Fix #7980: SAM type should check trait parameters

Author: liufengyun

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -4518,7 +4518,7 @@ object Types {
           case et: ExprType => true
           case _ => false
         }
-        if (tp.cls.is(Trait) || zeroParams(tp.cls.primaryConstructor.info)) tp // !!! needs to be adapted once traits have parameters
+        if (zeroParams(tp.cls.primaryConstructor.info)) tp
         else NoType
       case tp: AppliedType =>
         zeroParamClass(tp.superType)

tests/neg/i7980.scala

@@ -0,0 +1,7 @@
+trait Evidence[X]
+
+trait Trait[X : Evidence]
+  def method(x : X) : X
+
+given ev : Evidence[Int] = new Evidence[Int]{}
+val crash : Trait[Int] = (x: Int) => x          // error