Keep old rules under scala-2 compatibility mode

Issue a migration warning where the handling of braces changes

Author: odersky

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

@@ -1278,6 +1278,17 @@ object Parsers {
       if (in.token == COLONEOL) in.nextToken()
     }
 
+    def argumentStart(): Unit =
+      colonAtEOLOpt()
+      if in.isScala2CompatMode && in.isNewLine && in.next.token == LBRACE then
+        in.nextToken()
+        if in.indentWidth(in.offset) == in.currentRegion.indentWidth then
+          in.errorOrMigrationWarning(
+            i"""This opening brace will start a new statement in Scala 3.
+               |It needs to be indented to the right to keep being treated as
+               |an argument to the previous expression.${rewriteNotice()}""")
+          patch(source, Span(in.offset), "  ")
+
     def possibleTemplateStart(isNew: Boolean = false): Unit =
       in.observeColonEOL()
       if in.token == COLONEOL then
@@ -1468,7 +1479,7 @@ object Parsers {
     val refinedType: () => Tree = () => refinedTypeRest(withType())
 
     def refinedTypeRest(t: Tree): Tree = {
-      colonAtEOLOpt()
+      argumentStart()
       if (in.isNestedStart)
         refinedTypeRest(atSpan(startOffset(t)) { RefinedTypeTree(rejectWildcardType(t), refinement()) })
       else t
@@ -2222,7 +2233,7 @@ object Parsers {
     }
 
     def simpleExprRest(t: Tree, canApply: Boolean = true): Tree = {
-      if canApply then colonAtEOLOpt()
+      if (canApply) argumentStart()
       in.token match {
         case DOT =>
           in.nextToken()
@@ -2298,7 +2309,7 @@ object Parsers {
     /** ArgumentExprss ::= {ArgumentExprs}
      */
     def argumentExprss(fn: Tree): Tree = {
-      colonAtEOLOpt()
+      argumentStart()
       if (in.token == LPAREN || in.isNestedStart) argumentExprss(mkApply(fn, argumentExprs()))
       else fn
     }
@@ -3323,7 +3334,7 @@ object Parsers {
      */
     def selfInvocation(): Tree =
       atSpan(accept(THIS)) {
-        colonAtEOLOpt()
+        argumentStart()
         argumentExprss(mkApply(Ident(nme.CONSTRUCTOR), argumentExprs()))
       }
 

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

@@ -436,7 +436,7 @@ object Scanners {
       }
 
     def isContinuingParens() =
-      openParensTokens.contains(token) && !pastBlankLine && !isScala2Mode
+      openParensTokens.contains(token) && !pastBlankLine && !isScala2CompatMode
 
     /** The indentation width of the given offset */
     def indentWidth(offset: Offset): IndentWidth = {

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -129,6 +129,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/neg-custom-args/i3246.scala", scala2CompatMode),
       compileFile("tests/neg-custom-args/overrideClass.scala", scala2CompatMode),
       compileFile("tests/neg-custom-args/ovlazy.scala", scala2CompatMode.and("-migration", "-Xfatal-warnings")),
+      compileFile("tests/neg-custom-args/newline-braces.scala", scala2CompatMode.and("-migration", "-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/autoTuplingTest.scala", defaultOptions.andLanguageFeature("noAutoTupling")),
       compileFile("tests/neg-custom-args/nopredef.scala", defaultOptions.and("-Yno-predef")),
       compileFile("tests/neg-custom-args/noimports.scala", defaultOptions.and("-Yno-imports")),

tests/neg-custom-args/newline-braces.check

@@ -0,0 +1,7 @@
+-- Error: tests/neg-custom-args/newline-braces.scala:3:2 ---------------------------------------------------------------
+3 |  { x =>    // error (migration)
+  |  ^
+  |  This opening brace will start a new statement in Scala 3.
+  |  It needs to be indented to the right to keep being treated as
+  |  an argument to the previous expression.
+  |  This construct can be rewritten automatically under -rewrite.

tests/neg-custom-args/newline-braces.scala

@@ -0,0 +1,6 @@
+def f: List[Int] = {
+  List(1, 2, 3).map  // no newline inserted here in Scala-2 compat mode
+  { x =>    // error (migration)
+    x + 1
+  }
+}

tests/pos-scala2/indented-parens.scala

@@ -0,0 +1,13 @@
+val (x, y) =
+  try {
+    ???
+    (1, 2)
+  }
+  catch {
+    case e if !true =>
+      val x = 2
+      val foo = e match {
+        case e: java.io.IOException => ???
+      }
+      (1, 2)
+  }

tests/pos-scala2/newline-braces.scala

@@ -0,0 +1,6 @@
+def f: List[Int] = {
+  List(1, 2, 3).map  // no newline inserted here in Scala-2 compat mode
+  { x =>
+    x + 1
+  }
+}