SI-9178 Don't eta expand param-less method types to SAMs

Otherwise, we can end up with a subtle source incompatibility with
the pre-SAM regime. Arguably we should phase out eta expansion to
Function0 as well, but I'll leave that for another day.

1 files changed, 8 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 45ebbd532d..d06a4a5d80 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -862,7 +862,14 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           case Block(_, tree1) => tree1.symbol
           case _               => tree.symbol
         }
-        if (!meth.isConstructor && (isFunctionType(pt) || samOf(pt).exists)) { // (4.2)
+        def shouldEtaExpandToSam: Boolean = {
+          // SI-9536 don't adapt parameterless method types to a to SAM's, fall through to empty application
+          // instead for backwards compatiblity with 2.11. See comments of that ticket and SI-7187
+          // for analogous trouble with non-SAM eta expansion. Suggestions there are: a) deprecate eta expansion to Function0,
+          // or b) switch the order of eta-expansion and empty application in this adaptation.
+          !mt.params.isEmpty && samOf(pt).exists
+        }
+        if (!meth.isConstructor && (isFunctionType(pt) || shouldEtaExpandToSam)) { // (4.2)
           debuglog(s"eta-expanding $tree: ${tree.tpe} to $pt")
           checkParamsConvertible(tree, tree.tpe)
           val tree0 = etaExpand(context.unit, tree, this)