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                                   GCC Bugs

   The   latest   version   of  this  document  is  always  available  at
   [1]http://gcc.gnu.org/bugs.html.
     _________________________________________________________________

Table of Contents

     * [2]Reporting Bugs
          + [3]What we need
          + [4]What we DON'T want
          + [5]Where to post it
          + [6]Detailed bug reporting instructions
          + [7]Detailed bug reporting instructions for GNAT
          + [8]Detailed   bug   reporting   instructions   when  using  a
            precompiled header
     * [9]Frequently Reported Bugs in GCC
          + [10]C++
               o [11]Missing features
               o [12]Bugs fixed in the 3.4 series
          + [13]Fortran
     * [14]Non-bugs
          + [15]General
          + [16]C
          + [17]C++
               o [18]Common problems when upgrading the compiler
     _________________________________________________________________

                                Reporting Bugs

   The  main  purpose of a bug report is to enable us to fix the bug. The
   most  important  prerequisite  for  this  is  that  the report must be
   complete and self-contained.

   Before  you report a bug, please check the [19]list of well-known bugs
   and,  if  possible, try a current development snapshot. If you want to
   report  a  bug  with  versions of GCC before 3.4 we strongly recommend
   upgrading to the current release first.

   Before  reporting  that  GCC  compiles  your  code incorrectly, please
   compile  it  with  gcc -Wall and see whether this shows anything wrong
   with your code that could be the cause instead of a bug in GCC.

Summarized bug reporting instructions

   After  this  summary, you'll find detailed bug reporting instructions,
   that  explain  how to obtain some of the information requested in this
   summary.

  What we need

   Please  include  in  your  bug  report all of the following items, the
   first three of which can be obtained from the output of gcc -v:
     * the exact version of GCC;
     * the system type;
     * the options given when GCC was configured/built;
     * the complete command line that triggers the bug;
     * the compiler output (error messages, warnings, etc.); and
     * the  preprocessed  file (*.i*) that triggers the bug, generated by
       adding -save-temps to the complete compilation command, or, in the
       case  of  a  bug  report for the GNAT front end, a complete set of
       source files (see below).

  What we do not want

     * A source file that #includes header files that are left out of the
       bug report (see above)
     * That source file and a collection of header files.
     * An  attached archive (tar, zip, shar, whatever) containing all (or
       some :-) of the above.
     * A  code snippet that won't cause the compiler to produce the exact
       output  mentioned  in  the bug report (e.g., a snippet with just a
       few  lines  around  the one that apparently triggers the bug, with
       some   pieces   replaced  with  ellipses  or  comments  for  extra
       obfuscation :-)
     * The  location  (URL) of the package that failed to build (we won't
       download it, anyway, since you've already given us what we need to
       duplicate the bug, haven't you? :-)
     * An  error  that  occurs  only  some of the times a certain file is
       compiled,  such that retrying a sufficient number of times results
       in  a  successful  compilation;  this  is  a symptom of a hardware
       problem, not of a compiler bug (sorry)
     * Assembly  files  (*.s)  produced  by  the  compiler, or any binary
       files,   such   as  object  files,  executables,  core  files,  or
       precompiled header files
     * Duplicate  bug  reports,  or  reports of bugs already fixed in the
       development tree, especially those that have already been reported
       as fixed last week :-)
     * Bugs  in  the  assembler,  the  linker or the C library. These are
       separate  projects,  with separate mailing lists and different bug
       reporting procedures
     * Bugs  in  releases  or  snapshots  of  GCC  not  issued by the GNU
       Project. Report them to whoever provided you with the release
     * Questions  about  the  correctness  or  the  expected  behavior of
       certain constructs that are not GCC extensions. Ask them in forums
       dedicated to the discussion of the programming language

  Where to post it

   Please  submit  your  bug report directly to the [20]GCC bug database.
   Alternatively,  you  can  use  the  gccbug  script that mails your bug
   report to the bug database.
   Only  if  all  this  is absolutely impossible, mail all information to
   [21]gcc-bugs@gcc.gnu.org.

