Allow case in pattern bindings even without -Xsource:3

In scala#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.

spec/06-expressions.md

@@ -912,7 +912,7 @@ A semicolon preceding the `while` symbol of a do loop expression is ignored.
 Expr1          ::=  ‘for’ (‘(’ Enumerators ‘)’ | ‘{’ Enumerators ‘}’)
                        {nl} [‘yield’] Expr
 Enumerators    ::=  Generator {semi Generator}
-Generator      ::=  Pattern1 ‘<-’ Expr {[semi] Guard | semi Pattern1 ‘=’ Expr}
+Generator      ::=  [‘case’] Pattern1 ‘<-’ Expr {[semi] Guard | semi Pattern1 ‘=’ Expr}
 Guard          ::=  ‘if’ PostfixExpr
 ```
 
@@ -922,9 +922,15 @@ A _for comprehension_ `for ($\mathit{enums}\,$) yield $e$` evaluates
 expression $e$ for each binding generated by the enumerators $\mathit{enums}$
 and collects the results. An enumerator sequence always starts with a
 generator; this can be followed by further generators, value
-definitions, or guards.  A _generator_ `$p$ <- $e$`
-produces bindings from an expression $e$ which is matched in some way
-against pattern $p$. A _value definition_ `$p$ = $e$`
+definitions, or guards.
+
+A _generator_ `$p$ <- $e$` produces bindings from an expression $e$ which is
+matched in some way against pattern $p$. Optionally, `case` can appear in front
+of a generator pattern, this has no meaning in Scala 2 but will be [required in
+Scala 3 if `p` is not
+irrefutable](https://docs.scala-lang.org/scala3/reference/changed-features/pattern-bindings.html).
+
+A _value definition_ `$p$ = $e$`
 binds the value name $p$ (or several names in a pattern $p$) to
 the result of evaluating the expression $e$.  A _guard_
 `if $e$` contains a boolean expression which restricts
@@ -1762,4 +1768,4 @@ Finally:
 
  *  `e.m(x) = y` becomes `e.selectDynamic("m").update(x, y)`
 
-None of these methods are actually defined in the `scala.Dynamic`, so that users are free to define them with or without type parameters, or implicit arguments.
+None of these methods are actually defined in the `scala.Dynamic`, so that users are free to define them with or without type parameters, or implicit arguments.

spec/13-syntax-summary.md

@@ -172,7 +172,7 @@ grammar:
                       |  (Bindings | ([‘implicit’] id | ‘_’) ‘:’ CompoundType) ‘=>’ Block
 
   Enumerators       ::=  Generator {semi Generator}
-  Generator         ::=  Pattern1 ‘<-’ Expr {[semi] Guard | semi Pattern1 ‘=’ Expr}
+  Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr {[semi] Guard | semi Pattern1 ‘=’ Expr}
 
   CaseClauses       ::=  CaseClause { CaseClause }
   CaseClause        ::=  ‘case’ Pattern [Guard] ‘=>’ Block

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

@@ -1915,16 +1915,14 @@ self =>
       else generator(!isFirst, allowNestedIf)
 
     /** {{{
-     *  Generator ::= Pattern1 (`<-' | `=') Expr [Guard]
+     *  Generator ::= [`case'] Pattern1 (`<-' | `=') Expr [Guard]
      *  }}}
      */
     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
     ^
-four errors found
+two errors found

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
 }