Check user defined PolyFunction refinements

`PolyFunction` must be refined with an `apply` method that has a single
parameter list with no by-name nor varargs parameters. It may optionally
have type parameters. Some of these restrictions could be lifted later,
but for now these features are not properly handled by the compiler.

Fixes #8299
Fixes #18302

Author: nicolasstucki

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

@@ -1161,10 +1161,12 @@ class Definitions {
           Some(mt)
         case _ => None
 
-    private def isValidPolyFunctionInfo(info: Type)(using Context): Boolean =
+    def isValidPolyFunctionInfo(info: Type)(using Context): Boolean =
       def isValidMethodType(info: Type) = info match
         case info: MethodType =>
-          !info.resType.isInstanceOf[MethodOrPoly] // Has only one parameter list
+          !info.resType.isInstanceOf[MethodOrPoly] && // Has only one parameter list
+          !info.isVarArgsMethod &&
+          !info.paramInfos.exists(_.isInstanceOf[ExprType]) // No by-name parameters
         case _ => false
       info match
         case info: PolyType => isValidMethodType(info.resType)

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -381,13 +381,15 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
         case tree: ValDef =>
           registerIfHasMacroAnnotations(tree)
           checkErasedDef(tree)
+          Checking.checkPolyFunctionType(tree.tpt)
           val tree1 = cpy.ValDef(tree)(rhs = normalizeErasedRhs(tree.rhs, tree.symbol))
           if tree1.removeAttachment(desugar.UntupledParam).isDefined then
             checkStableSelection(tree.rhs)
           processValOrDefDef(super.transform(tree1))
         case tree: DefDef =>
           registerIfHasMacroAnnotations(tree)
           checkErasedDef(tree)
+          Checking.checkPolyFunctionType(tree.tpt)
           annotateContextResults(tree)
           val tree1 = cpy.DefDef(tree)(rhs = normalizeErasedRhs(tree.rhs, tree.symbol))
           processValOrDefDef(superAcc.wrapDefDef(tree1)(super.transform(tree1).asInstanceOf[DefDef]))
@@ -492,6 +494,9 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
           )
         case Block(_, Closure(_, _, tpt)) if ExpandSAMs.needsWrapperClass(tpt.tpe) =>
           superAcc.withInvalidCurrentClass(super.transform(tree))
+        case tree: RefinedTypeTree =>
+          Checking.checkPolyFunctionType(tree)
+          super.transform(tree)
         case _: Quote | _: QuotePattern =>
           ctx.compilationUnit.needsStaging = true
           super.transform(tree)

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

@@ -816,6 +816,31 @@ object Checking {
           else Feature.checkExperimentalFeature("features", imp.srcPos)
         case _ =>
   end checkExperimentalImports
+
+  /** Checks that PolyFunction only have valid refinements.
+   *
+   *  It only supports `apply` methods with one parameter list and optional type arguments.
+   */
+  def checkPolyFunctionType(tree: Tree)(using Context): Unit = new TreeTraverser {
+    def traverse(tree: Tree)(using Context): Unit = tree match
+      case tree: RefinedTypeTree if tree.tpe.derivesFrom(defn.PolyFunctionClass) =>
+        if tree.refinements.isEmpty then
+          reportNoRefinements(tree.srcPos)
+        tree.refinements.foreach {
+          case refinement: DefDef if refinement.name != nme.apply =>
+            report.error("PolyFunction only supports apply method refinements", refinement.srcPos)
+          case refinement: DefDef if !defn.PolyFunctionOf.isValidPolyFunctionInfo(refinement.tpe.widen) =>
+            report.error("Implementation restriction: PolyFunction apply must have exactly one parameter list and optionally type arguments. No by-name nor varags are allowed.", refinement.srcPos)
+          case _ =>
+        }
+      case _: RefTree if tree.symbol == defn.PolyFunctionClass =>
+        reportNoRefinements(tree.srcPos)
+      case _ =>
+        traverseChildren(tree)
+
+    def reportNoRefinements(pos: SrcPos) =
+      report.error("PolyFunction subtypes must refine the apply method", pos)
+  }.traverse(tree)
 }
 
 trait Checking {

tests/neg/i18302b.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i18302b.scala:3:32 ---------------------------------------------------------------------------------
+3 |def polyFun: PolyFunction { def apply(x: Int)(y: Int): Int } = // error
+  |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |Implementation restriction: PolyFunction apply must have exactly one parameter list and optionally type arguments. No by-name nor varags are allowed.

