Handle wildcard args when expanding partial functions

Because of the previous commit,
tests/fuzzy/AE-9a131723f09b9f77c99c52b709965e580a61706e.scala now passes
the typer, but then crashes in ExpandSAMs because we attempt to create
an anonymous class whose parent is `PartialFunction[_, Int]` and
tpd.ClassDef does not handle wildcard arguments. We avoid this issue by
getting the argument types for the parent from the closure DefDef whose
parameter and result types can never be wildcards.

Author: smarter

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

@@ -107,8 +107,8 @@ class ExpandSAMs extends MiniPhase {
     anon.rhs match {
       case PartialFunctionRHS(pf) =>
         val anonSym = anon.symbol
-
-        val parents = List(defn.AbstractPartialFunctionType.appliedTo(tpe.argInfos), defn.SerializableType)
+        val anonTpe = anon.tpe.widen
+        val parents = List(defn.AbstractPartialFunctionType.appliedTo(anonTpe.firstParamTypes.head, anonTpe.resultType), defn.SerializableType)
         val pfSym = ctx.newNormalizedClassSymbol(anonSym.owner, tpnme.ANON_CLASS, Synthetic | Final, parents, coord = tree.span)
 
         def overrideSym(sym: Symbol) = sym.copy(