Further investigation of RT#117043.

for_RT117043

Further investigation of RT#117043.

1) It is specced (via STD and from there RT#116607 and from there
S32-exceptions/misc.t) that sigilless my variables must have an
initializer.  I could not find an explanation as to why.

$ perl6 -e 'my (\a);'      # no error
$ perl6 -e 'my \a;'        # intended error
===SORRY!=== Error while compiling -e
Term definition requires an initializer
at -e:1
------> my \a⏏;

2) Whether this principle carries over to sigilless parameters
is hard to discern.

std: sub (\a?) { }
std 28329a7: OUTPUT«===SORRY!===␤Unable to parse signature at /tmp/866RutNr2q line 1:␤------> sub ⏏(\a?) { }␤Couldn't find final ')'; gave up at /tmp/866RutNr2q »
$ perl6 -e 'sub (\a?) { }' # Either LTA error, or broken
===SORRY!=== Error while compiling -e
Missing block
at -e:1
------> sub (\a⏏?) { }

3) Sigilless variables in general are apparently supposed to
just bind, when "assigned."  No spectests seem to try to write
to them.

$ perl6 -e 'my \a = 1; a = 2'
Cannot modify an immutable Int
  in block <unit> at -e:1
$ perl6 -e 'sub f (\a is copy) { a = 6; a.say }; f(5)'  # Seems to be equivalent
Cannot modify an immutable Int
  in sub f at -e:1
  in block <unit> at -e:1
$ perl6 -e 'my $b = 4; my \a = $b; $b = 2; a.perl.say;' # Further evidence
2
$ perl6 -e 'my \a = (); a = 3; a.perl.say;'             # This is odd
()
$ my \a := (); a = 3; a.perl.say;'                      # Same thing this way
()
$ perl6 -e 'my $a := (); $a = 3; $a.perl.say;'          # Expected behavior
Cannot assign to an immutable value
  in block <unit> at -e:1

4) Precedence (?) for behavior when a bind does not happen but a
declaration does:

$ perl6 -e 'my ($a, $b?) := 1,; $b.perl.say'
Mu

5) In any case if '=' means "bind" for sigilless variables,
then any mixture of sigiled and sigilless variables like:

my ($a, \b) = 3,4;

... means that the signature has to be iterated and the operation
(bind or assign) chosen based on whether the variable is sigilless
or not.

Internally this:

my (\x1) = 4;

...currently gets routed to a p6store:

- QAST::Op(p6store) (\\x1) = 4
  - QAST::Op(call &infix:<,>)
    - QAST::Var(lexical x1 :decl())
  - QAST::Want 4
    - QAST::WVal(Int)
    - Ii
    - QAST::IVal(4)

...and so this probably unintended behavior is possible:

$ perl6 -e 'my (\a) = 4; a.say; a = 5; a.say;'
(Any)
5

6) The list-declarator construct also admits this:

my (\c, \d) = @runtime-valued-array;

...which, if mandatory initialization is desired, must fail at
runtime based on the @runtime-valued-array.elems.