Error missing declared signature if where clause is used in proto

[zoffixznet]

See Newcomer Guide to Contributing to Core Perl 6 for tips on fixing this Issue

---

Normally, this message tells you what the declared signature is (in this case, (Int)):

$ perl6 -e 'sub z(Int) {}; z "x"'
===SORRY!=== Error while compiling -e
Calling z(Str) will never work with declared signature (Int)

But if you use a where clause and stick it into proto, the with declared signature part doesn't say what the declared signature is:

19:39 	Zoffix 	m: proto z($ where rand) {*}; say z "x"
19:39 	camelia 	rakudo-moar 9e6ae276a: OUTPUT: «5===SORRY!5=== Error while compiling <tmp>␤Calling z(Str) will never work with declared signature ␤at <tmp>:1␤------> 3proto z($ where rand) {*}; say 7⏏5z "x"␤»

[jsimonet]

The absence of signature comes from the fact that only multi signatures are shown (https://github.com/rakudo/rakudo/blob/master/src/Perl6/Optimizer.nqp#L2178).

But the error seems to be related with the fact we are trying to call a dispatcher proto which does not have multis:

$ perl6 -e 'proto z() {*}; say z'
===SORRY!=== Error while compiling -e
Calling z() will never work with declared signature 
at -e:1

$ perl6 -e 'proto z() {}; say z'
Nil

perl6 -e 'proto z() {*}; multi z() {}; say z'    
Nil

[jsimonet]

In addition, using a where clause causes the signatures to be evaluated at runtime. If a multi is specified, the error is a TypeCheck Exception.

perl6 --stagestats -e 'proto z( $ where rand ) {*}; multi z($ where rand){}; say z "x"'
Stage start      :   0.000
Stage parse      :   0.147
Stage syntaxcheck:   0.000
Stage ast        :   0.000
Stage optimize   :   0.004
Stage mast       :   0.009
Stage mbc        :   0.000
Stage moar       :   0.000
Constraint type check failed in binding to parameter '<anon>'; expected anonymous constraint to be met but got Str ("x")
  in block <unit> at -e line 1

[jsimonet]

Tried to implement a fix in PR #1868

[dolmen]

This looks fixed in rakudo star 2018.06:

$ perl6 -e 'proto z($ where rand) {*}; say z "x"'
===SORRY!=== Error while compiling -e
Calling z(Str) will never work with declared signature
at -e:1
------> proto z($ where rand) {*}; say ⏏z "x"

[zoffixznet]

Tried to implement a fix in PR #1868

Commented on the PR.

This looks fixed in rakudo star 2018.06:

That's still missing what the declared signature is.

[tinmarino]

I made PR #3558 creating a new Exception class. Then realized that #1868 handled it better. But got Blocked.
How to unblock it ?