fix SAM types when pt is a flexible type

Author: olhotak

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

@@ -4198,10 +4198,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       }
 
       // convert function literal to SAM closure
+      val pt1 = pt.stripFlexible
       tree match {
         case closure(Nil, id @ Ident(nme.ANON_FUN), _)
-        if defn.isFunctionNType(wtp) && !defn.isFunctionNType(pt) =>
-          pt match {
+        if defn.isFunctionNType(wtp) && !defn.isFunctionNType(pt1) =>
+          pt1 match {
             case SAMType(sam)
             if wtp <:< sam.toFunctionType(isJava = pt.classSymbol.is(JavaDefined)) =>
               // was ... && isFullyDefined(pt, ForceDegree.flipBottom)