Fix #3408 Implement SIP-27: Trailing Commas

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

@@ -1961,10 +1961,10 @@ object Parsers {
 
     /** ImportSelectors ::= `{' {ImportSelector `,'} (ImportSelector | `_') `}'
      */
-    def importSelectors(): List[Tree] =
-      if (in.token == RBRACE) Nil
+    def importSelectors(): List[Tree] = {
+      val sel = importSelector()
+      if (in.token == RBRACE) sel :: Nil
       else {
-        val sel = importSelector()
         sel :: {
           if (!isWildcardArg(sel) && in.token == COMMA) {
             in.nextToken()
@@ -1973,7 +1973,7 @@ object Parsers {
           else Nil
         }
       }
-
+    }
    /** ImportSelector ::= id [`=>' id | `=>' `_']
      */
     def importSelector(): Tree = {

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

@@ -390,7 +390,16 @@ object Scanners {
         val nextLastOffset = lastCharOffset
         lookahead()
         if (token != ELSE) reset(nextLastOffset)
+      } else if (token == COMMA){
+        val nextLastOffset = lastCharOffset
+        lookahead()
+        if (isAfterLineEnd() && (token == RPAREN || token == RBRACKET || token == RBRACE)) {
+          /* skip the trailing comma */
+        } else if (token == EOF) { // e.g. when the REPL is parsing "val List(x, y, _*,"
+          /* skip the trailing comma */
+        } else reset(nextLastOffset)
       }
+
     }
 
     /** Is current token first one after a newline? */

tests/neg/trailingCommas/trailingCommas.scala

@@ -0,0 +1,58 @@
+package foo
+
+// Multi-line only cases: make sure trailing commas are only supported when multi-line
+
+trait ArgumentExprs1 { validMethod(23, "bar", )(Ev0, Ev1) } // error // error
+trait ArgumentExprs2 { validMethod(23, "bar")(Ev0, Ev1, ) } // error // error
+trait ArgumentExprs3 { new ValidClass(23, "bar", )(Ev0, Ev1) } // error // error
+trait ArgumentExprs4 { new ValidClass(23, "bar")(Ev0, Ev1, ) } // error // error
+
+trait Params1 { def f(foo: Int, bar: String, )(implicit ev0: Ev0, ev1: Ev1, ) = 1 } // error // error // error
+
+trait Params2 { def f(foo: Int, bar: String, )(implicit ev0: Ev0, ev1: Ev1, ) = 1 } // error // error // error
+
+trait ClassParams1 { final class C(foo: Int, bar: String, )(implicit ev0: Ev0, ev1: Ev1) } // error
+trait ClassParams2 { final class C(foo: Int, bar: String)(implicit ev0: Ev0, ev1: Ev1, ) } // error
+
+trait SimpleExpr { (23, "bar", ) } // error
+trait TypeArgs { def f: ValidGeneric[Int, String, ] } // error
+
+trait TypeParamClause { type C[A, B, ] } // error
+trait FunTypeParamClause { def f[A, B, ] } // error
+
+trait SimpleType { def f: (Int, String, ) } // error
+trait FunctionArgTypes { def f: (Int, String, ) => Boolean } // error
+
+trait SimplePattern { val (foo, bar, ) = null: Any } // error
+
+trait ImportSelectors { import foo.{ Ev0, Ev1, } } // error
+
+trait Import { import foo.Ev0, foo.Ev1, } // error
+
+trait ValDcl { val foo, bar, = 23 } // error
+
+trait VarDef { var foo, bar, = _ } // error
+
+trait PatDef { val Foo(foo), Bar(bar), = bippy } // error
+
+
+// The Tuple 1 cases
+
+// the Tuple1 value case: make sure that the possible "(23, )" syntax for Tuple1 doesn't compile to "23"
+trait SimpleExpr2 { (23, ) } // error
+
+// the Tuple1 type case: make sure that the possible "(Int, )" syntax for Tuple1[Int] doesn't compile to "Int"
+trait SimpleType2 { def f: (Int, ) } // error
+
+// Test utilities
+object `package` {
+  sealed trait Ev0; implicit object Ev0 extends Ev0
+  sealed trait Ev1; implicit object Ev1 extends Ev1
+
+  class ValidClass(foo: Int, bar: String)(implicit ev0: Ev0, ev1: Ev1)
+  class ValidGeneric[A, B]
+  def validMethod(foo: Int, bar: String)(implicit ev0: Ev0, ev1: Ev1): Int = 1
+
+  case class Foo(foo: Any)
+  case class Bar(foo: Any)
+}

