Eta expand export forwarders that have contetx function types

Fixes #15181

Author: odersky

compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -385,7 +385,7 @@ object Erasure {
       case _: FunProto | AnyFunctionProto => tree
       case _ => tree.tpe.widen match
         case mt: MethodType if tree.isTerm =>
-          assert(mt.paramInfos.isEmpty)
+          assert(mt.paramInfos.isEmpty)//, i"bad adapt for $tree: $mt")
           adaptToType(tree.appliedToNone, pt)
         case tpw =>
           if (pt.isInstanceOf[ProtoType] || tree.tpe <:< pt)

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1176,7 +1176,8 @@ class Namer { typer: Typer =>
             import tpd._
             val ref = path.select(sym.asTerm)
             val ddef = tpd.DefDef(forwarder.asTerm, prefss =>
-                ref.appliedToArgss(adaptForwarderParams(Nil, sym.info, prefss)))
+                ref.appliedToArgss(adaptForwarderParams(Nil, sym.info, prefss))
+                  .etaExpandCFT(using ctx.withOwner(forwarder)))
             if forwarder.isInlineMethod then
               PrepareInlineable.registerInlineInfo(forwarder, ddef.rhs)
             buf += ddef.withSpan(span)

compiler/test/dotc/pos-test-pickling.blacklist

@@ -18,6 +18,7 @@ i7740b.scala
 i6507b.scala
 i12299a.scala
 i13871.scala
+i15181.scala
 
 # Tree is huge and blows stack for printing Text
 i7034.scala

tests/pos/i15181.scala

@@ -0,0 +1,9 @@
+object prelude:
+  //def scombine[A](x: A, y: A): Semigroup[A] ?=> A = Semigroup.scombine(x,y)
+  export Semigroup.*
+
+trait Semigroup[A]:
+  def scombine(x: A, y: A): A
+
+object Semigroup:
+  def scombine[A](x: A, y: A) = (s: Semigroup[A]) ?=> s.scombine(x,y)