Revert to Scala 2 rules for handling opening brace on next line

Fixes #16760

Author: odersky

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

@@ -1353,15 +1353,6 @@ object Parsers {
 
     def argumentStart(): Unit =
       colonAtEOLOpt()
-      if migrateTo3 && in.token == NEWLINE && in.next.token == LBRACE then
-        in.nextToken()
-        if in.indentWidth(in.offset) == in.currentRegion.indentWidth then
-          report.errorOrMigrationWarning(
-            em"""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()}""",
-            in.sourcePos(), from = `3.0`)
-          patch(source, Span(in.offset), "  ")
 
     def possibleTemplateStart(isNew: Boolean = false): Unit =
       in.observeColonEOL(inTemplate = true)

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

@@ -566,19 +566,20 @@ object Scanners {
        *   - current line starts with a token that can start a statement
        *   - current line does not start with a leading infix operator
        *  The answer is different for Scala-2 and Scala-3.
-       *   - In Scala 2: Only `{` is treated as continuing, irrespective of indentation.
-       *     But this is in fact handled by Parser.argumentStart which skips a NEWLINE,
-       *     so we always assume false here.
-       *   - In Scala 3: Only indented statements are treated as continuing, as long as
-       *     they start with `(`, `[` or `{`, or the last statement ends in a `return`.
+       *   - In Scala 2: Only `{` is treated as continuing, irrespective of indentation,
+       *     but not if it follows a `}`.
+       *   - In Scala 3: In addition, indented statements are treated as continuing, as long as
+       *     they start with `(`, or `[`, or the last statement ends in a `return`.
        *   The Scala 2 rules apply under source `3.0-migration` or under `-no-indent`.
        */
       inline def isContinuing =
-        lastWidth < nextWidth
-        && (openParensTokens.contains(token) || lastToken == RETURN)
+        ( token == LBRACE && lastToken != RBRACE
+          || lastWidth < nextWidth
+              && (token == LPAREN || token == LBRACKET || lastToken == RETURN)
+              && !migrateTo3
+              && !noindentSyntax
+        )
         && !pastBlankLine
-        && !migrateTo3
-        && !noindentSyntax
 
       currentRegion match
         case r: Indented =>

compiler/src/dotty/tools/dotc/reporting/trace.scala

@@ -55,7 +55,7 @@ trait TraceSyntax:
     apply(question, printer, {
       val showOp: T => String = inline if show == true then
         val showT = summonInline[Show[T]]
-        {
+        locally {
           given Show[T] = showT
           t => i"$t"
         }

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

@@ -149,7 +149,6 @@ class CompilationTests {
       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("-Xfatal-warnings")),
-      compileFile("tests/neg-custom-args/newline-braces.scala", scala2CompatMode.and("-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/autoTuplingTest.scala", defaultOptions.andLanguageFeature("noAutoTupling")),
       compileFile("tests/neg-custom-args/i1650.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes),

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

@@ -17,7 +17,7 @@ object Test {
 
   def i: Unit = {
     f("Foo")
-    {}    // ok, error in Scala 2
+    {}    // error, error in Scala 2
   }
 
   def j: Int = {

tests/neg/i12554a.scala

@@ -0,0 +1,13 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    def process(arg: Int)(block: => Unit): Int = arg
+    val x = process(1)
+    {
+    }
+    println(x)
+    val y = process(2)
+      ( ()
+      )
+    println(y)
+  }
+}

tests/new/i16760.scala

@@ -55,7 +55,7 @@ trait TraceSyntax:
     apply(question, printer, {
       val showOp: T => String = inline if show == true then
         val showT = summonInline[Show[T]]
-        {
+        locally {
           given Show[T] = showT
           t => i"$t"
         }