Allow observing an indent after conditional

Normally do not infer NEWLINE within parens,
but special case old syntax for conditionals,
so that it can observe indented syntax.
The mechanism is to inject an Indented region
when parsing a parenthesized condition which is
within an InParens region, such as an arg list.
The effect is not to advance past EOL after `(true)`.

Author: som-snytt

Diff for: 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 InBrk : Region => Region = _.match
+    case p: Scanners.InParens => Scanners.Indented(p.indentWidth, p.prefix, p)
+    case r => r
 
   def unimplementedExpr(using Context): Select =
     Select(scalaDot(nme.Predef), nme.???)
@@ -2325,8 +2328,10 @@ 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)))
+          var t: Tree =
+            inSepRegion(InBrk): // allow inferred NEWLINE for observeIndented below
+              atSpan(in.offset):
+                makeTupleOrParens(inParensWithCommas(commaSeparated(exprInParens)))
           if in.token != altToken then
             if toBeContinued(altToken) then
               t = inSepRegion(InCond) {

Diff for: 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)