fix: allow postfix setters under language.postfixOps (scala#23775)

Allow for postfix operators to be followed by assigns.
This enables the definition and use of the following syntax (more
precisely the parsing of the `>_=` method as a `postfix operator +
assign`):
```scala
val v = new Vector(1, 2, 3)
println(v) // prints <1, 2, 3>
v<1> = 10 // assign 10 to element at index 1
println(v) // prints <1, 10, 3>
println(v<1>) // prints: value at 1 is 10

// Definition of Vector:

class Vector(values: Int*) {
  val data = values.toArray
  class Getter(i: Int) {
    def `>_=`(x: Int) =
      data(i) = x
    def > : Int =
      data(i)
  }
  def < (i:Int) = new Getter(i)
  override def toString = data.mkString("<", ", ", ">")
}
```
[Cherry-picked de18af4]

Author: KacperFKorban

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

@@ -2179,6 +2179,7 @@ object Parsers {
      *                      |  ForExpr
      *                      |  [SimpleExpr `.'] id `=' Expr
      *                      |  PrefixOperator SimpleExpr `=' Expr
+     *                      |  InfixExpr id [nl] `=' Expr                  -- only if language.postfixOps is enabled
      *                      |  SimpleExpr1 ArgumentExprs `=' Expr
      *                      |  PostfixExpr [Ascription]
      *                      |  ‘inline’ InfixExpr MatchClause
@@ -2339,7 +2340,7 @@ object Parsers {
     def expr1Rest(t: Tree, location: Location): Tree =
       if in.token == EQUALS then
         t match
-          case Ident(_) | Select(_, _) | Apply(_, _) | PrefixOp(_, _) =>
+          case Ident(_) | Select(_, _) | Apply(_, _) | PrefixOp(_, _) | PostfixOp(_, _) =>
             atSpan(startOffset(t), in.skipToken()) {
               val loc = if location.inArgs then location else Location.ElseWhere
               Assign(t, subPart(() => expr(loc)))

docs/_docs/internals/syntax.md

@@ -240,6 +240,7 @@ Expr1             ::=  [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[
                     |  ForExpr
                     |  [SimpleExpr ‘.’] id ‘=’ Expr                             Assign(expr, expr)
                     |  PrefixOperator SimpleExpr ‘=’ Expr                       Assign(expr, expr)
+                    |  InfixExpr id [nl] `=' Expr                               Assign(expr, expr) -- only if language.postfixOps is enabled
                     |  SimpleExpr ArgumentExprs ‘=’ Expr                        Assign(expr, expr)
                     |  PostfixExpr [Ascription]
                     |  ‘inline’ InfixExpr MatchClause

docs/_docs/reference/syntax.md

@@ -240,6 +240,7 @@ Expr1             ::=  [‘inline’] ‘if’ ‘(’ Expr ‘)’ {nl} Expr [[
                     |  ForExpr
                     |  [SimpleExpr ‘.’] id ‘=’ Expr
                     |  PrefixOperator SimpleExpr ‘=’ Expr
+                    |  InfixExpr id [nl] `=' Expr                              -- only if language.postfixOps is enabled
                     |  SimpleExpr ArgumentExprs ‘=’ Expr
                     |  PostfixExpr [Ascription]
                     |  ‘inline’ InfixExpr MatchClause

tests/pos/vec-access-syntax.scala

@@ -0,0 +1,23 @@
+import scala.language.postfixOps
+
+class Vector(values: Int*) {
+  val data = values.toArray
+  class Getter(i: Int) {
+    def `>_=`(x: Int) =
+      data(i) = x
+    def > : Int =
+      data(i)
+  }
+  def < (i:Int) = new Getter(i)
+  override def toString = data.mkString("<", ", ", ">")
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val v = new Vector(1, 2, 3)
+    println(v) // prints <1, 2, 3>
+    v<1> = 10 // assign 10 to element at index 1
+    println(v) // prints <1, 10, 3>
+    println(v<1>) // prints: value at 1 is 10
+  }
+}