Don't assume leading infix if next line is indented less

Don't assume that an operator `op` is a leading infix operator if
`op` appears on its own line, and next line is indented less.

Fixes #12395

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -369,6 +369,8 @@ object Scanners {
       *   - it does not follow a blank line, and
       *   - it is followed by at least one whitespace character and a
       *     token that can start an expression.
+      *   - if the operator appears on its own line, the next line must have at least
+      *     the same indentation width as the operator. See pos/i12395 for a test where this matters.
       *  If a leading infix operator is found and the source version is `3.0-migration`, emit a change warning.
       */
     def isLeadingInfixOperator(nextWidth: IndentWidth = indentWidth(offset), inConditional: Boolean = true) =
@@ -397,7 +399,9 @@ object Scanners {
           // force a NEWLINE a after current token if it is on its own line
         lookahead.nextToken()
         assumeStartsExpr(lookahead)
-        || lookahead.token == NEWLINE && assumeStartsExpr(lookahead.next)
+        || lookahead.token == NEWLINE
+           && assumeStartsExpr(lookahead.next)
+           && indentWidth(offset) <= indentWidth(lookahead.next.offset)
       }
       && {
         currentRegion match

Diff for: docs/docs/reference/changed-features/operators.md

@@ -128,10 +128,12 @@ would have treated the continuations `++ " world"` or `|| xs.isEmpty` as separat
 
 To make this syntax work, the rules are modified to not infer semicolons in front of leading infix operators.
 A _leading infix operator_ is
- - a symbolic identifier such as `+`, or `approx_==`, or an identifier in backticks,
- - that starts a new line,
- - that precedes a token on the same or the next line that can start an expression,
- - and that is immediately followed by at least one whitespace character.
+ - a symbolic identifier such as `+`, or `approx_==`, or an identifier in backticks that
+ - starts a new line, and
+ - is not following a blank line, and
+ - is followed by at least one whitespace character and a token that can start an expression.
+ - Furthermore, if the operator appears on its own line, the next line must have at least
+   the same indentation width as the operator.
 
 Example:
 

Diff for: tests/pos/i12395.scala

@@ -0,0 +1,9 @@
+@main def main : Unit =
+  val x = 1
+
+  val y = x match
+    case 1 => 1
+    case _ =>
+      println("bad")
+      ???
+  println(x)