------- Comment #12 from fang at csl dot cornell dot edu 2006-10-23 03:34 ------- Subject: Re: [4.1 Regression] Internal compiler error on boost mpl test/apply.cpp
> There were PRs that showed that this extension (for those who don't > remember: we allowed to match templates with trailing default arguments to > match template template parameters with fewer template arguments) leads to > wrong code in some circumstances. It's also one of the more obscure and > less well thought out extensions, in my opinion. > > So my view of things is that the patch should be backported. But then I'm > not in charge of this, of course :-) My $.02, as a user: * This said extension was never given a period of deprecation before it was axed. * This extension is used in much existing code in practice, especially when using the STL container templates as template template arguments. (Since the Alloc param is always defaulted to std::allocator...) While I'm favor of removing extensions for the sake of standard conformance (and fixing wrong code), I think that a sudden change on the 4.1 release branch might cause some agony to those who aren't prepared to migrate to the 4.2-like behavior, in disabling the extension. The warning might even state the consequence of this extension: unexpected behavior or wrong code. Caveat emptor. I suggest adding such a diagnostic on existing release branches, to give fair warning when people are (often unknowingly) using the extension. Fortunately, it is easy to write an autoconf-macro to detect this feature, using the test case in http://gcc.gnu.org/gcc-4.2/changes.html. (and I have done so, if anyone would like it) End $.02. Fang -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28088
