Fix #9570: Warn on use of _ in quoted pattern

Author: nicolasstucki

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

@@ -278,6 +278,10 @@ trait QuotesAndSplices {
             }
             cpy.AppliedTypeTree(tree)(transform(tpt), args1)
         case tree: NamedDefTree =>
+          if tree.name.is(NameKinds.WildcardParamName) then
+            report.warning(
+              "Use of `_` for lambda in quoted pattern. Use explicit lambda instead or use `$_` to match any term.",
+              tree.srcPos)
           if tree.name.isTermName && !tree.nameSpan.isSynthetic && tree.name.startsWith("$") then
             report.error("Names cannot start with $ quote pattern ", tree.namePos)
           super.transform(tree)

compiler/test/dotty/tools/dotc/BootstrappedOnlyCompilationTests.scala

@@ -104,7 +104,10 @@ class BootstrappedOnlyCompilationTests extends ParallelTesting {
 
   @Test def negMacros: Unit = {
     implicit val testGroup: TestGroup = TestGroup("compileNegWithCompiler")
-    compileFilesInDir("tests/neg-macros", defaultOptions).checkExpectedErrors()
+    aggregateTests(
+      compileFilesInDir("tests/neg-macros", defaultOptions),
+      compileFile("tests/pos-macros/i9570.scala", defaultOptions.and("-Xfatal-warnings")),
+    ).checkExpectedErrors()
   }
 
   @Test def negWithCompiler: Unit = {

tests/pos-macros/i9570.scala

@@ -0,0 +1,24 @@
+import scala.quoted._
+
+object Macros {
+
+  object HList {
+    sealed trait HList
+    case class HCons[+HD, TL <: HList](hd: HD, tl: TL) extends HList
+    case object HNil extends HList
+
+    private def sizeImpl(e: Expr[HList], n:Int)(using qctx:QuoteContext): Expr[Int] = {
+      import qctx.tasty._
+      e match {
+        case '{HCons(_,$t)} => // error if run with fatal warinings in BootstrappedOnlyCompilationTests
+          sizeImpl(t,n+1)
+        case '{HNil} => Expr(n)
+      }
+    }
+
+    inline def size(inline expr: HList ): Int = {
+      ${sizeImpl('expr,0)}
+    }
+
+  }
+}

tests/run-macros/i9570/Macro_1.scala

@@ -0,0 +1,25 @@
+import scala.quoted._
+
+object Macros {
+
+  object HList {
+    sealed trait HList
+    case class HCons[+HD, TL <: HList](hd: HD, tl: TL) extends HList
+    case object HNil extends HList
+
+    private def sizeImpl(e: Expr[HList], n:Int)(using qctx:QuoteContext): Expr[Int] = {
+      import qctx.tasty._
+      e match {
+        case '{HCons($_,$t)} =>
+        //case '{HCons($a,$t)} =>
+          sizeImpl(t,n+1)
+        case '{HNil} => Expr(n)
+      }
+    }
+
+    inline def size(inline expr: HList ): Int = {
+      ${sizeImpl('expr,0)}
+    }
+
+  }
+}

tests/run-macros/i9570/Test_2.scala

@@ -0,0 +1,9 @@
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    import Macros.HList._
+
+    val hl0n = size( HCons(("1",1), HCons(("2",2), HCons(("3",3),HNil))) )
+    println(s"size(?) = $hl0n")
+  }
+}