Make indentation significant in old-style control syntax

Indentation was significant only after new-style if-then, while-do, for-yield, for-do.
This was prone to cause gotchas. We now treat indentation as significant also if the
conditions or enumerators of these constructs use parens or braces.

Author: odersky

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

@@ -1636,8 +1636,10 @@ object Parsers {
           if (rewriteToOldSyntax()) revertToParens(t)
           in.nextToken()
         }
-        else if (rewriteToNewSyntax(t.span))
-          dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
+        else
+          in.observeIndented()
+          if (rewriteToNewSyntax(t.span))
+            dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
         t
       }
       else {
@@ -2296,6 +2298,7 @@ object Parsers {
                 dropParensOrBraces(start, if (in.token == YIELD || in.token == DO) "" else "do")
               }
             }
+            in.observeIndented()
             res
           }
           else {

tests/pos/indent4.scala

@@ -0,0 +1,24 @@
+object testindent
+
+  if (true)
+    val x = 1
+    println(x)
+
+  while (false)
+    val x = 1
+    println(x)
+
+  for (x <- List(1, 2, 3))
+    val y = x
+    println(y)
+
+  for { x <- List(1, 2, 3) }
+    val y = x
+    println(y)
+
+  for {
+    x <- List(1, 2, 3)
+  }
+    val y = x
+    println(y)
+