Disallow : after return

Treat instead indentation as significant. I.e.

  return
    2         // one statement

  return
  println(2)  // two statements

Author: odersky

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

@@ -1971,7 +1971,6 @@ object Parsers {
         atSpan(in.skipToken()) { Throw(expr()) }
       case RETURN =>
         atSpan(in.skipToken()) {
-          colonAtEOLOpt()
           Return(if (isExprIntro) expr() else EmptyTree, EmptyTree)
         }
       case FOR =>
@@ -2240,10 +2239,6 @@ object Parsers {
         case MACRO =>
           val start = in.skipToken()
           MacroTree(simpleExpr())
-        case COLONEOL =>
-          syntaxError("':' not allowed here")
-          in.nextToken()
-          simpleExpr()
         case _ =>
           if (isLiteral) literal()
           else {

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

@@ -401,8 +401,8 @@ object Scanners {
         true
       }
 
-    def isContinuingParens() =
-      openParensTokens.contains(token)
+    def isContinuing(lastToken: Token) =
+      (openParensTokens.contains(token) || lastToken == RETURN)
       && !pastBlankLine
       && !migrateTo3
       && !noindentSyntax
@@ -504,7 +504,7 @@ object Scanners {
          && canEndStatTokens.contains(lastToken)
          && canStartStatTokens.contains(token)
          && !isLeadingInfixOperator()
-         && !(lastWidth < nextWidth && isContinuingParens())
+         && !(lastWidth < nextWidth && isContinuing(lastToken))
       then
         insert(if (pastBlankLine) NEWLINES else NEWLINE, lineOffset)
       else if indentIsSignificant then

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

@@ -273,7 +273,7 @@ object Tokens extends TokensCommon {
   final val closingRegionTokens = BitSet(RBRACE, RPAREN, RBRACKET, CASE) | statCtdTokens
 
   final val canStartIndentTokens: BitSet =
-    statCtdTokens | BitSet(COLONEOL, WITH, EQUALS, ARROW, CTXARROW, LARROW, WHILE, TRY, FOR, IF, THROW)
+    statCtdTokens | BitSet(COLONEOL, WITH, EQUALS, ARROW, CTXARROW, LARROW, WHILE, TRY, FOR, IF, THROW, RETURN)
 
   /** Faced with the choice between a type and a formal parameter, the following
    *  tokens determine it's a formal parameter.

tests/run/returns.scala

@@ -0,0 +1,16 @@
+def foo(x: Int): Int =
+  if x == 1 then
+    return
+      x
+  else
+    2
+
+def bar(): Unit =
+  return
+  assert(false)
+
+@main def Test =
+  assert(foo(1) == 1)
+  bar()
+
+