Don't constant-fold something producing a Seq.

Otherwise, if the value is used twice, we'll end up exhuasting it the
first time and it will explode every time afterwards. (We maybe want
to look at whether some things are bogusly marked PURE, but this will
fix all such problems and I can't think of a situation where you'd
ever want this misbehavior.)

Author: jnthn

src/Perl6/Optimizer.nqp

@@ -33,6 +33,7 @@ my class Symbols {
     has $!Routine;
     has $!Nil;
     has $!Failure;
+    has $!Seq;
 
     # Top routine, for faking it when optimizing post-inline.
     has $!fake_top_routine;
@@ -59,6 +60,7 @@ my class Symbols {
         $!Routine     := self.find_lexical('Routine');
         $!Nil         := self.find_lexical('Nil');
         $!Failure     := self.find_lexical('Failure');
+        $!Seq         := self.find_lexical('Seq');
         nqp::pop(@!block_stack);
     }
 
@@ -100,6 +102,7 @@ my class Symbols {
     method PseudoStash() { $!PseudoStash }
     method Nil()         { $!Nil }
     method Failure()     { $!Failure }
+    method Seq()         { $!Seq }
 
     # The following function is a nearly 1:1 copy of World.find_symbol.
     # Finds a symbol that has a known value at compile time from the
@@ -1420,7 +1423,7 @@ class Perl6::Optimizer {
                             $survived := 0;
                         }
                     }
-                    if $survived && !nqp::istype($ret_value, $!symbols.Failure) {
+                    if $survived && self.constant_foldable_type($ret_value) {
                         return $NULL if $!void_context && !$!in_declaration;
                         $*W.add_object($ret_value);
                         my $wval := QAST::WVal.new(:value($ret_value));
@@ -1552,6 +1555,13 @@ class Perl6::Optimizer {
         return NQPMu;
     }
 
+    method constant_foldable_type($value) {
+        !(
+            nqp::istype($value, $!symbols.Seq) ||
+            nqp::istype($value, $!symbols.Failure)
+        )
+    }
+
     my @native_assign_ops := ['', 'assign_i', 'assign_n', 'assign_s'];
     method optimize_nameless_call($op) {
         if +@($op) > 0 {