Allow indentation to work inside parens

Actually indentation was always enabled inside parens, but the rule which indentation
width to use was too simplistic and often required an extra line.

This is now fixed, so both of these work:

```scala
def test1 = g(1, x =>
    val y = x * x
    y * y
  )

def test2 = g(1,
  x =>
    val y = x * x
    y * y
  )
```

Author: odersky

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

@@ -462,7 +462,7 @@ object Scanners {
           indentPrefix = r.prefix
         case r =>
           indentIsSignificant = indentSyntax
-          if (r.knownWidth == null) r.knownWidth = nextWidth
+          r.proposeKnownWidth(nextWidth, lastToken)
           lastWidth = r.knownWidth
           newlineIsSeparating = r.isInstanceOf[InBraces]
 
@@ -1349,6 +1349,18 @@ object Scanners {
     /** The indentation width, Zero if not known */
     final def indentWidth: IndentWidth =
       if knownWidth == null then IndentWidth.Zero else knownWidth
+
+    def proposeKnownWidth(width: IndentWidth, lastToken: Token) =
+      if knownWidth == null then
+        this match
+          case InParens(_, _) if lastToken != LPAREN =>
+            useOuterWidth()
+          case _ =>
+            knownWidth = width
+
+    private def useOuterWidth(): Unit =
+      if enclosing.knownWidth == null then enclosing.useOuterWidth()
+      knownWidth = enclosing.knownWidth
   end Region
 
   case class InString(multiLine: Boolean, outer: Region) extends Region

docs/docs/reference/other-new-features/indentation.md

@@ -159,13 +159,39 @@ Indentation prefixes can consist of spaces and/or tabs. Indentation widths are t
 
 ### Indentation and Braces
 
-Indentation can be mixed freely with braces. For interpreting indentation inside braces, the following rules apply.
+Indentation can be mixed freely with braces and parentheses. For interpreting indentation inside braces and parentheses, the following rules apply.
 
  1. The assumed indentation width of a multiline region enclosed in braces is the
     indentation width of the first token that starts a new line after the opening brace.
 
- 2. On encountering a closing brace `}`, as many `<outdent>` tokens as necessary are
-    inserted to close all open indentation regions inside the pair of braces.
+ 2. The assumed indentation width of a multiline region inside parentheses is:
+
+     - if the opening parenthesis is at the end of a line, the indentation width of token following it,
+     - otherwise, the indentation width of the enclosing region.
+
+ 3. On encountering a closing brace `}` or parenthesis `)`, as many `<outdent>` tokens as necessary are
+    inserted to close all open indentation regions inside the pair of braces or parentheses.
+
+For instance, consider:
+```scala
+{
+   val x = f(x: Int, y =>
+      x * (
+         y + 1
+      ) +
+      (x +
+      x)
+   )
+}
+```
+ - Here, the indentation width of the region enclosed by the braces is 3 (i.e. the indentation width of the
+statement starting with `val`).
+ - The indentation width of the region in parentheses that follows `f` is also 3, since the opening
+   parenthesis is not at the end of a line.
+ - The indentation width of the region in parentheses around `y + 1` is 9
+   (i.e. the indentation width of `y + 1`).
+ - Finally, the indentation width of the last region in parentheses starting with `(x` is 6 (i.e. the indentation
+width of the indented region following the `=>`.
 
 ### Special Treatment of Case Clauses
 

tests/pos/indent-in-parens.scala

@@ -1,3 +1,16 @@
+def g(x: Int, op: Int => Int) = op(x)
+
+def test1 = g(1, x =>
+    val y = x * x
+    y * y
+  )
+
+def test2 = g(1,
+  x =>
+    val y = x * x
+    y * y
+  )
+
 def f(x: Int) =
   assert(
     if x > 0 then

tests/run/Course-2002-13.scala

@@ -179,7 +179,7 @@ class Parser(s: String) {
     val a = token;
     token = it.next;
     Con(a,
-	if (token equals "(") {
+  if (token equals "(") {
 	  token = it.next;
 	  val ts: List[Term] = if (token equals ")") List() else rep(term);
 	  if (token equals ")") token = it.next else syntaxError("`)` expected");