Let Array and Hash be smarter about parameterizing subclasses

First of all, if re-parameterizing a previously parameterized type
fall back to the firs non-mixin parent in MRO. This would make
`class A is Array[Int] {}; say A[Str:D];` produce an `A[Str:D]`, not
`Array[Int][Str:D]` as it used to be. `Array[Int].^parameterize(Str:D)`
would also become `Array[Str:D]`.

Note that methods defined in `Array::Types`, `Hash::Object`, and
`Hash::Typed` roles would still override subclass' methods. I.e. it is
not possible for the class `A` above to override method `new` or
`BIND-POS` because `Array::Typed` has them. A dev must take special care
if they want their own versions of these.

src/core.c/Array.rakumod

@@ -1461,11 +1461,12 @@ my class Array { # declared in BOOTSTRAP
             !! self.clone
     }
 
-    method ^parameterize(Mu:U \arr, Mu \of) {
+    method ^parameterize(Mu:U \type, Mu \of) {
         if nqp::isconcrete(of) {
-            die "Can not parameterize {arr.^name} with {of.raku}"
+            die "Can not parameterize {type.^name} with {of.raku}"
         }
         else {
+            my \arr = type.^mro.first(!*.^is_mixin);
             my $what := arr.^mixin(Array::Typed[of]);
             # needs to be done in COMPOSE phaser when that works
             $what.^set_name("{arr.^name}[{of.^name}]");

src/core.c/Array/Typed.rakumod

@@ -100,12 +100,12 @@ my role Array::Typed[::TValue] does Positional[TValue] {
     method is-generic { nqp::hllbool(callsame() || nqp::istrue(TValue.^archetypes.generic)) }
 
     multi method INSTANTIATE-GENERIC(::?CLASS:U: TypeEnv:D \type-environment) is raw {
-        Array.^parameterize: type-environment.instantiate(TValue)
+        self.WHAT.^parameterize: type-environment.instantiate(TValue)
     }
     multi method INSTANTIATE-GENERIC(::?CLASS:D: TypeEnv:D \type-environment) is raw {
         my Mu $descr := type-environment.instantiate(nqp::getattr(self, Array, '$!descriptor'));
         nqp::p6bindattrinvres(
-            Array.^parameterize(type-environment.instantiate(TValue)).new( |(self.elems ?? self !! Empty) ),
+            self.WHAT.^parameterize(type-environment.instantiate(TValue)).new( |(self.elems ?? self !! Empty) ),
             Array, '$!descriptor', $descr )
     }
 

src/core.c/Hash.rakumod

@@ -444,16 +444,18 @@ my class Hash { # declared in BOOTSTRAP
             !! self.clone
     }
 
-    method ^parameterize(Mu:U \hash, Mu \of, Mu \keyof = Str(Any)) {
+    method ^parameterize(Mu:U \type, Mu \of, Mu \keyof = Str(Any)) {
+
+        my \hash = type.^mro.first(!*.^is_mixin);
 
         # fast path
         if nqp::eqaddr(of,Mu) && nqp::eqaddr(keyof,Str(Any)) {
-            hash
+            type
         }
 
         # error checking
         elsif nqp::isconcrete(of) {
-            "Can not parameterize {hash.^name} with {of.raku}"
+            "Can not parameterize {type.^name} with {of.raku}"
         }
 
         # only constraint on type
@@ -467,7 +469,7 @@ my class Hash { # declared in BOOTSTRAP
 
         # error checking
         elsif nqp::isconcrete(keyof) {
-            "Can not parameterize {hash.^name} with {keyof.raku}"
+            "Can not parameterize {type.^name} with {keyof.raku}"
         }
 
         # no support for native types yet

src/core.c/Hash/Object.rakumod

@@ -267,13 +267,15 @@ my role Hash::Object[::TValue, ::TKey] does Associative[TValue] {
     }
 
     multi method INSTANTIATE-GENERIC(::?CLASS:U: TypeEnv:D \type-environment --> Associative) is raw {
-        Hash.^parameterize: type-environment.instantiate(TValue), type-environment.instantiate(TKey)
+        self.WHAT.^parameterize: type-environment.instantiate(TValue), type-environment.instantiate(TKey)
     }
 
     multi method INSTANTIATE-GENERIC(::?CLASS:D: TypeEnv:D \type-environment --> Associative) is raw {
         my Mu $descr := type-environment.instantiate( nqp::getattr(self, Hash, '$!descriptor') );
         nqp::p6bindattrinvres(
-            Hash.^parameterize( type-environment.instantiate(TValue), type-environment.instantiate(TKey) ).new(self),
+            self.WHAT.^parameterize(
+                type-environment.instantiate(TValue),
+                type-environment.instantiate(TKey) ).new(self),
             Hash, '$!descriptor', $descr )
     }
 

src/core.c/Hash/Typed.rakumod

@@ -37,12 +37,12 @@ my role Hash::Typed[::TValue] does Associative[TValue] {
     }
 
     multi method INSTANTIATE-GENERIC(::?CLASS:U: TypeEnv:D \type-environment --> Associative) is raw {
-        Hash.^parameterize: type-environment.instantiate(TValue)
+        self.WHAT.^parameterize: type-environment.instantiate(TValue)
     }
     multi method INSTANTIATE-GENERIC(::?CLASS:D: TypeEnv:D \type-environment --> Associative) is raw {
         my Mu $descr := type-environment.instantiate( nqp::getattr(self, Hash, '$!descriptor') );
         nqp::p6bindattrinvres(
-            Hash.^parameterize( type-environment.instantiate(TValue) ).new(self), Hash, '$!descriptor', $descr )
+            self.WHAT.^parameterize( type-environment.instantiate(TValue) ).new(self), Hash, '$!descriptor', $descr )
     }
 
     multi method raku(::?CLASS:D \SELF:) {