chore: have a better error message when context bounds are not allowed

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

@@ -68,8 +68,8 @@ object Parsers {
       this == Given || this == ExtensionFollow
     def acceptsVariance =
       this == Class || this == CaseClass || this == Hk
-    def acceptsCtxBounds =
-      !(this == Type || this == Hk)
+    def acceptsCtxBounds(using Context) =
+      !(this == Type || this == Hk) || (sourceVersion.enablesNewGivens && this == Type)
     def acceptsWildcard =
       this == Type || this == Hk
 
@@ -3549,10 +3549,12 @@ object Parsers {
             else ident().toTypeName
           val isCap = gobbleHat()
           val hkparams = typeParamClauseOpt(ParamOwner.Hk)
-          val bounds =
-            if paramOwner.acceptsCtxBounds then typeAndCtxBounds(name)
-            else if sourceVersion.enablesNewGivens && paramOwner == ParamOwner.Type then typeAndCtxBounds(name)
-            else typeBounds()
+          val bounds = typeAndCtxBounds(name) match
+            case bounds: TypeBoundsTree => bounds
+            case bounds: ContextBounds if paramOwner.acceptsCtxBounds => bounds
+            case ContextBounds(bounds, cxBounds) =>
+              for cbound <- cxBounds do  report.error(IllegalContextBounds(), cbound.srcPos)
+              bounds
           val res = TypeDef(name, lambdaAbstract(hkparams, bounds)).withMods(mods)
           if isCap then
             res.pushAttachment(CaptureVar, ())

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -227,6 +227,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case MatchIsNotPartialFunctionID // errorNumber: 211
   case OnlyFullyDependentAppliedConstructorTypeID // errorNumber: 212
   case PointlessAppliedConstructorTypeID // errorNumber: 213
+  case IllegalContextBoundsID // errorNumber: 214
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3510,3 +3510,11 @@ final class OnlyFullyDependentAppliedConstructorType()(using Context)
     i"Applied constructor type can only be used with classes where all parameters in the first parameter list are tracked"
 
   override protected def explain(using Context): String = ""
+
+final class IllegalContextBounds(using Context) extends SyntaxMsg(IllegalContextBoundsID):
+  override protected def msg(using Context): String = 
+    i"Context bounds are not allowed in this position"
+
+  override protected def explain(using Context): String = ""
+
+end IllegalContextBounds

tests/neg/i22552.check

@@ -1,7 +1,7 @@
--- [E040] Syntax Error: tests/neg/i22552.scala:3:10 --------------------------------------------------------------------
+-- [E214] Syntax Error: tests/neg/i22552.scala:3:14 --------------------------------------------------------------------
 3 |  type A[X: TC]                     // error
-  |          ^
-  |          ']' expected, but ':' found
+  |            ^^
+  |            Context bounds are not allowed in this position
 -- Error: tests/neg/i22552.scala:4:15 ----------------------------------------------------------------------------------
 4 |  type C = [X: TC] =>> List[X]      // error
   |               ^^

tests/neg/i22660.check

@@ -0,0 +1,4 @@
+-- [E214] Syntax Error: tests/neg/i22660.scala:2:21 --------------------------------------------------------------------
+2 |type Foo[T: Equatable] // error
+  |            ^^^^^^^^^
+  |            Context bounds are not allowed in this position

tests/neg/i22660.scala

@@ -0,0 +1,2 @@
+trait Equatable[T]
+type Foo[T: Equatable] // error