fix: Check if a PolyFunction TypeTree has no ByName parameters

Author: KacperFKorban

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

@@ -1829,12 +1829,11 @@ class NotAPath(tp: Type, usage: String)(using Context) extends TypeMsg(NotAPathI
     if sym.isAllOf(Flags.InlineParam) then
       i"""
          |Inline parameters are not considered immutable paths and cannot be used as
-         |singleton types. 
-         | 
+         |singleton types.
+         |
          |Hint: Removing the `inline` qualifier from the `${sym.name}` parameter
          |may help resolve this issue."""
     else ""
-    
 
 class WrongNumberOfParameters(tree: untpd.Tree, foundCount: Int, pt: Type, expectedCount: Int)(using Context)
   extends SyntaxMsg(WrongNumberOfParametersID) {

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

@@ -1914,11 +1914,26 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       .showing(i"desugared fun $tree --> $desugared with pt = $pt", typr)
   }
 
+  /** Check that the PolyFunction doesn't have by-name parameters.
+   *  Return the unchanged tree if it's valid, or EmptyTree otherwise.
+   */
+  private def checkPolyTypeTree(tree: untpd.Tree)(using Context): untpd.Tree =
+    val untpd.PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun @ untpd.Function(vparamTypes, res)) = tree: @unchecked
+    var tree1 = tree
+    vparamTypes.foreach:
+      case t: ByNameTypeTree =>
+        report.error("By-name parameters are not supported in Polymorphic Functions", t.srcPos)
+        tree1 = untpd.EmptyTree
+      case _ =>
+    tree1
 
   def typedPolyFunction(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val tree1 = desugar.normalizePolyFunction(tree)
-    if (ctx.mode is Mode.Type) typed(desugar.makePolyFunctionType(tree1), pt)
-    else typedPolyFunctionValue(tree1, pt)
+    checkPolyTypeTree(tree1) match
+      case tree2: untpd.PolyFunction =>
+        if (ctx.mode is Mode.Type) typed(desugar.makePolyFunctionType(tree2), pt)
+        else typedPolyFunctionValue(tree2, pt)
+      case untpd.EmptyTree => TypeTree(NoType)
 
   def typedPolyFunctionValue(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val untpd.PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun) = tree: @unchecked

tests/neg/21538.check

@@ -3,8 +3,8 @@
   |                                             ^^^^^^^^^^
   |                                      (value : V) is not a valid singleton type, since it is not an immutable path
   |                                      Inline parameters are not considered immutable paths and cannot be used as
-  |                                      singleton types. 
-  |                                       
+  |                                      singleton types.
+  |
   |                                      Hint: Removing the `inline` qualifier from the `value` parameter
   |                                      may help resolve this issue.
   |

tests/neg/i21652.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i21652.scala:1:15 ----------------------------------------------------------------------------------
+1 |def k: [A] => (=> A) => A = // error
+  |               ^^^^
+  |               By-name parameters are not supported in Polymorphic Functions

tests/neg/i21652.scala

@@ -0,0 +1,2 @@
+def k: [A] => (=> A) => A = // error
+  [A] => a => a