Re: [C++/78252] libiberty demangler crash with lambda (auto)

[Markus Trippelsdorf <markus at trippelsdorf dot de>]

On 2016.11.30 at 14:06 -0500, Nathan Sidwell wrote:
> This patch fixes a problem in libiberty's symbol demangler.  With a
> templated forwarding function such as std::forward, we can end up emitting
> mangled function names that encode lambda information.  Lambdas with auto
> argument types have a synthesized templated operator(), and g++ uses that
> when mangling the lambda.
> 
> Unfortunately g++ doesn't notice the template parameters there mean 'auto'
> and emits regular template parameter references. (This is a bug, see below.)
> 
> But, as the forwarding function itself is a template, and the lambda is part
> of a template parameter substitution, we can end up with the demangler
> recursing unboundedly.  In other cases we can fail to demangle (returning
> null), or demangle to an unexpected type (substituting the current template
> parameter type into the place of the 'auto').
> 
> This patch fixes the demangler by noting when it's printing the argument
> types of a lambda.  In that case whenever we encounter a template parameter
> reference we emit 'auto', and also inhibit some &/&& smushing that needs
> checking.  AFAICT, once inside a lambda argument list we cannot encounter
> template parameter references that actually refer to an enclosing template
> argument list. That means we don't have the problem of disabling this
> additional check within the argument list printing.  I don't think we can
> meet a nested lambda type either, but the ++...-- idiom seemed safer to me.
> 
> We cannot do this substitution when parsing the mangled name, because g++
> applies the usual squangling back references as-if there really was a
> template parameter reference.  Later squangling references to the type
> containing the lambda argument may or may not require the reference to be to
> an enclosing template argument, or be auto, depending on the context of the
> squangle reference.
> 
> I've also included a c++ testcase to check the mangling of the lambdas that
> cause this.  While this is a g++ bug, it's an ABI-affecting one, and we
> shouldn't change the behaviour unintentionally.  I've not investigated why
> the mangler's failing to check is_auto, and will look at that later.  I
> imagine a fix will be -fabi-version dependent. I have filed 78621 to track
> it.

Thanks. This patch also fixes:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70909

-- 
Markus