Backport Refine condition of leading infix operator

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -432,18 +432,31 @@ trait Scanners extends ScannersCommon {
         token = nl
       }
 
+      /* A leading infix operator must be followed by a lexically suitable expression.
+       * Usually any simple expr will do. However, if the op is backtick style, make
+       * sure it is not followed by a binary op, which suggests the backticked identifier
+       * is a reference.
+       */
+      def followedByInfixRHS: Boolean = {
+        val current = token
+        def isOp: Boolean = token == IDENTIFIER && isOperatorPart(name.charAt(name.length - 1))
+        def isCandidateInfixRHS: Boolean =
+          isSimpleExprIntroToken(token) &&
+          (current != BACKQUOTED_IDENT || !isOp || nme.raw.isUnary(name))
+        lookingAhead {
+          isCandidateInfixRHS ||
+          token == NEWLINE && { nextToken() ; isCandidateInfixRHS }
+        }
+      }
+
       /* A leading symbolic or backquoted identifier is treated as an infix operator
        * if it is followed by at least one ' ' and a token on the same line
        * that can start an expression.
        */
       def isLeadingInfixOperator =
         allowLeadingInfixOperators &&
         (token == BACKQUOTED_IDENT || token == IDENTIFIER && isOperatorPart(name.charAt(name.length - 1))) &&
-        ch <= ' ' && lookingAhead {
-          // force a NEWLINE after current token if it is on its own line
-          isSimpleExprIntroToken(token) ||
-          token == NEWLINE && { nextToken() ; isSimpleExprIntroToken(token) }
-        }
+        ch <= ' ' && followedByInfixRHS
 
       /* Insert NEWLINE or NEWLINES if
        * - we are after a newline

test/files/neg/t12071.scala

@@ -33,4 +33,10 @@ object C {
   def `test-1`: Int = 23
   def `test-2`: Int = 42
   def compareTo(x: Int) = println("lol")
+
+  var `test-3`: List[Int] = Nil
+
+  // since ++ is not unary, test-3 is not taken as an operator; this test doesn't fix y above.
+  def yy = List.empty[Int] ++
+    `test-3` ++ `test-3`
 }

test/files/pos/i11371.scala

@@ -0,0 +1,21 @@
+// scalac: -Xsource:3
+//
+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}")
+  }
+}