error for refutable for-generator patterns in 3.4

Author: bishabosha

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

@@ -2771,7 +2771,7 @@ object Parsers {
       atSpan(startOffset(pat), accept(LARROW)) {
         val checkMode =
           if casePat then GenCheckMode.FilterAlways
-          else if sourceVersion.isAtLeast(`future`) then GenCheckMode.Check
+          else if sourceVersion.isAtLeast(`3.4`) then GenCheckMode.Check
           else if sourceVersion.isAtLeast(`3.2`) then GenCheckMode.CheckAndFilter
           else GenCheckMode.FilterNow  // filter on source version < 3.2, for backward compat
         GenFrom(pat, subExpr(), checkMode)

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

@@ -923,7 +923,13 @@ trait Checking {
               |
               |If $usage is intentional, this can be communicated by $fix,
               |which $addendum.$rewriteMsg"""),
-        pos, warnFrom = `3.2`, errorFrom = `future`)
+        pos,
+        warnFrom = `3.2`,
+        // we tighten for-comprehension without `case` to error in 3.4,
+        // but we keep pat-defs as warnings for now ("@unchecked"),
+        // until we propose an alternative way to assert exhaustivity to the typechecker.
+        errorFrom = if isPatDef then `future` else `3.4`
+      )
       false
     }