Fix #7060: Make single-parameter constrained more versatile

Author: odersky

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

@@ -503,7 +503,8 @@ object ProtoTypes {
   }
 
   /**  Same as `constrained(tl, EmptyTree)`, but returns just the created type lambda */
-  def constrained(tl: TypeLambda)(implicit ctx: Context): TypeLambda = constrained(tl, EmptyTree)._1
+  def constrained(tl: TypeLambda)(implicit ctx: Context): TypeLambda =
+    constrained(tl, EmptyTree, alwaysAddTypeVars = ctx.typerState.isCommittable)._1
 
   def newTypeVar(bounds: TypeBounds)(implicit ctx: Context): TypeVar = {
     val poly = PolyType(DepParamName.fresh().toTypeName :: Nil)(

tests/neg/i7060.scala

@@ -0,0 +1,19 @@
+object PostConditions1 {
+
+  import PostConditions.{ensure, res, Box}
+
+  val v = List(1, 2, 4).sum.ensure(Box(10) => res == 10) // error: not a legal formal parameter
+  println(v)
+}
+
+object PostConditions {
+
+  class Box[T](val t: T)
+
+  def res[T] given (b: Box[T]): T = b.t
+
+  def (e: T) ensure[T](cond: given Box[T] => Boolean): T = {
+    if (cond given Box(e)) e
+    else throw new AssertionError("condition not fulfilled")
+  }
+}