Detailed bug reporting instructions

   Please  refer to the [22]next section when reporting bugs in GNAT, the
   Ada  compiler,  or  to the [23]one after that when reporting bugs that
   appear when using a precompiled header.

   In  general, all the information we need can be obtained by collecting
   the  command  line  below,  as well as its output and the preprocessed
   file it generates.

     gcc -v -save-temps all-your-options source-file

   The  only  excuses  to not send us the preprocessed sources are (i) if
   you've  found  a  bug  in the preprocessor, (ii) if you've reduced the
   testcase  to a small file that doesn't include any other file or (iii)
   if  the  bug appears only when using precompiled headers. If you can't
   post  the  preprocessed sources because they're proprietary code, then
   try to create a small file that triggers the same problem.

   Since  we're  supposed  to  be  able  to re-create the assembly output
   (extension  .s),  you usually should not include it in the bug report,
   although  you  may want to post parts of it to point out assembly code
   you consider to be wrong.

   Please  avoid  posting  an archive (.tar, .shar or .zip); we generally
   need   just  a  single  file  to  reproduce  the  bug  (the  .i/.ii/.f
   preprocessed  file),  and,  by  storing  it in an archive, you're just
   making our volunteers' jobs harder. Only when your bug report requires
   multiple source files to be reproduced should you use an archive. This
   is,  for  example,  the  case  if  you are using INCLUDE directives in
   Fortran  code,  which  are  not processed by the preprocessor, but the
   compiler.  In that case, we need the main file and all INCLUDEd files.
   In  any  case, make sure the compiler version, error message, etc, are
   included  in  the  body  of  your  bug  report  as plain text, even if
   needlessly duplicated as part of an archive.

Detailed bug reporting instructions for GNAT

   See  the  [24]previous  section for bug reporting instructions for GCC
   language implementations other than Ada.

   Bug  reports  have  to  contain  at least the following information in
   order to be useful:
     * the exact version of GCC, as shown by "gcc -v";
     * the system type;
     * the options when GCC was configured/built;
     * the  exact  command  line passed to the gcc program triggering the
       bug  (not  just  the flags passed to gnatmake, but gnatmake prints
       the parameters it passed to gcc)
     * a collection of source files for reproducing the bug, preferably a
       minimal set (see below);
     * a description of the expected behavior;
     * a description of actual behavior.

   If  your  code  depends  on  additional  source files (usually package
   specifications), submit the source code for these compilation units in
   a  single  file that is acceptable input to gnatchop, i.e. contains no
   non-Ada  text. If the compilation terminated normally, you can usually
   obtain a list of dependencies using the "gnatls -d main_unit" command,
   where  main_unit  is the file name of the main compilation unit (which
   is also passed to gcc).

   If  you  report  a  bug  which causes the compiler to print a bug box,
   include that bug box in your report, and do not forget to send all the
   source files listed after the bug box along with your report.

   If  you  use gnatprep, be sure to send in preprocessed sources (unless
   you have to report a bug in gnatprep).

   When  you  have  checked that your report meets these criteria, please
   submit  it  according  to  our [25]generic instructions. (If you use a
   mailing  list  for  reporting,  please  include  an "[Ada]" tag in the
   subject.)

Detailed bug reporting instructions when using a precompiled header

   If  you're  encountering  a  bug  when using a precompiled header, the
   first thing to do is to delete the precompiled header, and try running
   the  same GCC command again. If the bug happens again, the bug doesn't
   really  involve  precompiled  headers,  please report it without using
   them by following the instructions [26]above.

   If  you've  found  a  bug  while  building  a  precompiled header (for
   instance,   the  compiler  crashes),  follow  the  usual  instructions
   [27]above.

   If  you've  found  a  real  precompiled header bug, what we'll need to
   reproduce  it  is  the  sources  to build the precompiled header (as a
   single .i file), the source file that uses the precompiled header, any
   other  headers  that  source file includes, and the command lines that
   you used to build the precompiled header and to use it.

