Backport Allow infix operators on their own line

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

@@ -438,11 +438,11 @@ trait Scanners extends ScannersCommon {
        */
       def isLeadingInfixOperator =
         allowLeadingInfixOperators &&
-        (token == BACKQUOTED_IDENT ||
-         token == IDENTIFIER && isOperatorPart(name.charAt(name.length - 1))) &&
-        (ch == ' ') && lookingAhead {
+        (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)
+          isSimpleExprIntroToken(token) ||
+          token == NEWLINE && { nextToken() ; isSimpleExprIntroToken(token) }
         }
 
       /* Insert NEWLINE or NEWLINES if

test/files/pos/leading-infix-op.scala

@@ -0,0 +1,19 @@
+
+// scalac: -Xsource:3
+
+trait T {
+  def f(x: Int): Boolean =
+    x < 0
+    ||
+    x > 0
+    &&
+    x != 3
+
+  def g(x: Option[Int]) = x match {
+    case Some(err) =>
+      println("hi")
+      ???
+    case None =>
+      ???
+  }
+}

test/files/run/multiLineOps.scala

@@ -1,12 +1,13 @@
 // scalac: -Xsource:3
 //
-// without backticks, "not found: value +"
+// was: without backticks, "not found: value +" (but parsed here as +a * 6, where backticks fool the lexer)
+// now: + is taken as "solo" infix op
 //
 object Test extends App {
   val a = 7
   val x = 1
     + //
     `a` * 6
 
-  assert(x == 1)
+  assert(x == 1 + 42, x)  // was: 1
 }