Backport "Fail when a poly function value has a different number of t…

…ype params than the expected poly function" to 3.5.2 (#21459)

Backports #21248 to the 3.5.2 branch.

PR submitted by the release tooling.
[skip ci]

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

@@ -848,7 +848,7 @@ class Namer { typer: Typer =>
       else
         try
           completeInCreationContext(denot)
-          if (denot.isCompleted) registerIfChild(denot)
+          if (denot.isCompleted) registerIfChildInCreationContext(denot)
         catch
           case ex: CompilationUnit.SuspendException =>
             val completer = SuspendCompleter()
@@ -937,10 +937,12 @@ class Namer { typer: Typer =>
         denot.markAbsent()
     end invalidateIfClashingSynthetic
 
-    /** If completed symbol is an enum value or a named class, register it as a child
+    /** Intentionally left without `using Context` parameter.
+     *  This action should be performed in the context of where the completer was created.
+     *  If completed symbol is an enum value or a named class, register it as a child
      *  in all direct parent classes which are sealed.
      */
-    def registerIfChild(denot: SymDenotation)(using Context): Unit = {
+    def registerIfChildInCreationContext(denot: SymDenotation): Unit = {
       val sym = denot.symbol
 
       def register(child: Symbol, parentCls: ClassSymbol) = {
@@ -964,7 +966,7 @@ class Namer { typer: Typer =>
       end if
     }
 
-    /** Intentionally left without `implicit ctx` parameter. We need
+    /** Intentionally left without `using Context` parameter. We need
      *  to pick up the context at the point where the completer was created.
      */
     def completeInCreationContext(denot: SymDenotation): Unit = {

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

@@ -1895,32 +1895,34 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val untpd.Function(vparams: List[untpd.ValDef] @unchecked, body) = fun: @unchecked
     val dpt = pt.dealias
 
-    // If the expected type is a polymorphic function with the same number of
-    // type and value parameters, then infer the types of value parameters from the expected type.
-    val inferredVParams = dpt match
-      case defn.PolyFunctionOf(poly @ PolyType(_, mt: MethodType))
-      if tparams.lengthCompare(poly.paramNames) == 0 && vparams.lengthCompare(mt.paramNames) == 0 =>
-        vparams.zipWithConserve(mt.paramInfos): (vparam, formal) =>
-          // Unlike in typedFunctionValue, `formal` cannot be a TypeBounds since
-          // it must be a valid method parameter type.
-          if vparam.tpt.isEmpty && isFullyDefined(formal, ForceDegree.failBottom) then
-            cpy.ValDef(vparam)(tpt = new untpd.InLambdaTypeTree(isResult = false, (tsyms, vsyms) =>
-              // We don't need to substitute `mt` by `vsyms` because we currently disallow
-              // dependencies between value parameters of a closure.
-              formal.substParams(poly, tsyms.map(_.typeRef)))
-            )
-          else vparam
-      case _ =>
-        vparams
-
-    val resultTpt = dpt match
+    dpt match
       case defn.PolyFunctionOf(poly @ PolyType(_, mt: MethodType)) =>
-        untpd.InLambdaTypeTree(isResult = true, (tsyms, vsyms) =>
-          mt.resultType.substParams(mt, vsyms.map(_.termRef)).substParams(poly, tsyms.map(_.typeRef)))
-      case _ => untpd.TypeTree()
-
-    val desugared = desugar.makeClosure(tparams, inferredVParams, body, resultTpt, tree.span)
-    typed(desugared, pt)
+        if tparams.lengthCompare(poly.paramNames) == 0 && vparams.lengthCompare(mt.paramNames) == 0 then
+          // If the expected type is a polymorphic function with the same number of
+          // type and value parameters, then infer the types of value parameters from the expected type.
+          val inferredVParams = vparams.zipWithConserve(mt.paramInfos): (vparam, formal) =>
+            // Unlike in typedFunctionValue, `formal` cannot be a TypeBounds since
+            // it must be a valid method parameter type.
+            if vparam.tpt.isEmpty && isFullyDefined(formal, ForceDegree.failBottom) then
+              cpy.ValDef(vparam)(tpt = new untpd.InLambdaTypeTree(isResult = false, (tsyms, vsyms) =>
+                // We don't need to substitute `mt` by `vsyms` because we currently disallow
+                // dependencies between value parameters of a closure.
+                formal.substParams(poly, tsyms.map(_.typeRef)))
+              )
+            else vparam
+          val resultTpt =
+            untpd.InLambdaTypeTree(isResult = true, (tsyms, vsyms) =>
+              mt.resultType.substParams(mt, vsyms.map(_.termRef)).substParams(poly, tsyms.map(_.typeRef)))
+          val desugared = desugar.makeClosure(tparams, inferredVParams, body, resultTpt, tree.span)
+          typed(desugared, pt)
+        else
+          val msg =
+            em"""|Provided polymorphic function value doesn't match the expected type $dpt.
+                 |Expected type should be a polymorphic function with the same number of type and value parameters."""
+          errorTree(EmptyTree, msg, tree.srcPos)
+      case _ =>
+        val desugared = desugar.makeClosure(tparams, vparams, body, untpd.TypeTree(), tree.span)
+        typed(desugared, pt)
   end typedPolyFunctionValue
 
   def typedClosure(tree: untpd.Closure, pt: Type)(using Context): Tree = {

tests/neg/i20533.check

@@ -0,0 +1,5 @@
+-- Error: tests/neg/i20533.scala:5:8 -----------------------------------------------------------------------------------
+5 |    [X] => (x, y) => Map(x -> y) // error
+  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |    Provided polymorphic function value doesn't match the expected type [X, Y] => (x$1: X, x$2: Y) => Map[X, Y].
+  |    Expected type should be a polymorphic function with the same number of type and value parameters.

tests/neg/i20533.scala

@@ -0,0 +1,6 @@
+def mapF(h: [X, Y] => (X, Y) => Map[X, Y]): Unit = ???
+
+def test =
+  mapF(
+    [X] => (x, y) => Map(x -> y) // error
+  )

tests/pos/i21154/A.scala

@@ -0,0 +1,6 @@
+import Z.*
+
+object A:
+  val a: Option[AOptions] = ???
+  val b: Option[BOptions] = ???
+  val c: Option[COptions] = ???

tests/pos/i21154/Z.scala

@@ -0,0 +1,9 @@
+//> using options -Ytest-pickler-check
+
+// in the original issue https://github.com/scala/scala3/issues/21154, the non-deterministic tasty
+// depends on the order of compilation of files, the use-site (A.scala) has to come first,
+// and the file defining the enum has to come second (Z.scala), A.scala in namer will force Z to complete.
+enum Z:
+  case AOptions()
+  case BOptions()
+  case COptions()