   Please don't send us the actual precompiled header. It is likely to be
   very large and we can't use it to reproduce the problem.
     _________________________________________________________________

                        Frequently Reported Bugs in GCC

   This  is  a  list of bugs in GCC that are reported very often, but not
   yet  fixed.  While  it  is  certainly  better  to  fix bugs instead of
   documenting  them,  this  document  might  save  people  the effort of
   writing a bug report when the bug is already well-known.

   There  are many reasons why a reported bug doesn't get fixed. It might
   be  difficult  to  fix, or fixing it might break compatibility. Often,
   reports  get  a  low  priority  when there is a simple work-around. In
   particular, bugs caused by invalid code have a simple work-around: fix
   the code.
     _________________________________________________________________

C++

  Missing features

   The export keyword is not implemented.
          Most  C++ compilers (G++ included) do not yet implement export,
          which   is  necessary  for  separate  compilation  of  template
          declarations   and  definitions.  Without  export,  a  template
          definition  must be in scope to be used. The obvious workaround
          is  simply  to  place  all  definitions  in  the header itself.
          Alternatively,   the   compilation   unit  containing  template
          definitions may be included from the header.

  Bugs fixed in the 3.4 series

   The  following  bugs are present up to (and including) GCC 3.3.x. They
   have been fixed in 3.4.0.

   Two-stage name-lookup.
          GCC   did   not   implement  two-stage  name-lookup  (also  see
          [28]below).

   Covariant return types.
          GCC did not implement non-trivial covariant returns.

   Parse errors for "simple" code.
          GCC gave parse errors for seemingly simple code, such as

struct A
{
  A();
  A(int);
};

struct B
{
  B(A);
  B(A,A);
  void foo();
};

A bar()
{
  B b(A(),A(1));  // Variable b, initialized with two temporaries
  B(A(2)).foo();  // B temporary, initialized with A temporary
  return (A());   // return A temporary
}

          Although  being  valid  code,  each  of  the three lines with a
          comment  was  rejected  by  GCC.  The  work-arounds  for  older
          compiler versions proposed below do not change the semantics of
          the programs at all.

          The problem in the first case was that GCC started to parse the
          declaration  of  b as a function called b returning B, taking a
          function returning A as an argument. When it encountered the 1,
          it  was  too  late.  To  show  the compiler that this should be
          really  an  expression,  a comma operator with a dummy argument
          could be used:

B b((0,A()),A(1));

          The  work-around  for  simpler cases like the second one was to
          add  additional  parentheses  around  the expressions that were
          mistaken as declarations:

(B(A(2))).foo();

          In the third case, however, additional parentheses were causing
          the  problems:  The  compiler  interpreted  A()  as  a function
          (taking no arguments, returning A), and (A()) as a cast lacking
          an  expression  to  be  casted,  hence  the  parse  error.  The
          work-around was to omit the parentheses:

return A();

          This  problem  occurred  in  a  number  of  variants;  in throw
          statements,   people   also   frequently   put  the  object  in
          parentheses.
     _________________________________________________________________

Fortran

   G77  bugs  are  documented  in  the  G77 manual rather than explicitly
   listed  here.  Please see [29]Known Causes of Trouble with GNU Fortran
   in the G77 manual.
     _________________________________________________________________

                                   Non-bugs

   The  following are not actually bugs, but are reported often enough to
   warrant a mention here.

   It  is  not  always a bug in the compiler, if code which "worked" in a
   previous  version,  is now rejected. Earlier versions of GCC sometimes
   were less picky about standard conformance and accepted invalid source
   code.  In addition, programming languages themselves change, rendering
   code  invalid  that  used  to be conforming (this holds especially for
   C++).  In  either  case,  you  should update your code to match recent
   language standards.
     _________________________________________________________________

General

   Problems with floating point numbers - the [30]most often reported
          non-bug.
          In  a  number  of  cases, GCC appears to perform floating point
          computations incorrectly. For example, the C++ program

#include <iostream>

int main()
{
  double a = 0.5;
  double b = 0.01;
  std::cout << (int)(a / b) << std::endl;
  return 0;
}

          might  print 50 on some systems and optimization levels, and 49
          on others.

          This  is  the result of rounding: The computer cannot represent
          all real numbers exactly, so it has to use approximations. When
          computing  with  approximation,  the computer needs to round to
          the nearest representable number.

