Fix #12729: Don't encode <init> and <clinit> only.

In addition, reject `<init>` and `<clinit>` in the parser, so that
users don't write them in source code.

Author: mlachkar

.scalafmt.conf

@@ -342,8 +342,8 @@ object Names {
 
     override def encode: SimpleName = {
       val dontEncode =
-        length >= 3 &&
-        head == '<' && last == '>' && isIdentifierStart(apply(1))
+        length >= 6 &&
+        head == '<' && (this == StdNames.nme.CONSTRUCTOR || this == StdNames.nme.STATIC_CONSTRUCTOR)
       if (dontEncode) this else NameTransformer.encode(this)
     }
 

compiler/src/dotty/tools/dotc/core/Names.scala

@@ -978,6 +978,10 @@ object Parsers {
     def ident(): TermName =
       if (isIdent) {
         val name = in.name
+        if name == nme.CONSTRUCTOR || name == nme.STATIC_CONSTRUCTOR then
+          report.error(
+            i"""Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden""",
+            in.sourcePos())
         in.nextToken()
         name
       }
@@ -996,6 +1000,7 @@ object Parsers {
 
     private def makeIdent(tok: Token, offset: Offset, name: Name) = {
       val tree = Ident(name)
+
       if (tok == BACKQUOTED_IDENT) tree.pushAttachment(Backquoted, ())
 
       // Make sure that even trees with parsing errors have a offset that is within the offset

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

@@ -0,0 +1,7 @@
+class Test(i: Int):
+  val `<init>` = "init" // error: Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden
+  val `<clinit>` = "clinit" // error: Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden
+  class `<init>`: // error: Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden
+    def `<init>`(in: String) = ??? // error: Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden
+  class `<clinit>`: // error: Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden
+    def `<clinit>`(in: String) = ??? // error: Illegal backquoted identifier: `<init>` and `<clinit>` are forbidden

tests/neg/i12729.scala

@@ -0,0 +1,3 @@
+object Test:
+  val `<x>` = "hello!"
+  def main(args: Array[String]): Unit = println(`<x>`)