[6.d] Set non-DefiniteHOW default default for DefiniteHOWs

Per 6.d-prep[^1]: On :U/:D type constrained variables, params, attributes,
and parametarized Arrays/Hashes, use the base type as the default
default in 6.d instead of continuing to use the uninitializeable
DefiniteHOW as the type, which prevents use of `.=` and other constructs.

Fixes R#1493. Does not address enums, which is currently blocked by R#2297

[1] https://github.com/perl6/6.d-prep/blob/dffa2642419a0f481591b9b64b602bfc4c4eb66b/TODO/FEATURES.md#make-default-defaults-for-definitehows-be-normal-types
[2] #1493
[3] #2297

src/Perl6/Actions.nqp

@@ -3572,7 +3572,7 @@ class Perl6::Actions is HLL::Actions does STDActions {
                     }
                     elsif $<initializer><sym> eq '.=' {
                         my $type := nqp::defined($*OFTYPE)
-                          ?? $*OFTYPE.ast !! $*W.find_symbol: ['Any'];
+                          ?? $*W.maybe-definite-how-base($*OFTYPE.ast) !! $*W.find_symbol: ['Any'];
                         my $dot_equals := $initast;
                         $dot_equals.unshift(QAST::WVal.new(:value($type)));
                         $dot_equals.returns($type);
@@ -3723,7 +3723,7 @@ class Perl6::Actions is HLL::Actions does STDActions {
 
             $init-qast.unshift:
               QAST::WVal.new: value => nqp::defined($*OFTYPE)
-                ?? $*OFTYPE.ast !! $*W.find_symbol: ['Mu']
+                ?? $*W.maybe-definite-how-base($*OFTYPE.ast) !! $*W.find_symbol: ['Mu']
             if $<term_init><sym> eq '.=';
 
             my $qast;
@@ -5324,9 +5324,13 @@ class Perl6::Actions is HLL::Actions does STDActions {
         my $Mu := $W.find_symbol: ['Mu'];
         my $type := nqp::defined($*OFTYPE) ?? $*OFTYPE.ast !! $Mu;
         if $<initializer><sym> eq '.=' {
-            $value_ast.unshift(QAST::WVal.new(:value($type)));
+            my $init-type := $*W.maybe-definite-how-base: $type;
+            $value_ast.unshift: QAST::WVal.new: :value($init-type);
+            $value_ast.returns: $init-type;
+        }
+        else {
+            $value_ast.returns($type);
         }
-        $value_ast.returns($type);
 
         my $con_block := $W.pop_lexpad();
         my $value;

src/Perl6/World.nqp

@@ -1661,7 +1661,7 @@ class Perl6::World is HLL::World {
                 %info<bind_constraint> := self.parameterize_type_with_args($/,
                     %info<bind_constraint>, [$vtype], nqp::hash());
                 %info<value_type>      := $vtype;
-                %info<default_value>   := $vtype;
+                %info<default_value>   := self.maybe-definite-how-base: $vtype;
             }
             else {
                 %info<container_type> := %info<container_base>;
@@ -1728,7 +1728,8 @@ class Perl6::World is HLL::World {
                 %info<bind_constraint> := self.parameterize_type_with_args($/,
                     %info<bind_constraint>, @value_type, nqp::hash());
                 %info<value_type>      := @value_type[0];
-                %info<default_value>   := @value_type[0];
+                %info<default_value>
+                    := self.maybe-definite-how-base: @value_type[0];
             }
             else {
                 %info<container_type> := %info<container_base>;
@@ -1769,7 +1770,8 @@ class Perl6::World is HLL::World {
             if @value_type {
                 %info<bind_constraint> := @value_type[0];
                 %info<value_type>      := @value_type[0];
-                %info<default_value>   := @value_type[0];
+                %info<default_value>
+                    := self.maybe-definite-how-base: @value_type[0];
             }
             else {
                 %info<bind_constraint> := self.find_symbol(['Mu'], :setting-only);
@@ -1780,6 +1782,13 @@ class Perl6::World is HLL::World {
         }
         %info
     }
+    method maybe-definite-how-base ($v) {
+        # returns the value itself, unless it's a DefiniteHOW, in which case,
+        # it returns its base type. Behaviour available in 6.d and later only.
+        ! $*W.lang-ver-before('d') && nqp::eqaddr($v.HOW,
+            $*W.find_symbol: ['Metamodel','DefiniteHOW'], :setting-only
+        ) ?? $v.HOW.base_type: $v !! $v
+    }
 
     # Installs one of the magical lexicals ($_, $/ and $!). Uses a cache to
     # avoid massive duplication of containers and container descriptors.

src/core/Exception.pm6

@@ -2289,11 +2289,16 @@ my class X::TypeCheck::Assignment is X::TypeCheck {
     method message {
         my $to = $.symbol.defined && $.symbol ne '$'
             ?? " to $.symbol" !! "";
-        my $expected = $.expected =:= $.got
-            ?? "expected type $.expectedn cannot be itself " ~
-               "(perhaps Nil was assigned to a :D which had no default?)"
+        my $is-itself := $.expected =:= $.got;
+        my $expected = $is-itself
+            ?? "expected type $.expectedn cannot be itself"
             !! "expected $.expectedn but got $.gotn";
-        self.priors() ~ "Type check failed in assignment$to; $expected";
+        my $maybe-Nil := $is-itself
+          || nqp::istype($.expected.HOW, Metamodel::DefiniteHOW)
+          && $.expected.^base_type =:= $.got
+          ?? ' (perhaps Nil was assigned to a :D which had no default?)' !! '';
+
+        self.priors() ~ "Type check failed in assignment$to; $expected$maybe-Nil"
     }
 }
 my class X::TypeCheck::Argument is X::TypeCheck {