          This  is  not a bug in the compiler, but an inherent limitation
          of  the  floating  point types. Please study [31]this paper for
          more information.
     _________________________________________________________________

C

   Increment/decrement operator (++/--) not working as expected - a
          [32]problem with many variations.
          The following expressions have unpredictable results:

x[i]=++i
foo(i,++i)
i*(++i)                 /* special case with foo=="operator*" */
std::cout << i << ++i   /* foo(foo(std::cout,i),++i)          */

          since  the i without increment can be evaluated before or after
          ++i.

          The  C  and C++ standards have the notion of "sequence points".
          Everything  that happens between two sequence points happens in
          an  unspecified order, but it has to happen after the first and
          before  the second sequence point. The end of a statement and a
          function   call  are  examples  for  sequence  points,  whereas
          assignments and the comma between function arguments are not.

          Modifying a value twice between two sequence points as shown in
          the following examples is even worse:

i=++i
foo(++i,++i)
(++i)*(++i)               /* special case with foo=="operator*" */
std::cout << ++i << ++i   /* foo(foo(std::cout,++i),++i)        */

          This  leads  to  undefined  behavior  (i.e. the compiler can do
          anything).

   Casting does not work as expected when optimization is turned on.
          This  is  often  caused by a violation of aliasing rules, which
          are  part of the ISO C standard. These rules say that a program
          is invalid if you try to access a variable through a pointer of
          an  incompatible  type.  This  is  happening  in  the following
          example  where a short is accessed through a pointer to integer
          (the code assumes 16-bit shorts and 32-bit ints):

#include <stdio.h>

int main()
{
  short a[2];

  a[0]=0x1111;
  a[1]=0x1111;

  *(int *)a = 0x22222222; /* violation of aliasing rules */

  printf("%x %x\n", a[0], a[1]);
  return 0;
}

          The  aliasing  rules  were  designed  to  allow  compilers more
          aggressive  optimization. Basically, a compiler can assume that
          all  changes to variables happen through pointers or references
          to  variables  of  a  type compatible to the accessed variable.
          Dereferencing  a  pointer  that  violates  the  aliasing  rules
          results in undefined behavior.

          In  the  case  above,  the  compiler  may assume that no access
          through  an  integer pointer can change the array a, consisting
          of  shorts. Thus, printf may be called with the original values
          of a[0] and a[1]. What really happens is up to the compiler and
          may change with architecture and optimization level.

          Recent  versions  of  GCC  turn on the option -fstrict-aliasing
          (which  allows  alias-based optimizations) by default with -O2.
          And some architectures then really print "1111 1111" as result.
          Without   optimization   the   executable   will  generate  the
          "expected" output "2222 2222".

          To  disable  optimizations  based  on alias-analysis for faulty
          legacy  code,  the option -fno-strict-aliasing can be used as a
          work-around.

          The option -Wstrict-aliasing (which is included in -Wall) warns
          about some - but not all - cases of violation of aliasing rules
          when -fstrict-aliasing is active.

          To  fix  the  code above, you can use a union instead of a cast
          (note  that  this  is a GCC extension which might not work with
          other compilers):

#include <stdio.h>

int main()
{
  union
  {
    short a[2];
    int i;
  } u;

  u.a[0]=0x1111;
  u.a[1]=0x1111;

  u.i = 0x22222222;

  printf("%x %x\n", u.a[0], u.a[1]);
  return 0;
}

          Now the result will always be "2222 2222".

          For  some  more insight into the subject, please have a look at
          [33]this article.

   Cannot use preprocessor directive in macro arguments.
          Let  me  guess...  you  used an older version of GCC to compile
          code that looks something like this:

  memcpy(dest, src,
#ifdef PLATFORM1
         12
#else
         24
#endif
        );

          and you got a whole pile of error messages:

test.c:11: warning: preprocessing directive not recognized within macro arg
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c: In function `foo':
test.c:6: undefined or invalid # directive
test.c:8: undefined or invalid # directive
test.c:9: parse error before `24'
test.c:10: undefined or invalid # directive

          This  is  because your C library's <string.h> happens to define
          memcpy  as  a  macro - which is perfectly legitimate. In recent
          versions of glibc, for example, printf is among those functions
          which are implemented as macros.

