From 09b5ded130587e5336a0686e1b24a6536580c7e3 Mon Sep 17 00:00:00 2001
From: Guillaume Martres <smarter@ubuntu.com>
Date: Tue, 30 Mar 2021 14:15:08 +0200
Subject: [PATCH] Support `case` in pattern bindings under -Xsource:3

Just like in Scala 3.0, adding this keyword doesn't change anything, but
it will be required in future versions of Scala 3 for non-exhaustive
patterns in a for comprehension. We would like to start issuing warnings
by default in Scala 3 for code which does not use `case` in those
situations, but to not hamper cross-compilation we need Scala 2 to also
support that keyword.

For details, see:
https://dotty.epfl.ch/docs/reference/changed-features/pattern-bindings.html
---
 .../scala/tools/nsc/ast/parser/Parsers.scala  |  8 ++++++-
 .../scala/tools/nsc/ast/parser/Scanners.scala | 10 ++++++++
 .../neg/for-comprehension-case-future.check   |  7 ++++++
 .../neg/for-comprehension-case-future.scala   | 24 +++++++++++++++++++
 test/files/neg/for-comprehension-case.check   | 13 ++++++++++
 test/files/neg/for-comprehension-case.scala   | 14 +++++++++++
 6 files changed, 75 insertions(+), 1 deletion(-)
 create mode 100644 test/files/neg/for-comprehension-case-future.check
 create mode 100644 test/files/neg/for-comprehension-case-future.scala
 create mode 100644 test/files/neg/for-comprehension-case.check
 create mode 100644 test/files/neg/for-comprehension-case.scala

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index 055ed8c8fb39..7866f6d40b5d 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -1926,6 +1926,12 @@ 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.")
+        in.skipCASE()
+      }
+
       val hasVal = in.token == VAL
       if (hasVal)
         in.nextToken()
@@ -1944,7 +1950,7 @@ self =>
         else syntaxError(in.offset, msg("unsupported", "just remove `val`"))
       }
 
-      if (hasEq && eqOK) in.nextToken()
+      if (hasEq && eqOK && !hasCase) in.nextToken()
       else accept(LARROW)
       val rhs = expr()
 
diff --git a/src/compiler/scala/tools/nsc/ast/parser/Scanners.scala b/src/compiler/scala/tools/nsc/ast/parser/Scanners.scala
index a4f8efc43eea..0f41d3903c16 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Scanners.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Scanners.scala
@@ -393,6 +393,16 @@ trait Scanners extends ScannersCommon {
       case _ =>
     }
 
+    /** Advance beyond a case token without marking the CASE in sepRegions.
+     *  This method should be called to skip beyond CASE tokens that are
+     *  not part of matches, i.e. no ARROW is expected after them.
+     */
+    def skipCASE(): Unit = {
+      assert(token == CASE, s"Internal error: skipCASE() called on non-case token $token")
+      nextToken()
+      sepRegions = sepRegions.tail
+    }
+
     /** Produce next token, filling TokenData fields of Scanner.
      */
     def nextToken(): Unit = {
diff --git a/test/files/neg/for-comprehension-case-future.check b/test/files/neg/for-comprehension-case-future.check
new file mode 100644
index 000000000000..9ce9a9456882
--- /dev/null
+++ b/test/files/neg/for-comprehension-case-future.check
@@ -0,0 +1,7 @@
+for-comprehension-case-future.scala:22: error: '<-' expected but '=' found.
+      case y = x + 1
+             ^
+for-comprehension-case-future.scala:23: error: illegal start of simple expression
+    } yield x + y
+    ^
+2 errors
diff --git a/test/files/neg/for-comprehension-case-future.scala b/test/files/neg/for-comprehension-case-future.scala
new file mode 100644
index 000000000000..05602e537759
--- /dev/null
+++ b/test/files/neg/for-comprehension-case-future.scala
@@ -0,0 +1,24 @@
+// scalac: -Xsource:3
+//
+class A {
+  // ok
+  val a =
+    for {
+      case Some(x) <- List(Some(1), None)
+      y = x + 1
+    } yield x + y
+
+  // ok
+  val b =
+    for {
+      Some(x) <- List(Some(1), None)
+      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
+}
diff --git a/test/files/neg/for-comprehension-case.check b/test/files/neg/for-comprehension-case.check
new file mode 100644
index 000000000000..2e86e5d367b0
--- /dev/null
+++ b/test/files/neg/for-comprehension-case.check
@@ -0,0 +1,13 @@
+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.
+      case y = x + 1
+             ^
+for-comprehension-case.scala:13: error: illegal start of simple expression
+    } yield x+y
+    ^
+4 errors
diff --git a/test/files/neg/for-comprehension-case.scala b/test/files/neg/for-comprehension-case.scala
new file mode 100644
index 000000000000..55e8d44a40e3
--- /dev/null
+++ b/test/files/neg/for-comprehension-case.scala
@@ -0,0 +1,14 @@
+class A {
+  // fail
+  val a =
+    for {
+      case Some(x) <- List(Some(1), None)
+    } yield x
+
+  // fail
+  val b =
+    for {
+      Some(x) <- List(Some(1), None)
+      case y = x + 1
+    } yield x+y
+}