Allow observing an indent after conditional

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

@@ -105,6 +105,9 @@ object Parsers {
   private val InCase: Region => Region = Scanners.InCase(_)
   private val InCond: Region => Region = Scanners.InParens(LPAREN, _)
   private val InFor : Region => Region = Scanners.InBraces(_)
+  private val InOldCond: Region => Region = // old-style Cond to allow indent when InParens, see #22608
+    case p: Scanners.InParens => Scanners.Indented(p.indentWidth, p.prefix, p)
+    case r => r
 
   def unimplementedExpr(using Context): Select =
     Select(scalaDot(nme.Predef), nme.???)
@@ -2325,25 +2328,25 @@ object Parsers {
     def condExpr(altToken: Token): Tree =
       val t: Tree =
         if in.token == LPAREN then
-          var t: Tree = atSpan(in.offset):
-            makeTupleOrParens(inParensWithCommas(commaSeparated(exprInParens)))
-          if in.token != altToken then
-            if toBeContinued(altToken) then
-              t = inSepRegion(InCond) {
+          inSepRegion(InOldCond): // allow inferred NEWLINE for observeIndented below
+            atSpan(in.offset):
+              makeTupleOrParens(inParensWithCommas(commaSeparated(exprInParens)))
+          .pipe: t =>
+            if in.token == altToken then t
+            else if toBeContinued(altToken) then
+              inSepRegion(InCond):
                 expr1Rest(
                   postfixExprRest(
                     simpleExprRest(t, Location.ElseWhere),
                     Location.ElseWhere),
                   Location.ElseWhere)
-              }
             else
               if rewriteToNewSyntax(t.span) then
-                dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
+                dropParensOrBraces(t.span.start, tokenString(altToken))
               in.observeIndented()
               return t
-          t
         else if in.isNestedStart then
-          try expr() finally newLinesOpt()
+          expr().tap(_ => newLinesOpt())
         else
           inSepRegion(InCond)(expr())
       if rewriteToOldSyntax(t.span.startPos) then revertToParens(t)

tests/pos/i22608.scala

@@ -0,0 +1,48 @@
+
+def f(i: Int) = i
+def g(i: Int, j: Int) = i+j
+
+def t =
+  val y = f(
+    if (true)// then
+      val x = 1
+      5
+    else 7
+  )
+  y
+
+def u(j: Int) =
+  val y = g(
+    if (true)// then
+      val x = 1
+      5
+    else 7,
+    j
+  )
+  y
+
+def b(k: Boolean): Int =
+  f(
+    if (
+       k
+    && b(!k) > 0
+    ) then 27
+    else 42
+  )
+
+def p(b: Boolean) =
+  import collection.mutable.ListBuffer
+  val xs, ys = ListBuffer.empty[String]
+  (if (b)
+    xs
+  else
+    ys) += "hello, world"
+  (xs.toString, ys.toString)
+
+def q(b: Boolean) =
+  import collection.mutable.ListBuffer
+  val xs, ys = ListBuffer.empty[String]
+  (if (b)
+   then xs
+   else ys) += "hello, world"
+  (xs.toString, ys.toString)