Re: [patch] c++/58051 Implement Core 1579

[Jonathan Wakely <jwakely at redhat dot com>]

On 01/07/14 15:06 +0200, Markus Trippelsdorf wrote:
> On 2014.06.26 at 14:06 +0100, Jonathan Wakely wrote:
> > DR1579 relaxes [class.copy]/32 so that expressions in return
> > statements can be looked up as rvalues even when they aren't the same
> > type as the function return type.
> >
> > Implementing that seems as simple as removing the restriction on the
> > types. Tested x86_64-linux, no regressions.
>
> This patch cause yet another LLVM build error:
>
> FAILED: /var/tmp/gcc_test/usr/local/bin/g++   -DCLANG_ENABLE_ARCMT -DCLANG_ENABLE_REWRITER -DCLANG_ENABLE_STATIC_ANALYZER -DGTEST_HAS_RTTI=0 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DclangFrontend_EXPORTS -fPIC -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -Wwrite-strings -Wno-missing-field-initializers -pedantic -Wno-long-long -Wno-maybe-uninitialized -Wnon-virtual-dtor -Wno-comment -std=c++11 -ffunction-sections -fdata-sections -fno-common -Woverloaded-virtual -Wcast-qual -fno-strict-aliasing -O2  -DNDEBUG -pipe -fPIC -Itools/clang/lib/Frontend -I/var/tmp/llvm-project/llvm/tools/clang/lib/Frontend -I/var/tmp/llvm-project/llvm/tools/clang/include -Itools/clang/include -Iinclude -I/var/tmp/llvm-project/llvm/include    -fno-exceptions -fno-rtti -MMD -MT tools/clang/lib/Frontend/CMakeFiles/clangFrontend.dir/CompilerInvocation.cpp.o -MF "tools/clang/lib/Frontend/CMakeFiles/clangFrontend.dir/CompilerInvocation.cpp.o.d" -o tools/clang/lib/Frontend/CMakeFiles/clangFrontend.dir/CompilerInvocation.cpp.o -c /var/tmp/llvm-project/llvm/tools/clang/lib/Frontend/CompilerInvocation.cpp
> In file included from /var/tmp/llvm-project/llvm/tools/clang/include/clang/Basic/DiagnosticOptions.h:14:0,
>                 from /var/tmp/llvm-project/llvm/tools/clang/include/clang/Frontend/CompilerInvocation.h:13,
>                 from /var/tmp/llvm-project/llvm/tools/clang/lib/Frontend/CompilerInvocation.cpp:10:
> /var/tmp/llvm-project/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h: In instantiation of âllvm::IntrusiveRefCntPtr<T>::IntrusiveRefCntPtr(llvm::IntrusiveRefCntPtr<X>&&) [with X = clang::vfs::OverlayFileSystem; T = clang::vfs::FileSystem]â:
> /var/tmp/llvm-project/llvm/tools/clang/lib/Frontend/CompilerInvocation.cpp:2047:10:   required from here
> /var/tmp/llvm-project/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h:137:8: error: âclang::vfs::OverlayFileSystem* llvm::IntrusiveRefCntPtr<clang::vfs::OverlayFileSystem>::Objâ is private
>     T* Obj;
>        ^
> /var/tmp/llvm-project/llvm/include/llvm/ADT/IntrusiveRefCntPtr.h:158:13: error: within this context
>       S.Obj = 0;
>             ^
>
>
> Reduced:
>
> markus@x4 llvm_build % cat CompilerInvocation.ii
> template <typename T> class A
> {
>  T Obj;
>
> public:
>  T element_type;
>  A (T *);
>  template <class X> A (A<X> &&p1) { p1.Obj; }
>  template <class X> A (A<X> &);
> };
>
> class B
> {
> public:
>  B (A<int>);
> };
> A<int> fn1 ()
> {
>  A<B> a (new B (0));
>  return a;
> }
>
> markus@x4 llvm_build % /var/tmp/gcc_test/usr/local/bin/g++ -c -std=c++11 CompilerInvocation.ii
> CompilerInvocation.ii: In instantiation of âA<T>::A(A<X>&&) [with X = B; T = int]â:
> CompilerInvocation.ii:20:10:   required from here
> CompilerInvocation.ii:3:5: error: âB A<B>::Objâ is private
>   T Obj;
>     ^
> CompilerInvocation.ii:8:38: error: within this context
>   template <class X> A (A<X> &&p1) { p1.Obj; }

The error looks correct, A<T> cannot access private members of A<X>.

My patch only affects return statements and you get exactly the same
error if you change the code so there's no return statement:

void fn2 ()
{
 A<B> a (new B (0));
 A<int> aa( static_cast<A<B>&&>(a) );
}