Fail when a poly function value has a different number of type

params than the expected poly function

Author: KacperFKorban

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1912,32 +1912,34 @@ 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 = {

Diff for: 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.

Diff for: 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
+  )