allow old syntax when source version < 3.2

Author: griggt

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

@@ -521,9 +521,9 @@ object desugar {
         val enumCompanionRef = TermRefTree()
         val enumImport =
           Import(enumCompanionRef, enumCases.flatMap(caseIds).map(
-            enumCase => 
+            enumCase =>
               ImportSelector(enumCase.withSpan(enumCase.span.startPos))
-            ) 
+            )
           )
         (enumImport :: enumStats, enumCases, enumCompanionRef)
       }
@@ -1134,7 +1134,7 @@ object desugar {
       val matchExpr =
         if (tupleOptimizable) rhs
         else
-          val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids)) 
+          val caseDef = CaseDef(pat, EmptyTree, makeTuple(ids))
           Match(makeSelector(rhs, MatchCheck.IrrefutablePatDef), caseDef :: Nil)
       vars match {
         case Nil if !mods.is(Lazy) =>
@@ -1155,11 +1155,11 @@ object desugar {
           val restDefs =
             for (((named, tpt), n) <- vars.zipWithIndex if named.name != nme.WILDCARD)
             yield
-              if mods.is(Lazy) then 
+              if mods.is(Lazy) then
                 DefDef(named.name.asTermName, Nil, tpt, selector(n))
                   .withMods(mods &~ Lazy)
                   .withSpan(named.span)
-              else 
+              else
                 valDef(
                   ValDef(named.name.asTermName, tpt, selector(n))
                     .withMods(mods)
@@ -1582,7 +1582,7 @@ object desugar {
         if gen.checkMode == GenCheckMode.FilterAlways then // pattern was prefixed by `case`
           false
         else
-          gen.checkMode == GenCheckMode.Ignore
+          gen.checkMode != GenCheckMode.FilterNow
           || isVarBinding(gen.pat)
           || isIrrefutable(gen.pat, gen.expr)
 

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

@@ -168,6 +168,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   enum GenCheckMode {
     case Ignore       // neither filter nor check since filtering was done before
     case Check        // check that pattern is irrefutable
+    case FilterNow    // filter out non-matching elements if we are not in 3.2 or later
     case FilterAlways // filter out non-matching elements since pattern is prefixed by `case`
   }
 

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2516,7 +2516,8 @@ object Parsers {
       atSpan(startOffset(pat), accept(LARROW)) {
         val checkMode =
           if casePat then GenCheckMode.FilterAlways
-          else GenCheckMode.Check
+          else if sourceVersion.isAtLeast(`3.2`) && !sourceVersion.isMigrating then GenCheckMode.Check
+          else GenCheckMode.FilterNow  // filter on source version < 3.2, for backward compat
         GenFrom(pat, subExpr(), checkMode)
       }
 

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

@@ -814,6 +814,7 @@ trait Checking {
     def check(pat: Tree, pt: Type): Boolean = (pt <:< pat.tpe) || fail(pat, pt)
 
     def recur(pat: Tree, pt: Type): Boolean =
+      !(sourceVersion.isAtLeast(`3.2`) && !sourceVersion.isMigrating) ||
       pt.hasAnnotation(defn.UncheckedAnnot) || {
         patmatch.println(i"check irrefutable $pat: ${pat.tpe} against $pt")
         pat match {