          Versions  of  GCC  prior to 3.3 did not allow you to put #ifdef
          (or any other preprocessor directive) inside the arguments of a
          macro. The code therefore would not compile.

          As of GCC 3.3 this kind of construct is always accepted and the
          preprocessor  will  probably  do  what  you expect, but see the
          manual for detailed semantics.

          However,  this  kind  of code is not portable. It is "undefined
          behavior"  according  to  the  C standard; that means different
          compilers  may  do  different  things  with  it.  It  is always
          possible  to rewrite code which uses conditionals inside macros
          so that it doesn't. You could write the above example

#ifdef PLATFORM1
   memcpy(dest, src, 12);
#else
   memcpy(dest, src, 24);
#endif

          This  is  a bit more typing, but I personally think it's better
          style in addition to being more portable.

   Cannot initialize a static variable with stdin.
          This  has  nothing to do with GCC, but people ask us about it a
          lot. Code like this:

#include <stdio.h>

FILE *yyin = stdin;

          will  not  compile  with  GNU  libc,  because  stdin  is  not a
          constant.  This  was  done  deliberately,  to make it easier to
          maintain  binary  compatibility  when the type FILE needs to be
          changed. It is surprising for people used to traditional Unix C
          libraries, but it is permitted by the C standard.

          This  construct  commonly  occurs  in  code  generated  by  old
          versions  of  lex  or yacc. We suggest you try regenerating the
          parser  with  a current version of flex or bison, respectively.
          In   your  own  code,  the  appropriate  fix  is  to  move  the
          initialization to the beginning of main.

          There  is  a  common  misconception that the GCC developers are
          responsible  for  GNU  libc.  These  are  in  fact two entirely
          separate  projects; please check the [34]GNU libc web pages for
          details.
     _________________________________________________________________

C++

   Nested classes can access private members and types of the containing
          class.
          Defect  report  45 clarifies that nested classes are members of
          the  class  they  are  nested  in, and so are granted access to
          private members of that class.

   G++ emits two copies of constructors and destructors.
          In   general   there  are  three  types  of  constructors  (and
          destructors).

         1. The complete object constructor/destructor.
         2. The base object constructor/destructor.
         3. The allocating constructor/deallocating destructor.

          The  first  two  are  different,  when virtual base classes are
          involved.

   Global destructors are not run in the correct order.
          Global  destructors should be run in the reverse order of their
          constructors  completing. In most cases this is the same as the
          reverse  order  of  constructors  starting, but sometimes it is
          different,  and that is important. You need to compile and link
          your  programs  with  --use-cxa-atexit. We have not turned this
          switch  on  by  default,  as  it  requires  a cxa aware runtime
          library (libc, glibc, or equivalent).

   Classes in exception specifiers must be complete types.
          [15.4]/1  tells you that you cannot have an incomplete type, or
          pointer  to  incomplete  (other than cv void *) in an exception
          specification.

   Exceptions don't work in multithreaded applications.
          You  need  to  rebuild g++ and libstdc++ with --enable-threads.
          Remember,  C++ exceptions are not like hardware interrupts. You
          cannot  throw  an  exception  in  one  thread  and  catch it in
          another.  You  cannot  throw an exception from a signal handler
          and catch it in the main thread.

   Templates, scoping, and digraphs.
          If  you  have a class in the global namespace, say named X, and
          want to give it as a template argument to some other class, say
          std::vector, then std::vector<::X> fails with a parser error.

          The  reason  is that the standard mandates that the sequence <:
          is  treated  as if it were the token [. (There are several such
          combinations   of  characters  -  they  are  called  digraphs.)
          Depending  on  the  version,  the compiler then reports a parse
          error  before the character : (the colon before X) or a missing
          closing bracket ].

          The  simplest  way to avoid this is to write std::vector< ::X>,
          i.e.  place  a  space between the opening angle bracket and the
          scope operator.

