[C11] Implement WG14 N1285 (temporary lifetimes) (#133472)

This feature largely models the same behavior as in C++11. It is
technically a breaking change between C99 and C11, so the paper is not
being backported to older language modes.

One difference between C++ and C is that things which are rvalues in C
are often lvalues in C++ (such as the result of a ternary operator or a
comma operator).

Fixes #96486

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -162,6 +162,18 @@ C23 Feature Support
 - Fixed a bug where you could not cast a null pointer constant to type
   ``nullptr_t``. Fixes #GH133644.
 
+C11 Feature Support
+^^^^^^^^^^^^^^^^^^^
+- Implemented `WG14 N1285 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1285.htm>`_
+  which introduces the notion of objects with a temporary lifetime. When an
+  expression resulting in an rvalue with structure or union type and that type
+  contains a member of array type, the expression result is an automatic storage
+  duration object with temporary lifetime which begins when the expression is
+  evaluated and ends at the evaluation of the containing full expression. This
+  functionality is also implemented for earlier C language modes because the
+  C99 semantics will never be implemented (it would require dynamic allocations
+  of memory which leaks, which users would not appreciate).
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 

clang/lib/CodeGen/CGDecl.cpp

@@ -2265,11 +2265,18 @@ void CodeGenFunction::pushDestroy(QualType::DestructionKind dtorKind,
               cleanupKind & EHCleanup);
 }
 
+void CodeGenFunction::pushLifetimeExtendedDestroy(
+    QualType::DestructionKind dtorKind, Address addr, QualType type) {
+  CleanupKind cleanupKind = getCleanupKind(dtorKind);
+  pushLifetimeExtendedDestroy(cleanupKind, addr, type, getDestroyer(dtorKind),
+                              cleanupKind & EHCleanup);
+}
+
 void CodeGenFunction::pushDestroy(CleanupKind cleanupKind, Address addr,
                                   QualType type, Destroyer *destroyer,
                                   bool useEHCleanupForArray) {
-  pushFullExprCleanup<DestroyObject>(cleanupKind, addr, type,
-                                     destroyer, useEHCleanupForArray);
+  pushFullExprCleanup<DestroyObject>(cleanupKind, addr, type, destroyer,
+                                     useEHCleanupForArray);
 }
 
 // Pushes a destroy and defers its deactivation until its

clang/lib/CodeGen/CGExpr.cpp

@@ -393,55 +393,49 @@ pushTemporaryCleanup(CodeGenFunction &CGF, const MaterializeTemporaryExpr *M,
     }
   }
 
-  CXXDestructorDecl *ReferenceTemporaryDtor = nullptr;
-  if (const RecordType *RT =
-          E->getType()->getBaseElementTypeUnsafe()->getAs<RecordType>()) {
-    // Get the destructor for the reference temporary.
-    auto *ClassDecl = cast<CXXRecordDecl>(RT->getDecl());
-    if (!ClassDecl->hasTrivialDestructor())
-      ReferenceTemporaryDtor = ClassDecl->getDestructor();
-  }
+  QualType::DestructionKind DK = E->getType().isDestructedType();
+  if (DK != QualType::DK_none) {
+    switch (M->getStorageDuration()) {
+    case SD_Static:
+    case SD_Thread: {
+      CXXDestructorDecl *ReferenceTemporaryDtor = nullptr;
+      if (const RecordType *RT =
+              E->getType()->getBaseElementTypeUnsafe()->getAs<RecordType>()) {
+        // Get the destructor for the reference temporary.
+        if (auto *ClassDecl = dyn_cast<CXXRecordDecl>(RT->getDecl());
+            ClassDecl && !ClassDecl->hasTrivialDestructor())
+          ReferenceTemporaryDtor = ClassDecl->getDestructor();
+      }
 
-  if (!ReferenceTemporaryDtor)
-    return;
+      if (!ReferenceTemporaryDtor)
+        return;
 