tests/pos/trailingCommas/trailingCommas.scala

@@ -0,0 +1,155 @@
+package foo
+
+trait ArgumentExprs1 {
+  def f(foo: Int, bar: String)(implicit ev0: Ev0, ev1: Ev1) = 1
+  f(
+    23,
+    "bar",
+  )(
+    Ev0,
+    Ev1,
+  )
+
+  // test arg exprs in the presence of varargs
+  def g(x: Int, y: Int*) = 1
+  g(1,2,
+  )
+  g(1,List(2, 3): _*,
+  )
+}
+
+trait ArgumentExprs2 {
+  class C(foo: Int, bar: String)(implicit ev0: Ev0, ev1: Ev1)
+  new C(
+    23,
+    "bar",
+  )(
+    Ev0,
+    Ev1,
+  )
+}
+
+trait Params {
+  def f(
+         foo: Int,
+         bar: String,
+       )(implicit
+         ev0: Ev0,
+         ev1: Ev1,
+       ): Unit
+}
+
+trait ClassParams {
+  class C(
+           foo: Int,
+           bar: String,
+         )(implicit
+           ev0: Ev0,
+           ev1: Ev1,
+         )
+
+  // test class params in the precense of varargs
+  case class D(i: Int*,
+              )
+}
+
+trait SimpleExpr1 {
+  def f: (Int, String) = (
+    23,
+    "bar",
+  )
+
+  // the Tuple1 value case, the trailing comma is ignored so the type is Int and the value 23
+  def g: Int = (
+    23,
+    )
+}
+
+trait TypeArgs {
+  class C[A, B]
+  def f: C[
+    Int,
+    String,
+    ]
+}
+
+trait TypeParamClause {
+  class C[
+  A,
+  B,
+  ]
+}
+
+trait FunTypeParamClause {
+  def f[
+  A,
+  B,
+  ]: Unit
+}
+
+trait SimpleType {
+  def f: (
+    Int,
+      String,
+    )
+
+  // the Tuple1 type case, the trailing comma is ignored so the type is Int and the value 23
+  def g: (
+    Int,
+    ) = 23
+}
+
+trait FunctionArgTypes {
+  def f: (
+    Int,
+      String,
+    ) => Boolean
+}
+
+trait SimplePattern {
+  val (
+    foo,
+    bar,
+    ) = null: Any
+
+  // test '@' syntax in patterns
+  Some(1) match {
+    case Some(x @ 1,
+    ) => x
+  }
+
+  // test ': _*' syntax in patterns
+  List(1, 2, 3) match {
+    case List(1, 2, x : _*,
+    ) => 1
+  }
+
+  // test varargs in patterns
+  val List(x, y, z: _*,
+  ) = 42 :: 17 :: Nil
+}
+
+trait ImportSelectors {
+  import foo.{
+    Ev0,
+    Ev1,
+  }
+}
+
+trait Bindings {
+  def g(f: (Int, String) => Boolean): Unit
+
+  g((
+      foo,
+      bar,
+    ) => true)
+}
+
+// Import, ids, ValDcl, VarDcl, VarDef, PatDef use commas, but not inside paren, bracket or brace,
+// so they don't support an optional trailing comma
+
+// test utilities
+object `package` {
+  sealed trait Ev0; implicit object Ev0 extends Ev0
+  sealed trait Ev1; implicit object Ev1 extends Ev1
+}

tests/run/trailingCommas/trailingCommas.check

@@ -0,0 +1,2 @@
+42
+17

tests/run/trailingCommas/trailingCommas.scala

@@ -0,0 +1,8 @@
+object Test {
+  val List(x, y, z: _ *,
+  ) = 42 :: 17 :: Nil
+  def main(args: Array[String]): Unit = {
+    Console.println(x)
+    Console.println(y)
+  }
+}