SI-4043 Share code in etaExpand and inferPolyAlternatives

Refactors duplication between these methods. I've left the
old code with assertions that the old and new results are
equivalent, I'll remove these crutches in the subsequent commit.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Infer.scala

@@ -1449,6 +1449,8 @@ trait Infer extends Checkable {
           val tparams = newAsSeenFromMap(pre, hd.owner) mapOver hd.typeParams
           val bounds  = tparams map (_.tpeHK) // see e.g., #1236
           val tpe     = PolyType(tparams, OverloadedType(AntiPolyType(pre, bounds), alts))
+          val tpe1 = typeFunAnonSeenFrom(pre, hd, hd.typeParams, (bounds: List[Type]) => OverloadedType(AntiPolyType(pre, bounds), alts))
+          assert(tpe.matches(tpe1) && tpe1.matches(tpe1))
           finish(sym setInfo tpe, tpe)
       }
       matchingLength.alternatives match {

src/reflect/scala/reflect/internal/Types.scala

@@ -2184,10 +2184,13 @@ trait Types
       else {
         val map = newAsSeenFromMap(pre, sym.owner)
         val tpars1 = map.mapOver(tpars)
-        if (tpars eq tpars1)
+        val tp = if (tpars eq tpars1)
           typeFunAnon(tpars, copyTypeRef(this, pre, sym, tpars map (_.tpeHK)))
         else
           typeFunAnon(tpars1, copyTypeRef(this, pre, sym, tpars map (_.tpeHK.substSym(tpars, tpars1))))
+        val tp1 = typeFunAnonSeenFrom(pre, sym, tpars, bounds => copyTypeRef(this, pre, sym, bounds))
+        assert(tp.matches(tp) && tp1.matches(tp1))
+        tp
       } // todo: also beta-reduce?
     }
 
@@ -3617,6 +3620,15 @@ trait Types
   /** A creator for a type functions, assuming the type parameters tps already have the right owner. */
   def typeFun(tps: List[Symbol], body: Type): Type = PolyType(tps, body)
 
+  def typeFunAnonSeenFrom(pre: Type, sym: Symbol, tpars: List[Symbol], body: List[Type] => Type): Type = {
+    val map = newAsSeenFromMap(pre, sym.owner)
+    val tpars1 = map.mapOver(tpars)
+    if (tpars eq tpars1)
+      typeFunAnon(tpars, body(tpars map (_.tpeHK)))
+    else
+      typeFunAnon(tpars1, body(tpars map (_.tpeHK.substSym(tpars, tpars1))))
+  }
+
   /** A creator for existential types. This generates:
    *
    *  tpe1 where { tparams }