LMF cannot run trait's "initializer" (constructor)

Thus, rule out traits that have a constructor (which we use
as a proxy for having potentially side-effecting statements),
and create an anonymous subclass for them at compile time.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Erasure.scala b/src/compiler/scala/tools/nsc/transform/Erasure.scala
index cdf3e18b5a..ebb55afca9 100644
--- a/src/compiler/scala/tools/nsc/transform/Erasure.scala
+++ b/src/compiler/scala/tools/nsc/transform/Erasure.scala
@@ -1213,6 +1213,10 @@ abstract class Erasure extends AddInterfaces
       // When we delay sam expansion until after explicit outer & lambda lift, we could decide there whether
       // to expand sam at compile time or use LMF, and this implementation restriction could be lifted.
       sym.isStatic &&
+      // HACK: this is to rule out traits with an effectful initializer.
+      // The constructor only exists if the trait's template has statements.
+      // Sadly, we can't be more precise without access to the tree that defines the SAM's owner.
+      !sym.primaryConstructor.exists &&
       (sym.isInterface || sym.info.decls.forall(mem => mem.isMethod || mem.isType)) // TODO OPT: && {sym setFlag INTERFACE; true})
 
     // we still need to check our ancestors even if the INTERFACE flag is set, as it doesn't take inheritance into account