Detect non Expr[..] splice patterns (#19944)

We compute the pattern using `Expr[pt]` of the prototype `pt` of the
splice, but this is not enough to reject non-matching patterns. The
quoted splices are encoded away before we get to the pattern matching
phase where we could potentially detect that they will not match.
Instead we check that all quote patterns are `Expr[..]` when typing the
pattern.

Fixes #19941

Author: nicolasstucki

compiler/src/dotty/tools/dotc/quoted/QuotePatterns.scala

@@ -28,7 +28,9 @@ object QuotePatterns:
   /** Check for restricted patterns */
   def checkPattern(quotePattern: QuotePattern)(using Context): Unit = new tpd.TreeTraverser {
     def traverse(tree: Tree)(using Context): Unit = tree match {
-      case _: SplicePattern =>
+      case tree: SplicePattern =>
+        if !tree.body.typeOpt.derivesFrom(defn.QuotedExprClass) then
+          report.error(i"Splice pattern must match an Expr[...]", tree.body.srcPos)
       case tdef: TypeDef if tdef.symbol.isClass =>
         val kind = if tdef.symbol.is(Module) then "objects" else "classes"
         report.error(em"Implementation restriction: cannot match $kind", tree.srcPos)

tests/neg-macros/i19941.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg-macros/i19941.scala:7:14 ---------------------------------------------------------------------------
+7 |    case '{ ${hd *: tl} : *:[Int, EmptyTuple] } => '{ ??? } // error
+  |              ^^^^^^^^
+  |              Splice pattern must match an Expr[...]

tests/neg-macros/i19941.scala

@@ -0,0 +1,7 @@
+import scala.quoted.*
+
+inline def expandMacro(inline from: Tuple): Any = ${ expandMacroImpl }
+
+def expandMacroImpl(using Quotes): Expr[?] =
+  '{ 1 *: EmptyTuple } match
+    case '{ ${hd *: tl} : *:[Int, EmptyTuple] } => '{ ??? } // error

tests/pos-macros/quotedPatterns-4.scala renamed to tests/neg-macros/quotedPatterns-4.scala

@@ -3,7 +3,7 @@ object Test {
   def impl(receiver: Expr[StringContext])(using qctx: scala.quoted.Quotes) = {
     import quotes.reflect.Repeated
     receiver match {
-      case '{ StringContext(${Repeated(parts, tpt)}*) } => // now OK
+      case '{ StringContext(${Repeated(parts, tpt)}*) } => // error: Repeated is not an Expr pattern
     }
   }
 }