V4 Lambda templates and implicit function templates.

[Adam Butcher]

On 10.09.2013 03:19, Adam Butcher wrote:
>  - Instantiation of generic conversion op ICEs if the call op 
> contains
>    declarations and hasn't already been instantiated.
>
This is not a complete enough description.  It only ICEs instantiating 
the call op through the decltype return of the conversion op if the 
return type of the call op is a deduced one (i.e. unspecified or 
specified explicitly as 'auto').  If the lambda call op is instantiated 
explicitly (i.e. the lambda is called) prior to using the conversion op 
then all is well.  It seems to only occur if there are variables 
declared within the lambda body or accessible via the lambda's 'this'.

Specifically, the ICE is in tsubst_decl (cp/pt.c:10839) asserting 
'gcc_unreachable' due to being 'cp_unevaluated_operand'.  The 
instantiation chain starts from attempting to 'mark_used' the call op in 
the decltype expression.

The same ICE can be caused in user code by attempting to take get 
decltype of a generic lambda call having a deduced return type and 
declarations:

This is fine:

    auto f = [] <typename T> (T) {};
    decltype (f (4.f)) *p;

This is not; it ICEs doing 'tsubst_decl' on the declaration 'x'.

    auto f = [] <typename T> (T) { int x; };
    decltype (f (4.f)) *p;

The conversion op is clearly not a factor here but can be removed from 
the equation completely by adding a capture.  The ICE still occurs.  In 
this case it occurs trying to do 'tsubst_decl' on the capture decl 'i'.

    int i = 0;
    auto f = [i] <typename T> (T) {};
    decltype (f (4.f)) *p;

ice.cpp: In instantiation of ‘main()::<lambda(T)> [with T = float]’:
ice.cpp:5:20:   required from here
ice.cpp:4:34: internal compiler error: in tsubst_decl, at cp/pt.c:10839
     auto f = [i] <typename T> (T) {};
                                   ^
Any ideas?  Looks like it's something to do with how the call operator 
is defined.  Is there some flag I'm missing in the generic case?

Cheers,
Adam