Emit an error for quoted pattern type variable after new

[Cherry-picked 05b26be]

Author: mbovel

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

@@ -232,6 +232,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnnecessaryNN // errorNumber: 216
   case ErasedNotPureID // errorNumber: 217
   case IllegalErasedDefID // errorNumber: 218
+  case CannotInstantiateQuotedTypeVarID // errorNumber: 219
 
   def errorNumber = ordinal - 1
 

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

@@ -3435,6 +3435,18 @@ final class QuotedTypeMissing(tpe: Type)(using Context) extends StagingMessage(Q
 
 end QuotedTypeMissing
 
+final class CannotInstantiateQuotedTypeVar(symbol: Symbol)(using patternCtx: Context) extends StagingMessage(CannotInstantiateQuotedTypeVarID):
+  override protected def msg(using Context): String =
+    i"""Quoted pattern type variable `${symbol.name}` cannot be instantiated.
+      |If you meant to refer to a class named `${symbol.name}`, wrap it in backticks.
+      |If you meant to introduce a binding, this is not allowed after `new`. You might
+      |want to use the lower-level `quotes.reflect` API instead.
+      |Read more about type variables in quoted pattern in the Scala documentation:
+      |https://docs.scala-lang.org/scala3/guides/macros/quotes.html#type-variables-in-quoted-patterns
+    """
+
+  override protected def explain(using Context): String = ""
+
 final class DeprecatedAssignmentSyntax(key: Name, value: untpd.Tree)(using Context) extends SyntaxMsg(DeprecatedAssignmentSyntaxID):
   override protected def msg(using Context): String =
     i"""Deprecated syntax: since 3.7 this is interpreted as a named tuple with one element,

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

@@ -222,7 +222,7 @@ trait QuotesAndSplices {
         if ctx.mode.is(Mode.InPatternAlternative) then
           report.error(IllegalVariableInPatternAlternative(tree.name), tree.srcPos)
         val typeSym = inContext(quotePatternOuterContext(ctx)) {
-          newSymbol(ctx.owner, tree.name.toTypeName, Case, typeSymInfo, NoSymbol, tree.span)
+          newSymbol(ctx.owner, tree.name.toTypeName, Synthetic | Case, typeSymInfo, NoSymbol, tree.span)
         }
         addQuotedPatternTypeVariable(typeSym)
         Bind(typeSym, untpd.Ident(nme.WILDCARD).withType(typeSymInfo)).withSpan(tree.span)

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

@@ -1237,6 +1237,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case _ =>
         var tpt1 = typedType(tree.tpt)
         val tsym = tpt1.tpe.underlyingClassRef(refinementOK = false).typeSymbol
+        if ctx.mode.isQuotedPattern && tpt1.tpe.typeSymbol.isAllOf(Synthetic | Case) then
+          val errorTp = errorType(CannotInstantiateQuotedTypeVar(tpt1.tpe.typeSymbol), tpt1.srcPos)
+          return cpy.New(tree)(tpt1).withType(errorTp)
         if tsym.is(Package) then
           report error(em"$tsym cannot be instantiated", tpt1.srcPos)
         tpt1 = tpt1.withType(ensureAccessible(tpt1.tpe, superAccess = false, tpt1.srcPos))

tests/neg-macros/i22616.check

@@ -0,0 +1,16 @@
+-- [E219] Staging Issue Error: tests/neg-macros/i22616.scala:13:22 -----------------------------------------------------
+13 |          case '{ new caseName(${ Expr(name) }) } => Some(caseName(name))  // error // error
+   |                      ^^^^^^^^
+   |                  Quoted pattern type variable `caseName` cannot be instantiated.
+   |                  If you meant to refer to a class named `caseName`, wrap it in backticks.
+   |                  If you meant to introduce a binding, this is not allowed after `new`. You might
+   |                  want to use the lower-level `quotes.reflect` API instead.
+   |                  Read more about type variables in quoted pattern in the Scala documentation:
+   |                  https://docs.scala-lang.org/scala3/guides/macros/quotes.html#type-variables-in-quoted-patterns
+   |                      
+-- [E006] Not Found Error: tests/neg-macros/i22616.scala:13:67 ---------------------------------------------------------
+13 |          case '{ new caseName(${ Expr(name) }) } => Some(caseName(name))  // error // error
+   |                                                                   ^^^^
+   |                                                                   Not found: name
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg-macros/i22616.scala

@@ -0,0 +1,18 @@
+import scala.quoted.*
+
+final case class caseName(name: String) extends scala.annotation.Annotation
+object caseName {
+
+  given FromExpr[caseName] =
+    new FromExpr[caseName] {
+      override def unapply(x: Expr[caseName])(using Quotes): Option[caseName] =
+        val y: Int = 42
+        x match {
+          case '{ caseName(${ Expr(name) }) }     => Some(caseName(name))
+          // with/without the following line...
+          case '{ new caseName(${ Expr(name) }) } => Some(caseName(name))  // error // error
+          case _                                  => println(x.show); None
+        }
+    }
+
+}