Better disambiguation of leading operators

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

@@ -77,6 +77,9 @@ object Scanners {
 
     /** Is current token first one after a newline? */
     def isAfterLineEnd: Boolean = lineOffset >= 0
+
+    def isOperator =
+      token == IDENTIFIER && isOperatorPart(name(name.length - 1))
   }
 
   abstract class ScannerCommon(source: SourceFile)(using Context) extends CharArrayReader with TokenData {
@@ -348,12 +351,27 @@ object Scanners {
       && (isWhitespace(ch) || ch == LF)
       && !pastBlankLine
       && {
+        // Is current lexeme  assumed to start an expression?
+        // This is the case if the lexime is one of the tokens that
+        // starts an expression. Furthermore, if the previous token is
+        // in backticks, the lexeme may not be a binary operator.
+        // I.e. in
+        //
+        //   a
+        //   `x` += 1
+        //
+        // `+=` is not assumed to start an expression since it follows an identifier
+        // in backticks and is a binary operator. Hence, `x` is not classified as a
+        // leading infix operator.
+        def assumeStartsExpr(lexeme: TokenData) =
+          canStartExprTokens.contains(lexeme.token)
+          && (token != BACKQUOTED_IDENT || !lexeme.isOperator || nme.raw.isUnary(lexeme.name))
         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)
+        assumeStartsExpr(lookahead)
+        || lookahead.token == NEWLINE && assumeStartsExpr(lookahead.next)
       }
       && {
         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}")
+  }
+}

tests/run/i7031.scala

@@ -2,8 +2,7 @@
   val a = 5
   val x = 1
 
-    + //
-    `a` * 6
+    + `a` * 6
 
   assert(x == 1, x)
 }