Add escape hatch Selectable.WithoutPreciseParameterTypes

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

@@ -762,6 +762,7 @@ class Definitions {
   @tu lazy val LanguageDeprecatedModule: Symbol = requiredModule("scala.language.deprecated")
   @tu lazy val NonLocalReturnControlClass: ClassSymbol = requiredClass("scala.runtime.NonLocalReturnControl")
   @tu lazy val SelectableClass: ClassSymbol = requiredClass("scala.Selectable")
+  @tu lazy val WithoutPreciseParameterTypesClass: Symbol = requiredClass("scala.Selectable.WithoutPreciseParameterTypes")
 
   @tu lazy val ReflectPackageClass: Symbol = requiredPackage("scala.reflect.package").moduleClass
   @tu lazy val ClassTagClass: ClassSymbol = requiredClass("scala.reflect.ClassTag")

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

@@ -1778,10 +1778,14 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         // then the symbol referred to in the subtype must have a signature that coincides
         // in its parameters with the refinement's signature. The reason for the check
         // is that if the refinement does not refer to a member symbol, we will have to
-        // resort to reflection to invoke the member. And reflection needs to know exact
-        // erased parameter types. See neg/i12211.scala.
+        // resort to reflection to invoke the member. And Java reflection needs to know exact
+        // erased parameter types. See neg/i12211.scala. Other reflection algorithms could
+        // conceivably dispatch without knowning precise parameter signatures. One can signal
+        // this by inheriting from the `scala.reflect.SignatureCanBeImprecise` marker trait,
+        // in which case the signature test is elided.
         def sigsOK(symInfo: Type, info2: Type) =
           tp2.underlyingClassRef(refinementOK = true).member(name).exists
+          || tp2.derivesFrom(defn.WithoutPreciseParameterTypesClass)
           || symInfo.isInstanceOf[MethodType]
               && symInfo.signature.consistentParams(info2.signature)
 

docs/docs/reference/changed-features/structural-types-spec.md

@@ -100,12 +100,19 @@ conversion that can turn `v` into a `Selectable`, and the selection methods coul
   val b: { def put(x: String): Unit } = a  // error
   b.put("abc") // looks for a method with a `String` parameter
   ```
-  The second to last line is not well-typed, since the erasure of the parameter type of `put` in class `Sink` is `Object`, but the erasure of `put`'s parameter in the type of `b` is `String`. This additional condition is necessary, since we will have to resort to reflection to call a structural member like `put` in the type of `b` above. The condition ensures that the statically known parameter types of the refinement correspond up to erasure to the parameter types of the selected call target at runtime.
-
-  The usual reflection dispatch algorithms need to know exact erased parameter types. For instance, if the example above would typecheck, the call
+  The second to last line is not well-typed,
+  since the erasure of the parameter type of `put` in class `Sink` is `Object`,
+  but the erasure of `put`'s parameter in the type of `b` is `String`.
+  This additional condition is necessary, since we will have to resort
+  to some (as yet unknown) form of reflection to call a structural member
+  like `put` in the type of `b` above. The condition ensures that the statically
+  known parameter types of the refinement correspond up to erasure to the
+  parameter types of the selected call target at runtime.
+
+  Most reflection dispatch algorithms need to know exact erased parameter types. For instance, if the example above would typecheck, the call
   `b.put("abc")` on the last line would look for a method `put` in the runtime type of `b` that takes a `String` parameter. But the `put` method is the one from class `Sink`, which takes an `Object` parameter. Hence the call would fail at runtime with a `NoSuchMethodException`.
 
-  One might hope for a "more intelligent" reflexive dispatch algorithm that does not require exact parameter type matching. Unfortunately, this can always run into ambiguities. For instance, continuing the example above, we might introduce a new subclass `Sink1` of `Sink` and change the definition of `a` as follows:
+  One might hope for a "more intelligent" reflexive dispatch algorithm that does not require exact parameter type matching. Unfortunately, this can always run into ambiguities, as long as overloading is a possibility. For instance, continuing the example above, we might introduce a new subclass `Sink1` of `Sink` and change the definition of `a` as follows:
 
   ```scala
   class Sink1[A] extends Sink[A] { def put(x: "123") = ??? }
@@ -116,6 +123,20 @@ conversion that can turn `v` into a `Selectable`, and the selection methods coul
   types `Object` and `String`, respectively. Yet dynamic dispatch still needs to go
   to the first `put` method, even though the second looks like a better match.
 
