Do not remove InPatternAlternative when typing subexpressions

We use typedExpr to type the contents of a quoted pattern. In that
case it must not have the Pattern mode mut should keep the
InPatternAlternative because it will be needed in the pattern located
in the splices.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -89,7 +89,7 @@ object Mode {
   /** Don't suppress exceptions thrown during show */
   val PrintShowExceptions: Mode = newMode(18, "PrintShowExceptions")
 
-  val PatternOrTypeBits: Mode = Pattern | Type | InPatternAlternative
+  val PatternOrTypeBits: Mode = Pattern | Type
 
   /** We are elaborating the fully qualified name of a package clause.
    *  In this case, identifiers should never be imported.

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

@@ -243,17 +243,6 @@ trait QuotesAndSplices {
         res
       }
 
-      def checkAlternativeBinds(pat0: Tree): Unit =
-        def rec(pat: Tree): Unit =
-          pat match
-            case Typed(pat, _) => rec(pat)
-            case UnApply(_, _, pats) => pats.foreach(rec)
-            case pat: Bind =>
-              report.error(IllegalVariableInPatternAlternative(pat.symbol.name), pat.withSpan(pat.nameSpan))
-              rec(pat.body)
-            case _ =>
-        if ctx.mode.is(Mode.InPatternAlternative) then rec(pat0)
-
       val patBuf = new mutable.ListBuffer[Tree]
       val freshTypePatBuf = new mutable.ListBuffer[Tree]
       val freshTypeBindingsBuff = new mutable.ListBuffer[Tree]
@@ -265,7 +254,6 @@ trait QuotesAndSplices {
           val newSplice = ref(defn.QuotedRuntime_exprSplice).appliedToType(tpt1.tpe).appliedTo(Typed(pat, exprTpt))
           transform(newSplice)
         case Apply(TypeApply(fn, targs), Apply(sp, pat :: Nil) :: args :: Nil) if fn.symbol == defn.QuotedRuntimePatterns_patternHigherOrderHole =>
-          checkAlternativeBinds(pat)
           args match // TODO support these patterns. Possibly using scala.quoted.util.Var
             case SeqLiteral(args, _) =>
               for arg <- args; if arg.symbol.is(Mutable) do
@@ -278,7 +266,6 @@ trait QuotesAndSplices {
             patBuf += pat1
           }
         case Apply(fn, pat :: Nil) if fn.symbol.isExprSplice =>
-          checkAlternativeBinds(pat)
           try ref(defn.QuotedRuntimePatterns_patternHole.termRef).appliedToType(tree.tpe).withSpan(tree.span)
           finally {
             val patType = pat.tpe.widen

tests/neg-macros/i14696.scala

@@ -5,7 +5,7 @@ def test(a: Expr[Any])(using Quotes): Unit = a match
   case '{ $x1: String } | '{ ${_}: Int } => // error
   case '{ $x: String } | '{ $x: Int } => // error // error
   case '{ $x1: String } | '{ ${x2: Expr[Int]} } => // error // error
-  case '{ ${Str(x)}: String } | '{ ${y @ Str(z)}: Int } => // error // error // error
+  case '{ ${Str(x)}: String } | '{ ${y @ Str(z)}: Int } => // error // error
   case '{ val x: Int = ???; ${_}(x): String } | '{ '{ val x: String = ???; ${_}(x): String }} =>
   case '{ val x: Int = ???; $f(x): String } | '{ val x: String = ???; ${_}(x): String } => // error
   case '{ val x: Int = ???; $f(x): String } | '{ val x: String = ???; $f(x): String } => // error // error