   Copy constructor access check while initializing a reference.
          Consider this code:

class A
{
public:
  A();

private:
  A(const A&);   // private copy ctor
};

A makeA(void);
void foo(const A&);

void bar(void)
{
  foo(A());       // error, copy ctor is not accessible
  foo(makeA());   // error, copy ctor is not accessible

  A a1;
  foo(a1);        // OK, a1 is a lvalue
}

          Starting with GCC 3.4.0, binding an rvalue to a const reference
          requires   an   accessible  copy  constructor.  This  might  be
          surprising  at  first  sight,  especially  since  most  popular
          compilers do not correctly implement this rule.

          The C++ Standard says that a temporary object should be created
          in  this  context  and  its  contents filled with a copy of the
          object  we  are  trying  to bind to the reference; it also says
          that  the  temporary  copy  can  be  elided,  but  the semantic
          constraints  (eg.  accessibility) of the copy constructor still
          have to be checked.

          For   further   information,  you  can  consult  the  following
          paragraphs  of  the  C++  standard: [dcl.init.ref]/5, bullet 2,
          sub-bullet 1, and [class.temporary]/2.

  Common problems when upgrading the compiler

    ABI changes

   The C++ application binary interface (ABI) consists of two components:
   the  first  defines  how  the  elements  of  classes are laid out, how
   functions  are called, how function names are mangled, etc; the second
   part deals with the internals of the objects in libstdc++. Although we
   strive  for  a  non-changing ABI, so far we have had to modify it with
   each  major  release. If you change your compiler to a different major
   release you must recompile all libraries that contain C++ code. If you
   fail  to  do  so  you  risk  getting  linker  errors or malfunctioning
   programs. Some of our Java support libraries also contain C++ code, so
   you might want to recompile all libraries to be safe. It should not be
   necessary to recompile if you have changed to a bug-fix release of the
   same  version  of  the compiler; bug-fix releases are careful to avoid
   ABI changes. See also the [35]compatibility section of the GCC manual.

   Remark:  A  major  release  is  designated by a change to the first or
   second  component  of  the  two- or three-part version number. A minor
   (bug-fix)  release  is  designated  by a change to the third component
   only.  Thus  GCC 3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2
   are  bug-fix  releases  for  GCC  3.3.  With  the  3.4  series  we are
   introducing  a  new naming scheme; the first release of this series is
   3.4.0 instead of just 3.4.

    Standard conformance

   With  each  release,  we try to make G++ conform closer to the ISO C++
   standard  (available  at  [36]http://www.ncits.org/cplusplus.htm).  We
   have  also  implemented  some  of  the core and library defect reports
   (available at
   [37]http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html     &
   [38]http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html
   respectively).

   Non-conforming  legacy code that worked with older versions of GCC may
   be  rejected by more recent compilers. There is no command-line switch
   to   ensure   compatibility   in  general,  because  trying  to  parse
   standard-conforming  and  old-style code at the same time would render
   the   C++   frontend   unmaintainable.  However,  some  non-conforming
   constructs  are  allowed  when the command-line option -fpermissive is
   used.

   Two  milestones in standard conformance are GCC 3.0 (including a major
   overhaul  of the standard library) and the 3.4.0 version (with its new
   C++ parser).

    New in GCC 3.0

     * The  standard  library is much more conformant, and uses the std::
       namespace (which is now a real namespace, not an alias for ::).
     * The standard header files for the c library don't end with .h, but
       begin with c (i.e. <cstdlib> rather than <stdlib.h>). The .h names
       are still available, but are deprecated.
     * <strstream> is deprecated, use <sstream> instead.
     * streambuf::seekoff  &  streambuf::seekpos are private, instead use
       streambuf::pubseekoff & streambuf::pubseekpos respectively.
     * If std::operator << (std::ostream &, long long) doesn't exist, you
       need to recompile libstdc++ with --enable-long-long.

