Improve message for colon fusion

When the user accidentally writes `val x_: Int` where the colon
belongs to the identifier as an operator suffix, tell them so.

Author: som-snytt

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

@@ -4008,7 +4008,14 @@ object Parsers {
       val tpt = typedOpt()
       val rhs =
         if tpt.isEmpty || in.token == EQUALS then
-          accept(EQUALS)
+          if tpt.isEmpty && in.token != EQUALS then
+            lhs match
+            case Ident(name) :: Nil if name.endsWith(":") =>
+              val help = i"; identifier ends in colon, did you mean `${name.toSimpleName.dropRight(1)}`: in backticks?"
+              syntaxErrorOrIncomplete(ExpectedTokenButFound(EQUALS, in.token, suffix = help))
+            case _ => accept(EQUALS)
+          else
+            accept(EQUALS)
           val rhsOffset = in.offset
           subExpr() match
             case rhs0 @ Ident(name) if placeholderParams.nonEmpty && name == placeholderParams.head.name

tests/neg/i18020.scala

@@ -1,3 +1,4 @@
+
 import _root_.scala.StringContext // ok
 
 class Test :

tests/neg/i18020b.check

@@ -0,0 +1,26 @@
+-- [E040] Syntax Error: tests/neg/i18020b.scala:2:17 -------------------------------------------------------------------
+2 |class i18020(a_: Int): // error
+  |                 ^^^
+  |                 ':' expected, but identifier found
+-- [E040] Syntax Error: tests/neg/i18020b.scala:3:12 -------------------------------------------------------------------
+3 |  def f(b_: Int) = 42 // error
+  |            ^^^
+  |            ':' expected, but identifier found
+-- [E040] Syntax Error: tests/neg/i18020b.scala:4:10 -------------------------------------------------------------------
+4 |  def g_: Int = 27 // error
+  |          ^^^
+  |          '=' expected, but identifier found
+-- [E040] Syntax Error: tests/neg/i18020b.scala:6:12 -------------------------------------------------------------------
+6 |    val x_: Int = 1 // error
+  |            ^^^
+  |            '=' expected, but identifier found; identifier ends in colon, did you mean `x_`: in backticks?
+-- [E040] Syntax Error: tests/neg/i18020b.scala:7:12 -------------------------------------------------------------------
+7 |    val y_: Int = 2 // error
+  |            ^^^
+  |            '=' expected, but identifier found; identifier ends in colon, did you mean `y_`: in backticks?
+-- [E006] Not Found Error: tests/neg/i18020b.scala:8:4 -----------------------------------------------------------------
+8 |    x_ + y_ // error
+  |    ^^
+  |    Not found: x_ - did you mean x_:?
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i18020b.scala

@@ -0,0 +1,8 @@
+// problems with colon fusion, a harder challenge than cold fusion
+class i18020(a_: Int): // error
+  def f(b_: Int) = 42 // error
+  def g_: Int = 27 // error
+  def k =
+    val x_: Int = 1 // error
+    val y_: Int = 2 // error
+    x_ + y_ // error