r261297 - Implement the likely resolution of core issue 253.
Nico Weber via cfe-commits
cfe-commits at lists.llvm.org
Fri Feb 19 19:02:17 PST 2016
On Fri, Feb 19, 2016 at 8:01 PM, Richard Smith <richard at metafoo.co.uk>
wrote:
> On Fri, Feb 19, 2016 at 4:41 PM, Nico Weber <thakis at chromium.org> wrote:
> > On Fri, Feb 19, 2016 at 4:29 PM, Richard Smith via cfe-commits
> > <cfe-commits at lists.llvm.org> wrote:
> >>
> >> On Thu, Feb 18, 2016 at 5:52 PM, Nico Weber via cfe-commits
> >> <cfe-commits at lists.llvm.org> wrote:
> >> > Author: nico
> >> > Date: Thu Feb 18 19:52:46 2016
> >> > New Revision: 261297
> >> >
> >> > URL: http://llvm.org/viewvc/llvm-project?rev=261297&view=rev
> >> > Log:
> >> > Implement the likely resolution of core issue 253.
> >> >
> >> > C++11 requires const objects to have a user-provided constructor, even
> >> > for
> >> > classes without any fields. DR 253 relaxes this to say "If the
> implicit
> >> > default
> >> > constructor initializes all subobjects, no initializer should be
> >> > required."
> >> >
> >> > clang is currently the only compiler that implements this C++11 rule,
> >> > and e.g.
> >> > libstdc++ relies on something like DR 253 to compile in newer
> versions.
> >> > This
> >> > change makes it possible to build code that says `const vector<int>
> v;'
> >> > again
> >> > when using libstdc++5.2 and _GLIBCXX_DEBUG
> >> > (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60284).
> >> >
> >> > Fixes PR23381.
> >> >
> >> > http://reviews.llvm.org/D16552
> >> >
> >> > Modified:
> >> > cfe/trunk/include/clang/AST/DeclCXX.h
> >> > cfe/trunk/lib/AST/ASTImporter.cpp
> >> > cfe/trunk/lib/AST/DeclCXX.cpp
> >> > cfe/trunk/lib/Sema/SemaInit.cpp
> >> > cfe/trunk/lib/Serialization/ASTReaderDecl.cpp
> >> > cfe/trunk/lib/Serialization/ASTWriter.cpp
> >> > cfe/trunk/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp
> >> > cfe/trunk/test/CXX/dcl.decl/dcl.init/p6.cpp
> >> > cfe/trunk/test/CXX/drs/dr4xx.cpp
> >> > cfe/trunk/test/SemaCXX/attr-selectany.cpp
> >> > cfe/trunk/test/SemaCXX/constexpr-value-init.cpp
> >> > cfe/trunk/test/SemaCXX/cxx0x-cursory-default-delete.cpp
> >> > cfe/trunk/test/SemaCXX/illegal-member-initialization.cpp
> >> > cfe/trunk/www/cxx_dr_status.html
> >> >
> >> > Modified: cfe/trunk/include/clang/AST/DeclCXX.h
> >> > URL:
> >> >
> http://llvm.org/viewvc/llvm-project/cfe/trunk/include/clang/AST/DeclCXX.h?rev=261297&r1=261296&r2=261297&view=diff
> >> >
> >> >
> ==============================================================================
> >> > --- cfe/trunk/include/clang/AST/DeclCXX.h (original)
> >> > +++ cfe/trunk/include/clang/AST/DeclCXX.h Thu Feb 18 19:52:46 2016
> >> > @@ -378,6 +378,10 @@ class CXXRecordDecl : public RecordDecl
> >> > /// even if the class has a trivial default constructor.
> >> > bool HasUninitializedReferenceMember : 1;
> >> >
> >> > + /// \brief True if any non-mutable field whose type doesn't have
> a
> >> > user-
> >> > + /// provided default ctor also doesn't have an in-class
> >> > initializer.
> >> > + bool HasUninitializedFields : 1;
> >> > +
> >> > /// \brief These flags are \c true if a defaulted corresponding
> >> > special
> >> > /// member can't be fully analyzed without performing overload
> >> > resolution.
> >> > /// @{
> >> > @@ -1270,6 +1274,13 @@ public:
> >> > return !(data().HasTrivialSpecialMembers & SMF_Destructor);
> >> > }
> >> >
> >> > + /// \brief Determine whether declaring a const variable with this
> >> > type is ok
> >> > + /// per core issue 253.
> >> > + bool allowConstDefaultInit() const {
> >> > + return !data().HasUninitializedFields ||
> >> > + hasUserProvidedDefaultConstructor();
> >>
> >> hasUserProvidedDefaultConstructor() here is subtly incorrect. We
> >> shouldn't care whether there's a user-provided default constructor, we
> >> instead care whether the constructor that would have been chosen for
> >> initialization is defaulted (or equivalently, whether there *is* a
> >> defaulted default constructor, since if there is one, then either the
> >> initialization is ambiguous or it is chosen).
