Disallow opaque type aliases of context functions

We could allow them but they would not do what one probably expects (i.e. create
context closures). This is because abstract types upper-bounded by context functions
don't do that either (see neg/i16035a.scala), and we have to keep semantic equivalence
between the two.

Therefore, it's better to disallow them.

Fixes scala#16035

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

@@ -386,7 +386,7 @@ object Erasure {
       case _: FunProto | AnyFunctionProto => tree
       case _ => tree.tpe.widen match
         case mt: MethodType if tree.isTerm =>
-          assert(mt.paramInfos.isEmpty)//, i"bad adapt for $tree: $mt")
+          assert(mt.paramInfos.isEmpty, i"bad adapt for $tree: $mt")
           adaptToType(tree.appliedToNone, pt)
         case tpw =>
           if (pt.isInstanceOf[ProtoType] || tree.tpe <:< pt)

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

@@ -1460,6 +1460,15 @@ trait Checking {
             |CanThrow capabilities can only be generated $req.""",
         pat.srcPos)
 
+  /** Check that tree does not define a context fucntion type */
+  def checkNoContextFunctionType(tree: Tree)(using Context): Unit =
+    def recur(tp: Type): Unit = tp.dealias match
+      case tp: HKTypeLambda => recur(tp.resType)
+      case tp if defn.isContextFunctionType(tp) =>
+        report.error(em"context functon type cannot have opaque aliases", tree.srcPos)
+      case _ =>
+    recur(tree.tpe)
+
   /** (1) Check that every named import selector refers to a type or value member of the
    *  qualifier type.
    *  (2) Check that no import selector is renamed more than once.
@@ -1495,6 +1504,7 @@ trait ReChecking extends Checking {
   override def checkNoModuleClash(sym: Symbol)(using Context) = ()
   override def checkCanThrow(tp: Type, span: Span)(using Context): Tree = EmptyTree
   override def checkCatch(pat: Tree, guard: Tree)(using Context): Unit = ()
+  override def checkNoContextFunctionType(tree: Tree)(using Context): Unit = ()
   override def checkFeature(name: TermName, description: => String, featureUseSite: Symbol, pos: SrcPos)(using Context): Unit = ()
 }
 

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

@@ -2420,6 +2420,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case rhs =>
         typedType(rhs)
     checkFullyAppliedType(rhs1)
+    if sym.isOpaqueAlias then checkNoContextFunctionType(rhs1)
     assignType(cpy.TypeDef(tdef)(name, rhs1), sym)
   }
 

docs/_docs/reference/other-new-features/opaques-details.md

@@ -52,6 +52,7 @@ def id(x: o.T): o.T = x
 ```
 
 Opaque type aliases cannot be `private` and cannot be overridden in subclasses.
+Opaque type aliases cannot have a context function type as right-hand side.
 
 ## Type Parameters of Opaque Types
 

tests/neg/i16035.scala

@@ -0,0 +1,12 @@
+object Scope:
+  opaque type Uses[A, B] = A ?=> B // error
+  opaque type UsesAlt = [A, B] =>> A ?=> B // error
+
+  object Uses:
+    def apply[A, B](fn: A ?=> B): Uses[A, B] = fn
+
+import Scope.*
+val uses =
+  given Int = 1
+  Uses[Int, String](i ?=> s"*$i*")
+

tests/neg/i16035a.scala

@@ -0,0 +1,14 @@
+trait S:
+  type Uses[A, B] <: A ?=> B
+  object Uses:
+    def apply[A, B](fn: A ?=> B): Uses[A, B] = fn // error
+  val uses1 =
+    given Int = 1
+    Uses[Int, String](i ?=> s"*$i*")
+
+object I extends S:
+  type Uses[A, B] = A ?=> B
+  val uses2 =
+    given Int = 1
+    Uses[Int, String](i ?=> s"*$i*")
+

tests/pos/i16035.scala

@@ -0,0 +1,11 @@
+object Scope:
+  type Uses[A, B] = A ?=> B
+
+  object Uses:
+    def apply[A, B](fn: A ?=> B): Uses[A, B] = fn
+
+import Scope.*
+val uses =
+  given Int = 1
+  Uses[Int, String](i ?=> s"*$i*")
+