Fix #6999: Emit an error when using type splice in quoted val pattern

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1740,7 +1740,8 @@ object desugar {
         new TreeTraverser {
           def traverse(tree: untpd.Tree)(implicit ctx: Context): Unit = tree match {
             case Splice(expr) => collect(expr)
-            case TypSplice(expr) => collect(expr)
+            case TypSplice(expr) =>
+              ctx.error("Type splices cannot ve used in val patterns. Consider using `match` instead.", tree.sourcePos)
             case _ => traverseChildren(tree)
           }
         }.traverse(expr)

tests/neg/i6997b.scala

@@ -0,0 +1,14 @@
+package playground
+
+import scala.quoted.{_, given}, scala.quoted.matching._
+
+inline def mcr(x: => Any): Any = ${mcrImpl('x)}
+
+def mcrImpl(body: Expr[Any])(given ctx: QuoteContext): Expr[Any] = {
+  val '{$x: $t} = body // error
+  '{
+    val tmp: $t = $x.asInstanceOf[$t] // error // error
+    println(tmp)
+    tmp
+  }
+}

tests/pos/i6997b.scala → tests/pos/i6997c.scala

@@ -5,10 +5,10 @@ import scala.quoted.{_, given}, scala.quoted.matching._
 inline def mcr(x: => Any): Any = ${mcrImpl('x)}
 
 def mcrImpl(body: Expr[Any])(given ctx: QuoteContext): Expr[Any] = {
-  val '{$x: $t} = body
+  body match case '{$x: $t} =>
   '{
-    val tmp: $t = $x.asInstanceOf[$t]
+    val tmp: $t = $x
     println(tmp)
     tmp
   }
-}
+}

tests/run-staging/quote-type-matcher.scala

@@ -7,22 +7,22 @@ object Test {
   def main(args: Array[String]): Unit = withQuoteContext {
     val '[List[Int]] = '[List[Int]]
 
-    val '[List[$int]] = '[List[Int]]
-    println(int.show)
-    println()
+    '[List[Int]] match
+      case '[List[$int]] =>
+        println(int.show)
+        println()
 
-    {
-      val '[Function1[$t1, $r]] = '[Int => Double]
-      println(t1.show)
-      println(r.show)
-      println()
-    }
+    '[Int => Double] match
+      case  '[Function1[$t1, $r]] =>
+        println(t1.show)
+        println(r.show)
+        println()
+
+    '[(Int => Short) => Double] match
+      case '[Function1[Function1[$t1, $r0], $r]] =>
+        println(t1.show)
+        println(r0.show)
+        println(r.show)
 
-    {
-      val '[Function1[Function1[$t1, $r0], $r]] = '[(Int => Short) => Double]
-      println(t1.show)
-      println(r0.show)
-      println(r.show)
-    }
   }
 }