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

[Jonathan Wakely]

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 
‘llvm::IntrusiveRefCntPtr<T>::IntrusiveRefCntPtr(llvm::IntrusiveRefCntPtr<X>&&) 
[with X = clang::vfs::OverlayFileSystem; T = clang::vfs::FileSystem]’:
/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: 
‘clang::vfs::OverlayFileSystem* 
llvm::IntrusiveRefCntPtr<clang::vfs::OverlayFileSystem>::Obj’ 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 ‘A<T>::A(A<X>&&) [with X = B; T = 
int]’:
CompilerInvocation.ii:20:10:   required from here
CompilerInvocation.ii:3:5: error: ‘B A<B>::Obj’ 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) );
}