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

Author: nicolasstucki

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

@@ -882,6 +882,12 @@ trait Checking {
     }
   }
 
+  /** Checks that the name is allowed */
+  def checkValidName(vdef: ValDef)(using Context): Unit = {
+      if vdef.name.endsWith("_=") && vdef.symbol.owner.isClass then
+        ctx.error("Names of vals or vars may not end in `_=`", vdef.namePos)
+  }
+
   /** Check that `tree` can be right hand-side or argument to `inline` value or parameter. */
   def checkInlineConformant(tpt: Tree, tree: Tree, sym: Symbol)(using Context): Unit = {
     if sym.is(Inline, butNot = DeferredOrTermParamOrAccessor) && !ctx.erasedTypes && !Inliner.inInlineMethod then

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

@@ -1802,6 +1802,7 @@ class Typer extends Namer
     }
     val vdef1 = assignType(cpy.ValDef(vdef)(name, tpt1, rhs1), sym)
     checkSignatureRepeatedParam(sym)
+    checkValidName(vdef1)
     checkInlineConformant(tpt1, rhs1, sym)
     patchFinalVals(vdef1)
     vdef1.setDefTree

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
+}