Fix #7060: Make single-parameter constrained more versatile

Author: odersky

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

@@ -483,7 +483,7 @@ object ProtoTypes {
     val state = ctx.typerState
     val addTypeVars = alwaysAddTypeVars || !owningTree.isEmpty
     if (tl.isInstanceOf[PolyType])
-      assert(!(ctx.typerState.isCommittable && !addTypeVars),
+      assert(!ctx.typerState.isCommittable || addTypeVars,
         s"inconsistent: no typevars were added to committable constraint ${state.constraint}")
       // hk type lambdas can be added to constraints without typevars during match reduction
 
@@ -502,8 +502,8 @@ object ProtoTypes {
     (added, tvars)
   }
 
-  /**  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)(