Make simplify replace type parameters inside method types

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

@@ -175,7 +175,13 @@ object TypeOps:
         val normed = tp.tryNormalize
         if (normed.exists) normed else mapOver
       case tp: MethodicType =>
-        tp // See documentation of `Types#simplified`
+        // See documentation of `Types#simplified`
+        val addTypeVars = new TypeMap:
+          val constraint = ctx.typerState.constraint
+          def apply(t: Type): Type = t match
+            case t: TypeParamRef => constraint.typeVarOfParam(t).orElse(t)
+            case _ => this.mapOver(t)
+        addTypeVars(tp)
       case tp: SkolemType =>
         // Mapping over a skolem creates a new skolem which by definition won't
         // be =:= to the original one.

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

@@ -1890,7 +1890,8 @@ object Types {
      *  but its simplification is `Serializable`). This means that simplification
      *  should never be used in a `MethodicType`, because that could
      *  lead to a different `signature`. Since this isn't very useful anyway,
-     *  this method handles this by never simplifying inside a `MethodicType`.
+     *  this method handles this by never simplifying inside a `MethodicType`,
+     *  except for replacing type parameters with associated type variables.
      */
     def simplified(using Context): Type = TypeOps.simplify(this, null)
 

tests/pos/i15428.scala

@@ -0,0 +1,11 @@
+import reflect.Selectable.reflectiveSelectable
+
+trait Foo:
+  def f(): Long
+
+def h() = k((_: Foo) => ???)
+
+trait Bar[TB]
+given Bar[Foo] = ???
+
+def k[Tk, Ptr <: { def f(): Tk }](function: Ptr => Int)(using alloc: Bar[Ptr]): Tk = ???