r322334 - Add `__reference_binds_to_temporary` trait for checking safe reference initialization.
Eric Fiselier via cfe-commits
cfe-commits at lists.llvm.org
Thu Jan 11 16:09:37 PST 2018
Author: ericwf
Date: Thu Jan 11 16:09:37 2018
New Revision: 322334
URL: http://llvm.org/viewvc/llvm-project?rev=322334&view=rev
Log:
Add `__reference_binds_to_temporary` trait for checking safe reference initialization.
Summary:
The STL types `std::pair` and `std::tuple` can both store reference types. However their constructors cannot adequately check if the initialization of reference types is safe. For example:
```
std::tuple<std::tuple<int> const&> t = 42;
// The stored reference is already dangling.
```
Libc++ has a best effort attempts in tuple to diagnose this, but they're not able to handle all valid cases (If I'm not mistaken). For example initialization of a reference from the result of a class's conversion operator. Libc++ would benefit from having a builtin traits which can provide a much better implementation.
This patch introduce the `__reference_binds_to_temporary(T, U)` trait that determines whether a reference of type `T` bound to an expression of type `U` would bind to a materialized temporary object.
Note that the trait simply returns false if `T` is not a reference type instead of reporting it as an error.
```
static_assert(__is_constructible(int const&, long));
static_assert(__reference_binds_to_temporary(int const&, long));
```
Reviewers: majnemer, rsmith
Reviewed By: rsmith
Subscribers: compnerd, cfe-commits
Differential Revision: https://reviews.llvm.org/D29930
Modified:
cfe/trunk/docs/LanguageExtensions.rst
cfe/trunk/include/clang/Basic/TokenKinds.def
cfe/trunk/include/clang/Basic/TypeTraits.h
cfe/trunk/lib/Sema/SemaExprCXX.cpp
cfe/trunk/test/SemaCXX/type-traits.cpp
Modified: cfe/trunk/docs/LanguageExtensions.rst
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/docs/LanguageExtensions.rst?rev=322334&r1=322333&r2=322334&view=diff
==============================================================================
--- cfe/trunk/docs/LanguageExtensions.rst (original)
+++ cfe/trunk/docs/LanguageExtensions.rst Thu Jan 11 16:09:37 2018
@@ -1096,6 +1096,11 @@ The following type trait primitives are
* ``__is_constructible`` (MSVC 2013, clang)
* ``__is_nothrow_constructible`` (MSVC 2013, clang)
* ``__is_assignable`` (MSVC 2015, clang)
+* ``__reference_binds_to_temporary(T, U)`` (Clang): Determines whether a
+ reference of type ``T`` bound to an expression of type ``U`` would bind to a
+ materialized temporary object. If ``T`` is not a reference type the result
+ is false. Note this trait will also return false when the initialization of
+ ``T`` from ``U`` is ill-formed.
Blocks
======
Modified: cfe/trunk/include/clang/Basic/TokenKinds.def
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/TokenKinds.def?rev=322334&r1=322333&r2=322334&view=diff
==============================================================================
--- cfe/trunk/include/clang/Basic/TokenKinds.def (original)
+++ cfe/trunk/include/clang/Basic/TokenKinds.def Thu Jan 11 16:09:37 2018
@@ -464,6 +464,7 @@ TYPE_TRAIT_1(__has_unique_object_represe
TYPE_TRAIT_N(__is_trivially_constructible, IsTriviallyConstructible, KEYCXX)
TYPE_TRAIT_1(__is_trivially_copyable, IsTriviallyCopyable, KEYCXX)
TYPE_TRAIT_2(__is_trivially_assignable, IsTriviallyAssignable, KEYCXX)
+TYPE_TRAIT_2(__reference_binds_to_temporary, ReferenceBindsToTemporary, KEYCXX)
KEYWORD(__underlying_type , KEYCXX)
// Embarcadero Expression Traits
Modified: cfe/trunk/include/clang/Basic/TypeTraits.h
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/Basic/TypeTraits.h?rev=322334&r1=322333&r2=322334&view=diff
==============================================================================
--- cfe/trunk/include/clang/Basic/TypeTraits.h (original)
+++ cfe/trunk/include/clang/Basic/TypeTraits.h Thu Jan 11 16:09:37 2018
@@ -80,7 +80,8 @@ namespace clang {
BTT_IsAssignable,
BTT_IsNothrowAssignable,
BTT_IsTriviallyAssignable,
- BTT_Last = BTT_IsTriviallyAssignable,
+ BTT_ReferenceBindsToTemporary,
+ BTT_Last = BTT_ReferenceBindsToTemporary,
TT_IsConstructible,
TT_IsNothrowConstructible,
TT_IsTriviallyConstructible
Modified: cfe/trunk/lib/Sema/SemaExprCXX.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaExprCXX.cpp?rev=322334&r1=322333&r2=322334&view=diff
==============================================================================
--- cfe/trunk/lib/Sema/SemaExprCXX.cpp (original)
+++ cfe/trunk/lib/Sema/SemaExprCXX.cpp Thu Jan 11 16:09:37 2018
@@ -4645,11 +4645,14 @@ static bool evaluateTypeTrait(Sema &S, T
if (Kind <= UTT_Last)
return EvaluateUnaryTypeTrait(S, Kind, KWLoc, Args[0]->getType());
- if (Kind <= BTT_Last)
+ // Evaluate BTT_ReferenceBindsToTemporary alongside the IsConstructible
+ // traits to avoid duplication.
