Better disambiguation of leading operators

Classify something as a leading operator only if it is not followed in turn
by an operator. This means that

```scala
acc += (i * `i'`)
`i'` += 1
```
the `i'` is no longer classified as a leading operator.

Author: odersky

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

@@ -335,6 +335,9 @@ object Scanners {
       this.token = token
     }
 
+    def isOperator =
+      token == IDENTIFIER && isOperatorPart(name(name.length - 1))
+
     /** A leading symbolic or backquoted identifier is treated as an infix operator if
       *   - it does not follow a blank line, and
       *   - it is followed by at least one whitespace character and a
@@ -343,17 +346,17 @@ object Scanners {
       */
     def isLeadingInfixOperator(inConditional: Boolean = true) =
       allowLeadingInfixOperators
-      && (  token == BACKQUOTED_IDENT
-         || token == IDENTIFIER && isOperatorPart(name(name.length - 1)))
+      && (token == BACKQUOTED_IDENT || isOperator)
       && (isWhitespace(ch) || ch == LF)
       && !pastBlankLine
       && {
         val lookahead = LookaheadScanner()
         lookahead.allowLeadingInfixOperators = false
           // force a NEWLINE a after current token if it is on its own line
         lookahead.nextToken()
-        canStartExprTokens.contains(lookahead.token)
-        || lookahead.token == NEWLINE && canStartExprTokens.contains(lookahead.next.token)
+        (canStartExprTokens.contains(lookahead.token)
+         || lookahead.token == NEWLINE && canStartExprTokens.contains(lookahead.next.token)
+        ) && !lookahead.isOperator
       }
       && {
         if migrateTo3 then

tests/pos/i11371.scala

@@ -0,0 +1,19 @@
+object HelloWorld {
+  def whileLoop: Int = {
+    var i = 0
+    var acc = 0
+    while (i < 3) {
+      var `i'` = 0
+      while (`i'` < 4) {
+        acc += (i * `i'`)
+        `i'` += 1
+      }
+      i += 1
+    }
+    acc
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(s"hello world: ${whileLoop}")
+  }
+}