   If  you  get  lots  of  errors about things like cout not being found,
   you've most likely forgotten to tell the compiler to look in the std::
   namespace. There are several ways to do this:
     * Say std::cout at the call. This is the most explicit way of saying
       what you mean.
     * Say  using  std::cout; somewhere before the call. You will need to
       do  this  for  each  function  or  type  you  wish to use from the
       standard library.
     * Say  using  namespace  std; somewhere before the call. This is the
       quick-but-dirty  fix. This brings the whole of the std:: namespace
       into  scope. Never do this in a header file, as every user of your
       header file will be affected by this decision.

    New in GCC 3.4.0

   The  new  parser  brings  a lot of improvements, especially concerning
   name-lookup.
     * The  "implicit  typename"  extension  got  removed (it was already
       deprecated  since  GCC  3.1),  so  that  the following code is now
       rejected, see [14.6]:

template <typename> struct A
{
    typedef int X;
};

template <typename T> struct B
{
    A<T>::X          x;  // error
    typename A<T>::X y;  // OK
};

B<void> b;

     * For  similar reasons, the following code now requires the template
       keyword, see [14.2]:

template <typename> struct A
{
    template <int> struct X {};
};

template <typename T> struct B
{
    typename A<T>::X<0>          x;  // error
    typename A<T>::template X<0> y;  // OK
};

B<void> b;

     * We  now  have two-stage name-lookup, so that the following code is
       rejected, see [14.6]/9:

template <typename T> int foo()
{
    return i;  // error
}

     * This also affects members of base classes, see [14.6.2]:

template <typename> struct A
{
    int i, j;
};

template <typename T> struct B : A<T>
{
    int foo1() { return i; }       // error
    int foo2() { return this->i; } // OK
    int foo3() { return B<T>::i; } // OK
    int foo4() { return A<T>::i; } // OK

    using A<T>::j;
    int foo5() { return j; }       // OK
};

   In  addition  to  the  problems  listed  above,  the manual contains a
   section on [39]Common Misunderstandings with GNU C++.

References

   1. http://gcc.gnu.org/bugs.html
   2. http://gcc.gnu.org/bugs.html#report
   3. http://gcc.gnu.org/bugs.html#need
   4. http://gcc.gnu.org/bugs.html#dontwant
   5. http://gcc.gnu.org/bugs.html#where
   6. http://gcc.gnu.org/bugs.html#detailed
   7. http://gcc.gnu.org/bugs.html#gnat
   8. http://gcc.gnu.org/bugs.html#pch
   9. http://gcc.gnu.org/bugs.html#known
  10. http://gcc.gnu.org/bugs.html#cxx
  11. http://gcc.gnu.org/bugs.html#missing
  12. http://gcc.gnu.org/bugs.html#fixed34
  13. http://gcc.gnu.org/bugs.html#fortran
  14. http://gcc.gnu.org/bugs.html#nonbugs
  15. http://gcc.gnu.org/bugs.html#nonbugs_general
  16. http://gcc.gnu.org/bugs.html#nonbugs_c
  17. http://gcc.gnu.org/bugs.html#nonbugs_cxx
  18. http://gcc.gnu.org/bugs.html#upgrading
  19. http://gcc.gnu.org/bugs.html#known
  20. http://gcc.gnu.org/bugzilla/
  21. mailto:gcc-bugs@gcc.gnu.org
  22. http://gcc.gnu.org/bugs.html#gnat
  23. http://gcc.gnu.org/bugs.html#pch
  24. http://gcc.gnu.org/bugs.html#detailed
  25. http://gcc.gnu.org/bugs.html#where
  26. http://gcc.gnu.org/bugs.html#detailed
  27. http://gcc.gnu.org/bugs.html#detailed
  28. http://gcc.gnu.org/bugs.html#new34
  29. http://gcc.gnu.org/onlinedocs/gcc-3.4.6/g77/Trouble.html
  30. http://gcc.gnu.org/PR323
  31. http://www.validlab.com/goldberg/paper.ps
  32. http://gcc.gnu.org/PR11751
  33. http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html
  34. http://www.gnu.org/software/libc/
  35. http://gcc.gnu.org/onlinedocs/gcc/Compatibility.html
  36. http://www.ncits.org/cplusplus.htm
  37. http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html
  38. http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html
  39. http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Misunderstandings.html