Cover meta assign opts and bugs

Rakudo fix: rakudo/rakudo@b9b0838dd8
Closes rakudo/rakudo#1987 R#1987
Closes rakudo/rakudo#1989 R#1989
Covers rakudo/rakudo@b9b0838dd8

S03-metaops/misc.t

@@ -0,0 +1,67 @@
+use v6;
+use Test;
+
+plan 1;
+
+=begin pod
+
+=head1 DESCRIPTION
+
+Miscellaneous metaop tests that don't fit into other files in this directory.
+
+=end pod
+
+# Covers opts in https://github.com/rakudo/rakudo/commit/b9b0838dd8
+subtest 'cover metaop call simplification optimization' => {
+    plan 7;
+    subtest '(//=) +=' => {
+        plan 2;
+        my $a;
+        ($a //= 42) += 10;
+        is-deeply $a, 52, '(1)';
+        ($a //= 42) += 10;
+        is-deeply $a, 62, '(2)';
+    }
+    subtest 'R+= (||=)' => {
+        plan 2;
+        my $a;
+        10 R+= ($a ||= 42);
+        is-deeply $a, 52, '(1)';
+        10 R+= ($a ||= 42);
+        is-deeply $a, 62, '(2)';
+    }
+    subtest '(((R+= ([R-]=)) //=) +=)' => {
+        plan 1;
+        my $a = 12;
+        ((10 R+= ($a [R-]= 42)) //= 100) += 1000;
+        is-deeply $a, 1040, '(1)';
+    }
+    subtest '(((R+= ([R-]=)) //=) +=)' => {
+        plan 1;
+        my $a = 12;
+        ((10 R+= ($a [R-]= 42)) //= 100) += 1000;
+        is-deeply $a, 1040, '(1)';
+    }
+    subtest 'array var with ((||=) += )' => {
+        my @a = 20; (@a ||= 42) += 10; dd @a
+    }
+
+    # https://github.com/rakudo/rakudo/issues/1989
+    subtest 'metassign to array/hash returned from a method' => {
+        plan 2;
+        my class Foo { has @.a is rw; has %.h is rw };
+        my $o := Foo.new: :h{:42foo};
+        is-deeply ($o.a += <a b c>), [3], '+= to array';
+        is-deeply ($o.h ,= :100bar), {:42foo, :100bar}, ',= to hash';
+    }
+    subtest 'failure modes' => {
+        plan 5;
+        throws-like ｢my $a := 42; ($a //= 42)  += 10｣, X::Assignment::RO, '(1)';
+        throws-like ｢my $a := 42; ($a ||= 42) R+= 10｣, X::Assignment::RO, '(2)';
+        throws-like ｢my $a  = 42; ($a ||= 42) R+= 10｣, X::Assignment::RO, '(3)';
+        throws-like ｢my $a  = 42; 1000 += ((10 R+= ($a [R-]= 42)) //= 100)｣, X::Assignment::RO,
+            '(4)';
+        # https://github.com/rakudo/rakudo/issues/1987
+        throws-like '42.abs += 42', X::Assignment::RO, '(5)';
+    }
+}