Allow case in pattern bindings even without -Xsource:3

In #9558 (which shipped with 2.13.6) we added support for `case` bindings
under -Xsource:3. Since this parser change does not break any existing
code and since IntelliJ and scalameta/metals now understand this syntax
in Scala 2 code, it should be safe to enable it by default to further
ease cross-compilation between Scala 2 and 3.

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -1999,10 +1999,8 @@ self =>
     def generator(eqOK: Boolean, allowNestedIf: Boolean = true): List[Tree] = {
       val start  = in.offset
       val hasCase = in.token == CASE
-      if (hasCase) {
-        if (!currentRun.isScala3) syntaxError(in.offset, s"`case` keyword in for comprehension requires the -Xsource:3 flag.")
+      if (hasCase)
         in.skipCASE()
-      }
 
       val hasVal = in.token == VAL
       if (hasVal)

test/files/neg/for-comprehension-case.check

@@ -1,13 +1,7 @@
-for-comprehension-case.scala:5: error: `case` keyword in for comprehension requires the -Xsource:3 flag.
-      case Some(x) <- List(Some(1), None)
-      ^
-for-comprehension-case.scala:12: error: `case` keyword in for comprehension requires the -Xsource:3 flag.
-      case y = x + 1
-      ^
-for-comprehension-case.scala:12: error: '<-' expected but '=' found.
+for-comprehension-case.scala:20: error: '<-' expected but '=' found.
       case y = x + 1
              ^
-for-comprehension-case.scala:13: error: illegal start of simple expression
-    } yield x+y
+for-comprehension-case.scala:21: error: illegal start of simple expression
+    } yield x + y
     ^
-4 errors
+2 errors

test/files/neg/for-comprehension-case.scala

@@ -1,14 +1,22 @@
 class A {
-  // fail
+  // ok
   val a =
     for {
       case Some(x) <- List(Some(1), None)
-    } yield x
+      y = x + 1
+    } yield x + y
 
-  // fail
+  // ok
   val b =
     for {
       Some(x) <- List(Some(1), None)
-      case y = x + 1
+      Some(y) <- List(None, Some(2))
     } yield x+y
+
+  // fail
+  val c =
+    for {
+      case Some(x) <- List(Some(1), None)
+      case y = x + 1
+    } yield x + y
 }