tests/neg/i18302b.scala

@@ -0,0 +1,5 @@
+def test = polyFun(1)(2)
+
+def polyFun: PolyFunction { def apply(x: Int)(y: Int): Int } = // error
+  new PolyFunction:
+    def apply(x: Int)(y: Int): Int = x + y

tests/neg/i18302c.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i18302c.scala:4:32 ---------------------------------------------------------------------------------
+4 |def polyFun: PolyFunction { def foo(x: Int): Int } = // error
+  |                            ^^^^^^^^^^^^^^^^^^^^
+  |                            PolyFunction only supports apply method refinements

tests/neg/i18302c.scala

@@ -0,0 +1,5 @@
+import scala.reflect.Selectable.reflectiveSelectable
+
+def test = polyFun.foo(1)
+def polyFun: PolyFunction { def foo(x: Int): Int } = // error
+  new PolyFunction { def foo(x: Int): Int = x + 1 }

tests/neg/i18302d.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i18302d.scala:1:32 ---------------------------------------------------------------------------------
+1 |def polyFun: PolyFunction { def apply: Int } = // error
+  |                            ^^^^^^^^^^^^^^
+  |Implementation restriction: PolyFunction apply must have exactly one parameter list and optionally type arguments. No by-name nor varags are allowed.

tests/neg/i18302d.scala

@@ -0,0 +1,2 @@
+def polyFun: PolyFunction { def apply: Int } = // error
+  new PolyFunction { def apply: Int = 1 }

tests/neg/i18302e.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg/i18302e.scala:1:13 ---------------------------------------------------------------------------------
+1 |def polyFun: PolyFunction {  } = // error
+  |             ^^^^^^^^^^^^^^^^^
+  |             PolyFunction subtypes must refine the apply method
+-- Error: tests/neg/i18302e.scala:4:15 ---------------------------------------------------------------------------------
+4 |def polyFun(f: PolyFunction {  }) = () // error
+  |               ^^^^^^^^^^^^^^^^^
+  |               PolyFunction subtypes must refine the apply method

tests/neg/i18302e.scala

@@ -0,0 +1,4 @@
+def polyFun: PolyFunction {  } = // error
+  new PolyFunction {  }
+
+def polyFun(f: PolyFunction {  }) = () // error

tests/neg/i18302f.check

@@ -0,0 +1,12 @@
+-- Error: tests/neg/i18302f.scala:1:13 ---------------------------------------------------------------------------------
+1 |def polyFun: PolyFunction = // error
+  |             ^^^^^^^^^^^^
+  |             PolyFunction subtypes must refine the apply method
+-- Error: tests/neg/i18302f.scala:4:16 ---------------------------------------------------------------------------------
+4 |def polyFun2(a: PolyFunction) = () // error
+  |                ^^^^^^^^^^^^
+  |                PolyFunction subtypes must refine the apply method
+-- Error: tests/neg/i18302f.scala:6:14 ---------------------------------------------------------------------------------
+6 |val polyFun3: PolyFunction = // error
+  |              ^^^^^^^^^^^^
+  |              PolyFunction subtypes must refine the apply method

tests/neg/i18302f.scala

@@ -0,0 +1,7 @@
+def polyFun: PolyFunction = // error
+  new PolyFunction {  }
+
+def polyFun2(a: PolyFunction) = () // error
+
+val polyFun3: PolyFunction = // error
+  new PolyFunction {  }

tests/neg/i18302i.scala

@@ -0,0 +1,6 @@
+def polyFun1: Option[PolyFunction] = ??? // error
+def polyFun2: PolyFunction & Any = ??? // error
+def polyFun3: Any & PolyFunction = ??? // error
+def polyFun4: PolyFunction | Any = ??? // error
+def polyFun5: Any | PolyFunction = ??? // error
+def polyFun6(a: Any | PolyFunction) = ??? // error

tests/neg/i18302j.scala

@@ -0,0 +1,5 @@
+def polyFunByName: PolyFunction { def apply(thunk: => Int): Int } = // error
+  new PolyFunction { def apply(thunk: => Int): Int = 1 }
+
+def polyFunVarArgs: PolyFunction { def apply(args: Int*): Int } = // error
+  new PolyFunction { def apply(thunk: Int*): Int = 1 }

tests/neg/i8299.scala

@@ -0,0 +1,8 @@
+package example
+
+object Main {
+  def main(a: Array[String]): Unit = {
+    val p: PolyFunction = // error: PolyFunction subtypes must refine the apply method
+      [A] => (xs: List[A]) => xs.headOption
+  }
+}

tests/pos/i18302a.scala

@@ -0,0 +1,4 @@
+def test = polyFun(1)
+
+def polyFun: PolyFunction { def apply(x: Int): Int } =
+  new PolyFunction { def apply(x: Int): Int = x + 1 }