Fix #1044: handle TypevarsMissContext mode in wildApprox

When `wildApprox` encounters a PolyParam it approximates it by its
bounds in the current constraint set, but this is incorrect if
`TypevarsMissContext` is set, we might get the bounds of another use of
this `PolyType`. This is exactly what happened in i1044.scala where the
implicit view `refArrayOps` needs to be used twice with two different
type parameters.

The fix is to approximate a PolyParam by its original bounds in its
PolyType if `TypevarsMissContext` is set.

This fix was inspired by the approach of #1054.

Author: smarter

src/dotty/tools/dotc/typer/Implicits.scala

@@ -52,6 +52,8 @@ object Implicits {
             mt.paramTypes.length != 1 ||
             !(argType relaxed_<:< mt.paramTypes.head)(ctx.fresh.setExploreTyperState)
           case poly: PolyType =>
+            // We do not need to call ProtoTypes#constrained on `poly` because
+            // this method is always called with mode TypevarsMissContext set.
             poly.resultType match {
               case mt: MethodType =>
                 mt.isImplicit ||

src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -391,11 +391,15 @@ object ProtoTypes {
     case tp: TypeAlias => // default case, inlined for speed
       tp.derivedTypeAlias(wildApprox(tp.alias, theMap))
     case tp @ PolyParam(poly, pnum) =>
-      ctx.typerState.constraint.entry(tp) match {
-        case bounds: TypeBounds => wildApprox(WildcardType(bounds))
-        case NoType => WildcardType(wildApprox(poly.paramBounds(pnum)).bounds)
-        case inst => wildApprox(inst)
-      }
+      def unconstrainedApprox = WildcardType(wildApprox(poly.paramBounds(pnum)).bounds)
+      if (ctx.mode.is(Mode.TypevarsMissContext))
+        unconstrainedApprox
+      else
+        ctx.typerState.constraint.entry(tp) match {
+          case bounds: TypeBounds => wildApprox(WildcardType(bounds))
+          case NoType => unconstrainedApprox
+          case inst => wildApprox(inst)
+        }
     case MethodParam(mt, pnum) =>
       WildcardType(TypeBounds.upper(wildApprox(mt.paramTypes(pnum))))
     case tp: TypeVar =>

tests/pos/i1044.scala

@@ -0,0 +1,3 @@
+object Test {
+  val x = ???.getClass.getMethods.head.getParameterTypes.mkString(",")
+}