Restrict wildcard SAM criterion to proper classes

Traits are OK, since traits with wildcard arguments can be produced in source.

Author: odersky

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

@@ -1099,10 +1099,10 @@ class Typer extends Namer
             pt match {
               case SAMType(sam)
               if !defn.isFunctionType(pt) && mt <:< sam =>
+                def isWildcardClassSAM =
+                  !pt.classSymbol.is(Trait) && pt.argInfos.exists(_.isInstanceOf[TypeBounds])
                 val targetTpe =
-                  if isFullyDefined(pt, ForceDegree.all)
-                     && !pt.argInfos.exists(_.isInstanceOf[TypeBounds])
-                  then
+                  if isFullyDefined(pt, ForceDegree.all) && !isWildcardClassSAM then
                     pt
                   else if pt.isRef(defn.PartialFunctionClass) then
                     // Replace the underspecified expected type by one based on the closure method type

tests/pos/i8012.scala

@@ -0,0 +1,16 @@
+
+
+@FunctionalInterface
+abstract class Q[A] {
+  def apply(a: A): Int
+}
+
+trait T[A]
+
+class C extends Q[?]  // error: Type argument must be fully defined
+class D extends T[?]  // error: Type argument must be fully defined
+
+object O {
+  def m(i: Int): Int = i
+  val x: Q[_] = m // error: result type of lambda is an underspecified SAM type Q[?]
+}