LITERAL,
 
+  /// A CXXParenListInitExpr record.
+  EXPR_CXX_PAREN_LIST_INIT,
+
   EXPR_CXX_NULL_PTR_LITERAL, // CXXNullPtrLiteralExpr
   EXPR_CXX_TYPEID_EXPR,      // CXXTypeidExpr (of expr).
   EXPR_CXX_TYPEID_TYPE,      // CXXTypeidExpr (of type).

clang/lib/AST/ComputeDependence.cpp

@@ -831,6 +831,13 @@ ExprDependence clang::computeDependence(CXXFoldExpr *E) {
   return D;
 }
 
+ExprDependence clang::computeDependence(CXXParenListInitExpr *E) {
+  auto D = ExprDependence::None;
+  for (const auto *A : E->getInitExprs())
+    D |= A->getDependence();
+  return D;
+}
+
 ExprDependence clang::computeDependence(TypeTraitExpr *E) {
   auto D = ExprDependence::None;
   for (const auto *A : E->getArgs())

clang/lib/AST/Expr.cpp

@@ -3644,6 +3644,7 @@ bool Expr::HasSideEffects(const ASTContext &Ctx,
   case ShuffleVectorExprClass:
   case ConvertVectorExprClass:
   case AsTypeExprClass:
+  case CXXParenListInitExprClass:
     // These have a side-effect if any subexpression does.
     break;
 

clang/lib/AST/ExprCXX.cpp

@@ -1757,3 +1757,21 @@ CUDAKernelCallExpr *CUDAKernelCallExpr::CreateEmpty(const ASTContext &Ctx,
                            alignof(CUDAKernelCallExpr));
   return new (Mem) CUDAKernelCallExpr(NumArgs, HasFPFeatures, Empty);
 }
+
+CXXParenListInitExpr *
+CXXParenListInitExpr::Create(ASTContext &C, ArrayRef<Expr *> Args, QualType T,
+                             unsigned NumUserSpecifiedExprs,
+                             SourceLocation InitLoc, SourceLocation LParenLoc,
+                             SourceLocation RParenLoc) {
+  void *Mem = C.Allocate(totalSizeToAlloc<Expr *>(Args.size()));
+  return new (Mem) CXXParenListInitExpr(Args, T, NumUserSpecifiedExprs, InitLoc,
+                                        LParenLoc, RParenLoc);
+}
+
+CXXParenListInitExpr *CXXParenListInitExpr::CreateEmpty(ASTContext &C,
+                                                        unsigned NumExprs,
+                                                        EmptyShell Empty) {
+  void *Mem = C.Allocate(totalSizeToAlloc<Expr *>(NumExprs),
+                         alignof(CXXParenListInitExpr));
+  return new (Mem) CXXParenListInitExpr(Empty, NumExprs);
+}

clang/lib/AST/ExprClassification.cpp

@@ -445,6 +445,11 @@ static Cl::Kinds ClassifyInternal(ASTContext &Ctx, const Expr *E) {
   case Expr::SYCLUniqueStableNameExprClass:
     return Cl::CL_PRValue;
     break;
+
+  case Expr::CXXParenListInitExprClass:
+    if (isa<ArrayType>(E->getType()))
+      return Cl::CL_ArrayTemporary;
+    return Cl::CL_ClassTemporary;
   }
 
   llvm_unreachable("unhandled expression kind in classification");