> >>
> >> This causes a regression for a testcase such as:
> >>
> >> struct X { template<typename ...T> X(T...); int n; };
> >> const X x; // formerly OK, now bogus error
> >
> >
> > Hm, great point. For a single hop, this fixes it:
> >
> > Index: lib/Sema/SemaInit.cpp
> > ===================================================================
> > --- lib/Sema/SemaInit.cpp (revision 261298)
> > +++ lib/Sema/SemaInit.cpp (working copy)
> > @@ -3521,7 +3521,7 @@
> > // The 253 proposal is for example needed to process libstdc++
> headers in
> > 5.x.
> > CXXConstructorDecl *CtorDecl =
> cast<CXXConstructorDecl>(Best->Function);
> > if (Kind.getKind() == InitializationKind::IK_Default &&
> > - Entity.getType().isConstQualified()) {
> > + Entity.getType().isConstQualified() &&
> !CtorDecl->isUserProvided()) {
> > if (!CtorDecl->getParent()->allowConstDefaultInit()) {
> > if (!maybeRecoverWithZeroInitialization(S, Sequence, Entity))
> >
> Sequence.SetFailed(InitializationSequence::FK_DefaultInitOfConst);
> >
> >
> > However, it doesn't make this pass:
> >
> > struct X { template<typename ...T> X(T...); int n; };
> > struct Y { X x; };
> > const Y y;
> >
> > That didn't build before this change either, but it feels like this
> should
> > be ok after this change. I think what you're suggesting is to
> conceptually
> > do this instead:
> >
> > Index: include/clang/AST/DeclCXX.h
> > ===================================================================
> > --- include/clang/AST/DeclCXX.h (revision 261298)
> > +++ include/clang/AST/DeclCXX.h (working copy)
> > @@ -1277,8 +1277,10 @@
> > /// \brief Determine whether declaring a const variable with this
> type is
> > ok
> > /// per core issue 253.
> > bool allowConstDefaultInit() const {
> > - return !data().HasUninitializedFields ||
> > - hasUserProvidedDefaultConstructor();
> > + if (!data().HasUninitializedFields)
> > + return true;
> > + CXXConstructorDecl *CD = S.LookupDefaultConstructor(ClassDecl);
> > + return !CD->isDefaulted();
> > }
> >
> > /// \brief Determine whether this class has a destructor which has no
> >
> > Now AST can't access Sema of course, so one way to do this would be to
> look
> > up the default ctor for every record in sema when completeDefinition()
> for a
> > record is called and then do
> >
> > if (!CD->isDefaulted())
> > RD->setAllowConstDefaultInit(true);
> >
> > But looking up the constructor is a bit more expensive than the current
> > computation, so maybe it makes sense to go back to lazy computation of
> this
> > information? Do you have any preferences for how to implement this?
>
> We don't need to actually do overload resolution here. There are three
> cases:
>
> 0) Default initialization is ill-formed in some way => we don't care
> what this function returns
> 1) There is no defaulted default constructor => const default init is OK
> 2) There is a defaulted default constructor => default init must use
> it (any alternative would put us in case 0), const default init is not
> OK if we have uninitialized fields
>
> So we only need to know if there is either an implicit default
> constructor,
How would you know that in your example though? Actually, after reading the
code a bit more, how about this instead:
Index: lib/AST/DeclCXX.cpp
===================================================================
--- lib/AST/DeclCXX.cpp (revision 261301)
+++ lib/AST/DeclCXX.cpp (working copy)
@@ -1793,7 +1803,8 @@
// A default constructor for a class X is a constructor of class
// X that can be called without an argument.
return (getNumParams() == 0) ||
- (getNumParams() > 0 && getParamDecl(0)->hasDefaultArg());
+ (getNumParams() > 0 && getParamDecl(0)->hasDefaultArg()) ||
+ (getNumParams() == 1 && getParamDecl(0)->isParameterPack());
}
bool
Fixes the test cases, passes the test suite, and seems like a good change
to me. For example, in
#include <type_traits>
struct X {
template <typename... T,
typename = typename std::enable_if<sizeof...(T) != 0>::type>
X(T...);
int n;
};
struct Y { X x; };
const Y y;
the explicit parameter pack deletes the implicit default ctor even though
it's SFINAE'd out. (I tried to find a different example where this change
makes an observable difference but so far I've failed to find one. This
probably impacts other things though.)
> or one that was defaulted on its first declaration. We
> can either incrementally compute that in CXXRecordDecl, or move the
> caching to Sema and actually do the lookup. (The latter seems like it
> should generally not be a big deal, as we're doing more costly things
> to check the initialization anyway, and Sema caches special member
> lookups.)
>
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