Fix #9023: Check that names of vals and vars do not end with _=

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -389,6 +389,8 @@ class Namer { typer: Typer =>
         val name = checkNoConflict(tree.name, flags.is(Private), tree.span)
         tree match
           case tree: ValOrDefDef =>
+            if tree.isInstanceOf[ValDef] && !flags.is(Param) && name.endsWith("_=") then
+              ctx.error("Names of vals or vars may not end in `_=`", tree.namePos)
             if tree.unforcedRhs == EmptyTree
                && !flags.isOneOf(TermParamOrAccessor)
                && !tree.name.isConstructorName

tests/neg/i9023.scala

@@ -0,0 +1,19 @@
+class C {
+  var __= : Int = 42 // error
+  var x_= : Int = 42 // error
+}
+
+class D {
+  val __= : Int = 42 // error
+  val x_= : Int = 42 // error
+}
+
+class E {
+  lazy val __= : Int = 42 // error
+  lazy val x_= : Int = 42 // error
+}
+
+class F {
+  def __= : Int = 42
+  def x_= : Int = 42
+}