Colonpairs with indirect-lookup as variables fail to compile right

[zoffixznet]

Spotted here: https://stackoverflow.com/questions/44212696/why-doesnt-perl-6s-colon-pair-and-name-interpolation-work-together

Expected these two versions to be equivalent:

$ ./perl6 -e 'my $foo = "bar"; my %bar = :42foo, :70bar; dd [$foo => %::($foo)]'
[:bar({:bar(70), :foo(42)})]

$ ./perl6 -e 'my $foo = "bar"; my %bar = :42foo, :70bar; dd [:%::($foo)]'
===SORRY!=== Error while compiling -e
Variable '%::($foo)' is not declared
at -e:1
------> = "bar"; my %bar = :42foo, :70bar; dd [:⏏%::($foo)]

$ ./perl6 -v
This is Rakudo version 2018.01-204-gbeaad7b built on MoarVM version 2018.01-94-gc7774e3
implementing Perl 6.c.

The %::($foo) construct is already parsed as a variable and colonpairs can handle them, but looks like the colonpair token simply assumes it can strigify the Match of the variable to get its name: $<var><desigilname>.Str

[jnthn]

One thing to be aware of: :$foo is a named parameter in the context of an argument list, and we need to know our named parameter names at compile time.