-  // Call the destructor for the temporary.
-  switch (M->getStorageDuration()) {
-  case SD_Static:
-  case SD_Thread: {
-    llvm::FunctionCallee CleanupFn;
-    llvm::Constant *CleanupArg;
-    if (E->getType()->isArrayType()) {
-      CleanupFn = CodeGenFunction(CGF.CGM).generateDestroyHelper(
-          ReferenceTemporary, E->getType(),
-          CodeGenFunction::destroyCXXObject, CGF.getLangOpts().Exceptions,
-          dyn_cast_or_null<VarDecl>(M->getExtendingDecl()));
-      CleanupArg = llvm::Constant::getNullValue(CGF.Int8PtrTy);
-    } else {
-      CleanupFn = CGF.CGM.getAddrAndTypeOfCXXStructor(
-          GlobalDecl(ReferenceTemporaryDtor, Dtor_Complete));
-      CleanupArg = cast<llvm::Constant>(ReferenceTemporary.emitRawPointer(CGF));
+      llvm::FunctionCallee CleanupFn;
+      llvm::Constant *CleanupArg;
+      if (E->getType()->isArrayType()) {
+        CleanupFn = CodeGenFunction(CGF.CGM).generateDestroyHelper(
+            ReferenceTemporary, E->getType(), CodeGenFunction::destroyCXXObject,
+            CGF.getLangOpts().Exceptions,
+            dyn_cast_or_null<VarDecl>(M->getExtendingDecl()));
+        CleanupArg = llvm::Constant::getNullValue(CGF.Int8PtrTy);
+      } else {
+        CleanupFn = CGF.CGM.getAddrAndTypeOfCXXStructor(
+            GlobalDecl(ReferenceTemporaryDtor, Dtor_Complete));
+        CleanupArg =
+            cast<llvm::Constant>(ReferenceTemporary.emitRawPointer(CGF));
+      }
+      CGF.CGM.getCXXABI().registerGlobalDtor(
+          CGF, *cast<VarDecl>(M->getExtendingDecl()), CleanupFn, CleanupArg);
+    } break;
+    case SD_FullExpression:
+      CGF.pushDestroy(DK, ReferenceTemporary, E->getType());
+      break;
+    case SD_Automatic:
+      CGF.pushLifetimeExtendedDestroy(DK, ReferenceTemporary, E->getType());
+      break;
+    case SD_Dynamic:
+      llvm_unreachable("temporary cannot have dynamic storage duration");
     }
-    CGF.CGM.getCXXABI().registerGlobalDtor(
-        CGF, *cast<VarDecl>(M->getExtendingDecl()), CleanupFn, CleanupArg);
-    break;
-  }
-
-  case SD_FullExpression:
-    CGF.pushDestroy(NormalAndEHCleanup, ReferenceTemporary, E->getType(),
-                    CodeGenFunction::destroyCXXObject,
-                    CGF.getLangOpts().Exceptions);
-    break;
-
-  case SD_Automatic:
-    CGF.pushLifetimeExtendedDestroy(NormalAndEHCleanup,
-                                    ReferenceTemporary, E->getType(),
-                                    CodeGenFunction::destroyCXXObject,
-                                    CGF.getLangOpts().Exceptions);
-    break;
-
-  case SD_Dynamic:
-    llvm_unreachable("temporary cannot have dynamic storage duration");
   }
 }
 

clang/lib/CodeGen/CodeGenFunction.h

@@ -2249,6 +2249,8 @@ class CodeGenFunction : public CodeGenTypeCache {
   void pushLifetimeExtendedDestroy(CleanupKind kind, Address addr,
                                    QualType type, Destroyer *destroyer,
                                    bool useEHCleanupForArray);
+  void pushLifetimeExtendedDestroy(QualType::DestructionKind dtorKind,
+                                   Address addr, QualType type);
   void pushCallObjectDeleteCleanup(const FunctionDecl *OperatorDelete,
                                    llvm::Value *CompletePtr,
                                    QualType ElementType);

clang/lib/Sema/SemaInit.cpp

@@ -7650,11 +7650,14 @@ Sema::CreateMaterializeTemporaryExpr(QualType T, Expr *Temporary,
 }
 
 ExprResult Sema::TemporaryMaterializationConversion(Expr *E) {
-  // In C++98, we don't want to implicitly create an xvalue.
+  // In C++98, we don't want to implicitly create an xvalue. C11 added the
+  // same rule, but C99 is broken without this behavior and so we treat the
+  // change as applying to all C language modes.
   // FIXME: This means that AST consumers need to deal with "prvalues" that
   // denote materialized temporaries. Maybe we should add another ValueKind
   // for "xvalue pretending to be a prvalue" for C++98 support.
-  if (!E->isPRValue() || !getLangOpts().CPlusPlus11)
+  if (!E->isPRValue() ||
+      (!getLangOpts().CPlusPlus11 && getLangOpts().CPlusPlus))
     return E;
 
   // C++1z [conv.rval]/1: T shall be a complete type.

clang/test/C/C11/n1285.c

@@ -1,24 +1,82 @@
-// RUN: %clang_cc1 -verify=wrong -std=c99 %s
-// RUN: %clang_cc1 -verify=wrong -std=c11 %s
-// RUN: %clang_cc1 -verify=cpp -std=c++11 -x c++ %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,c -std=c99 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,c -std=c11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cpp -std=c++11 -x c++ %s
 
