Polymorphic closures cannot have dependencies between value params

Just like for monomorphic closures this isn't currently supported.

Author: smarter

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -195,6 +195,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case MatchTypeScrutineeCannotBeHigherKindedID // errorNumber: 179
   case AmbiguousExtensionMethodID // errorNumber 180
   case UnqualifiedCallToAnyRefMethodID // errorNumber: 181
+  case ClosureCannotHaveInternalParameterDependenciesID // errorNumber: 182
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2906,3 +2906,10 @@ class MatchTypeScrutineeCannotBeHigherKinded(tp: Type)(using Context)
   extends TypeMsg(MatchTypeScrutineeCannotBeHigherKindedID) :
     def msg(using Context) = i"the scrutinee of a match type cannot be higher-kinded"
     def explain(using Context) = ""
+
+class ClosureCannotHaveInternalParameterDependencies(mt: Type)(using Context)
+  extends TypeMsg(ClosureCannotHaveInternalParameterDependenciesID):
+    def msg(using Context) =
+      i"""cannot turn method type $mt into closure
+         |because it has internal parameter dependencies"""
+    def explain(using Context) = ""

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

@@ -1686,19 +1686,20 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 TypeTree(targetTpe)
               case _ =>
                 if (mt.isParamDependent)
-                  errorTree(tree,
-                    em"""cannot turn method type $mt into closure
-                        |because it has internal parameter dependencies""")
+                  errorTree(tree, ClosureCannotHaveInternalParameterDependencies(mt))
                 else if hasCaptureConversionArg(mt.resType) then
                   errorTree(tree,
                     em"""cannot turn method type $mt into closure
                         |because it has capture conversion skolem types""")
                 else
                   EmptyTree
             }
-          case _: PolyType =>
-            // Polymorphic SAMs are not currently supported (#6904).
-            EmptyTree
+          case poly @ PolyType(_, mt: MethodType) =>
+            if (mt.isParamDependent)
+              errorTree(tree, ClosureCannotHaveInternalParameterDependencies(poly))
+            else
+              // Polymorphic SAMs are not currently supported (#6904).
+              EmptyTree
           case tp =>
             if !tp.isErroneous then
               throw new java.lang.Error(i"internal error: closing over non-method $tp, pos = ${tree.span}")

tests/neg/polymorphic-functions.scala

@@ -2,4 +2,6 @@ object Test {
   val pv0: [T] => List[T] = ???        // error
   val pv1: Any = [T] => Nil            // error
   val pv2: [T] => List[T] = [T] => Nil // error // error
+
+  val intraDep = [T] => (x: T, y: List[x.type]) => List(y) // error
 }