+ if (Kind <= BTT_Last && Kind != BTT_ReferenceBindsToTemporary)
return EvaluateBinaryTypeTrait(S, Kind, Args[0]->getType(),
Args[1]->getType(), RParenLoc);
switch (Kind) {
+ case clang::BTT_ReferenceBindsToTemporary:
case clang::TT_IsConstructible:
case clang::TT_IsNothrowConstructible:
case clang::TT_IsTriviallyConstructible: {
@@ -4726,6 +4729,13 @@ static bool evaluateTypeTrait(Sema &S, T
if (Kind == clang::TT_IsConstructible)
return true;
+ if (Kind == clang::BTT_ReferenceBindsToTemporary) {
+ if (!T->isReferenceType())
+ return false;
+
+ return !Init.isDirectReferenceBinding();
+ }
+
if (Kind == clang::TT_IsNothrowConstructible)
return S.canThrow(Result.get()) == CT_Cannot;
Modified: cfe/trunk/test/SemaCXX/type-traits.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/type-traits.cpp?rev=322334&r1=322333&r2=322334&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/type-traits.cpp (original)
+++ cfe/trunk/test/SemaCXX/type-traits.cpp Thu Jan 11 16:09:37 2018
@@ -2225,6 +2225,7 @@ void constructible_checks() {
// PR25513
{ int arr[F(__is_constructible(int(int)))]; }
+ { int arr[T(__is_constructible(int const &, long))]; }
{ int arr[T(__is_constructible(ACompleteType))]; }
{ int arr[T(__is_nothrow_constructible(ACompleteType))]; }
@@ -2275,6 +2276,47 @@ void is_trivially_constructible_test() {
{ int arr[F(__is_trivially_constructible(const volatile void))]; }
}
+template <class T, class RefType = T &>
+struct ConvertsToRef {
+ operator RefType() const { return static_cast<RefType>(obj); }
+ mutable T obj = 42;
+};
+
+void reference_binds_to_temporary_checks() {
+ { int arr[F((__reference_binds_to_temporary(int &, int &)))]; }
+ { int arr[F((__reference_binds_to_temporary(int &, int &&)))]; }
+
+ { int arr[F((__reference_binds_to_temporary(int const &, int &)))]; }
+ { int arr[F((__reference_binds_to_temporary(int const &, int const &)))]; }
+ { int arr[F((__reference_binds_to_temporary(int const &, int &&)))]; }
+
+ { int arr[F((__reference_binds_to_temporary(int &, long &)))]; } // doesn't construct
+ { int arr[T((__reference_binds_to_temporary(int const &, long &)))]; }
+ { int arr[T((__reference_binds_to_temporary(int const &, long &&)))]; }
+ { int arr[T((__reference_binds_to_temporary(int &&, long &)))]; }
+
+ using LRef = ConvertsToRef<int, int &>;
+ using RRef = ConvertsToRef<int, int &&>;
+ using CLRef = ConvertsToRef<int, const int &>;
+ using LongRef = ConvertsToRef<long, long &>;
+ { int arr[T((__is_constructible(int &, LRef)))]; }
+ { int arr[F((__reference_binds_to_temporary(int &, LRef)))]; }
+
+ { int arr[T((__is_constructible(int &&, RRef)))]; }
+ { int arr[F((__reference_binds_to_temporary(int &&, RRef)))]; }
+
+ { int arr[T((__is_constructible(int const &, CLRef)))]; }
+ { int arr[F((__reference_binds_to_temporary(int &&, CLRef)))]; }
+
+ { int arr[T((__is_constructible(int const &, LongRef)))]; }
+ { int arr[T((__reference_binds_to_temporary(int const &, LongRef)))]; }
+
+ // Test that it doesn't accept non-reference types as input.
+ { int arr[F((__reference_binds_to_temporary(int, long)))]; }
+
+ { int arr[T((__reference_binds_to_temporary(const int &, long)))]; }
+}
+
void array_rank() {
int t01[T(__array_rank(IntAr) == 1)];
int t02[T(__array_rank(ConstIntArAr) == 2)];
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