+  For the cases where we can in fact implement reflection without knowing precise parameter types (for instance if static overloading is replaced by dynamically dispatched multi-methods), there is an escape hatch. For types that extend `scala.Selectable.WithoutPreciseParameterTypes` the signature check is omitted. Example:
+
+  ```scala
+  trait MultiMethodSelectable extends Selectable.WithoutPreciseParameterTypes:
+    // Assume this version of `applyDynamic` can be implemented without knowing
+    // precise parameter types `paramTypes`:
+    def applyDynamic(name: String, paramTypes: Class[_]*)(args: Any*): Any = ???
+
+  class Sink[A] extends MultiMethodSelectable:
+    def put(x: A): Unit = {}
+
+  val a = new Sink[String]
+  val b: MultiMethodSelectable { def put(x: String): Unit } = a  // OK
+  ```
 ## Differences with Scala 2 Structural Types
 
 - Scala 2 supports structural types by means of Java reflection. Unlike

library/src/scala/Selectable.scala

@@ -1,5 +1,7 @@
 package scala
 
+import scala.annotation.experimental
+
 /** A marker trait for objects that support structural selection via
  *  `selectDynamic` and `applyDynamic`
  *
@@ -34,3 +36,19 @@ object Selectable:
   implicit def reflectiveSelectableFromLangReflectiveCalls(x: Any)(
       using scala.languageFeature.reflectiveCalls): scala.reflect.Selectable =
     scala.reflect.Selectable.reflectiveSelectable(x)
+
+  /** A marker trait for subclasses of `Selectable` indicating
+   *  that precise parameter types are not needed for method dispatch. That is,
+   *  a class inheriting from this trait and implementing
+   *
+   *     def applyDynamic(name: String, paramTypes: Class[_]*)(args: Any*)
+   *
+   *  should dispatch to a method with the given `name` without having to rely
+   *  on the precise `paramTypes`. Subtypes of `WithoutPreciseParameterTypes`
+   *  can have more relaxed subtyping rules for refinements. They do not need
+   *  the additional restriction that the signatures of the refinement and
+   *  the definition that implements the refinment must match.
+   */
+  @experimental
+  trait WithoutPreciseParameterTypes extends Selectable
+end Selectable

library/src/scala/reflect/Selectable.scala

@@ -49,3 +49,4 @@ object Selectable:
 
   @inline // important for Scala.js
   private final class DefaultSelectable(override protected val selectedValue: Any) extends Selectable
+end Selectable

project/MiMaFilters.scala

@@ -27,5 +27,6 @@ object MiMaFilters {
     exclude[ReversedMissingMethodProblem]("scala.quoted.Quotes#reflectModule#SymbolMethods.typeMember"),
     exclude[ReversedMissingMethodProblem]("scala.quoted.Quotes#reflectModule#SymbolMethods.typeMembers"),
     exclude[ReversedMissingMethodProblem]("scala.quoted.Quotes#reflectModule#TermParamClauseMethods.isErased"),
+    exclude[MissingClassProblem]("scala.Selectable$WithoutPreciseParameterTypes")
   )
 }

tests/pos/i12211.scala

@@ -19,3 +19,14 @@ class BB[T]
 
 def test3: (a: AA) => (b: BB[a.type]) => BB[?] =
   (a: AA) => (b: BB[a.type]) => b
+
+@annotation.experimental // TODO: Remove once WithoutPreciseParameterTypes is no longer experimental
+trait RelaxedSelectable extends Selectable.WithoutPreciseParameterTypes:
+  def applyDynamic(name: String, paramTypes: Class[_]*)(args: Any*): Any = ???
+@annotation.experimental // TODO: Remove once WithoutPreciseParameterTypes is no longer experimental
+class Sink[A] extends RelaxedSelectable {
+  def put(x: A): Unit = {}
+}
+val a = new Sink[String]
+val b: RelaxedSelectable { def put(x: String): Unit } = a
+val _ = b.put("")