From jns@gellyfish.co.uk

Hi,
I was trying to make an array of hashes which had one member an
anonymous class for some tests, so I did​:

  my @​f = %{ class => class { has Int $.attr; }, }; say @​f.perl

Which gave rise to​:

  When invoking 2 '', provided outer frame 0x2b11cb0 (4 '<unit>') does
  not match expected static frame 0x2b11db8 (3 '')
    in block <unit> at -e line 1

Which was definitely unexpected and not something I had seen before.

Naming the class gives a different errror but suggests everything
inside the outer curlies is being interpreted as a block for some
reason​:

  my @​f = %{ class => class Foo { has Int $.attr; }, }; say @​f.perl

Giving​:

  Odd number of elements found where hash initializer expected​:
  Only saw​: -> ;; $_? is raw { #`(Block|50212200) ... }
    in block <unit> at -e line 1

Inlining the class does work like that does work without the additional
curlies​:

  my %f =  class => class  { has Int $.attr; }; say %f.perl

Giving​:

  {​:class(<anon|60919760>)}

Of course what I actually meant originally was​:

  my @​f = (%( "class" => class  { has Int $.attr; }, ),); say @​f.perl

So I guess at the very least it's a LTA error message but does seem to
point to some other bug.

From @zoffixznet

Golfed a bit​:

<Zoffix> m​: { foo => class {} }()
<camelia> rakudo-moar 266f34​: OUTPUT«When invoking 1 '', provided outer frame 0x322f4e0 (3 '<unit>') does not match expected static frame 0x322f5e8 (2 '')␤ in block <unit> at <tmp> line 1␤␤»

The RT System itself - Status changed from 'new' to 'open'

From @timo

This is caused by the way we differentiate between anonymous blocks and
hashes; we just parse it as a block first, and if at the end it turns
out it should be a hash instead, we change it around. I suppose at that
point the class will already have been defined and its stuff will point
at the "not actually a block any more" thing that we had as a "stand-in"
when we weren't sure yet whether it should be a hash or a block ...

As a status update: The first example from the OP no longer gives a weird error:

$ ./rakudo-m -e 'my @f = %{ class => class { has Int $.attr }, }; say @f.raku'
[:class(<anon|1>)]

So an Array with a Pair as the first (and only) element is created. That looks reasonable to me.