-/* WG14 N1285: No
+/* WG14 N1285: Clang 21
  * Extending the lifetime of temporary objects (factored approach)
  *
- * NB: we do not properly materialize temporary expressions in situations where
- * it would be expected; that is why the "no-diagnostics" marking is named
- * "wrong". We do issue the expected diagnostic in C++ mode.
+ * This paper introduced the notion of an object with a temporary lifetime. Any
+ * operation resulting in an rvalue of structure or union type which contains
+ * an array results in an object with temporary lifetime.
+ *
+ * Even though this is a change for C11, we treat it as a DR and apply it
+ * retroactively to earlier C language modes.
  */
 
-// wrong-no-diagnostics
+// C11 6.2.4p8: A non-lvalue expression with structure or union type, where the
+// structure or union contains a member with array type (including,
+// recursively, members of all contained structures and unions) refers to an
+// object with automatic storage duration and temporary lifetime. Its lifetime
+// begins when the expression is evaluated and its initial value is the value
+// of the expression. Its lifetime ends when the evaluation of the containing
+// full expression or full declarator ends. Any attempt to modify an object
+// with temporary lifetime results in undefined behavior.
 
 struct X { int a[5]; };
 struct X f(void);
 
-int foo(void) {
-  // FIXME: This diagnostic should be issued in C11 as well (though not in C99,
-  // as this paper was a breaking change between C99 and C11).
-  int *p = f().a; // cpp-warning {{temporary whose address is used as value of local variable 'p' will be destroyed at the end of the full-expression}}
-  return *p;
+union U { int a[10]; double d; };
+union U g(void);
+
+void sink(int *);
+
+int func_return(void) {
+  int *p = f().a; // expected-warning {{temporary whose address is used as value of local variable 'p' will be destroyed at the end of the full-expression}}
+  p = f().a;      // expected-warning {{object backing the pointer p will be destroyed at the end of the full-expression}}
+  p = g().a;      // expected-warning {{object backing the pointer p will be destroyed at the end of the full-expression}}
+  sink(f().a);    // Ok
+  return *f().a;  // Ok
+}
+
+int ternary(void) {
+  int *p = (1 ? (struct X){ 0 } : f()).a; // expected-warning {{temporary whose address is used as value of local variable 'p' will be destroyed at the end of the full-expression}}
+  int *r = (1 ? (union U){ 0 } : g()).a;  // expected-warning {{temporary whose address is used as value of local variable 'r' will be destroyed at the end of the full-expression}}
+  p = (1 ? (struct X){ 0 } : f()).a;      // expected-warning {{object backing the pointer p will be destroyed at the end of the full-expression}}
+  sink((1 ? (struct X){ 0 } : f()).a);    // Ok
+
+  // This intentionally gets one diagnostic in C and two in C++. In C, the
+  // compound literal results in an lvalue, not an rvalue as it does in C++. So
+  // only one branch results in a temporary in C but both branches do in C++.
+  int *q = 1 ? (struct X){ 0 }.a : f().a; // expected-warning {{temporary whose address is used as value of local variable 'q' will be destroyed at the end of the full-expression}} \
+                                             cpp-warning {{temporary whose address is used as value of local variable 'q' will be destroyed at the end of the full-expression}}
+  q = 1 ? (struct X){ 0 }.a : f().a; // expected-warning {{object backing the pointer q will be destroyed at the end of the full-expression}} \
+                                        cpp-warning {{object backing the pointer q will be destroyed at the end of the full-expression}}
+  q = 1 ? (struct X){ 0 }.a : g().a; // expected-warning {{object backing the pointer q will be destroyed at the end of the full-expression}} \
+                                        cpp-warning {{object backing the pointer q will be destroyed at the end of the full-expression}}
+  sink(1 ? (struct X){ 0 }.a : f().a); // Ok
+  return *(1 ? (struct X){ 0 }.a : f().a); // Ok
 }
 
+int comma(void) {
+  struct X x;
+  int *p = ((void)0, x).a; // c-warning {{temporary whose address is used as value of local variable 'p' will be destroyed at the end of the full-expression}}
+  p = ((void)0, x).a;      // c-warning {{object backing the pointer p will be destroyed at the end of the full-expression}}
+  sink(((void)0, x).a);    // Ok
+  return *(((void)0, x).a);// Ok
+}
+
+int cast(void) {
+  struct X x;
+  int *p = ((struct X)x).a;  // expected-warning {{temporary whose address is used as value of local variable 'p' will be destroyed at the end of the full-expression}}
+  p = ((struct X)x).a;       // expected-warning {{object backing the pointer p will be destroyed at the end of the full-expression}}
+  sink(((struct X)x).a);     // Ok
+  return *(((struct X)x).a); // Ok
+}
+
+int assign(void) {
+  struct X x, s;
+  int *p = (x = s).a;  // c-warning {{temporary whose address is used as value of local variable 'p' will be destroyed at the end of the full-expression}}
+  p = (x = s).a;       // c-warning {{object backing the pointer p will be destroyed at the end of the full-expression}}
+  sink((x = s).a);     // Ok
+  return *((x = s).a); // Ok
+}