Merge pull request #432 from scala/backport-lts-3.3-23190

Backport "chore: have a better error message when context bounds are not allowed" to 3.3 LTS

Author: tgodzik

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

@@ -67,7 +67,6 @@ object Parsers {
       this == Given || this == ExtensionFollow
     def acceptsVariance =
       this == Class || this == CaseClass || this == Type
-
   end ParamOwner
 
   enum ParseKind:
@@ -3283,7 +3282,7 @@ object Parsers {
         ok
 
       def typeParam(): TypeDef = {
-        val isAbstractOwner = paramOwner == ParamOwner.Type || paramOwner == ParamOwner.TypeParam
+        val isAbstractOwner = (paramOwner == ParamOwner.Type || paramOwner == ParamOwner.TypeParam)
         val start = in.offset
         var mods = annotsAsMods() | Param
         if paramOwner == ParamOwner.Class || paramOwner == ParamOwner.CaseClass then
@@ -3304,7 +3303,13 @@ object Parsers {
             }
             else ident().toTypeName
           val hkparams = typeParamClauseOpt(ParamOwner.Type)
-          val bounds = if (isAbstractOwner) typeBounds() else typeParamBounds(name)
+          // val bounds = if (isAbstractOwner) typeBounds() else typeParamBounds(name)
+          val bounds = typeParamBounds(name) match
+            case bounds: TypeBoundsTree => bounds
+            case bounds: ContextBounds if !isAbstractOwner => bounds
+            case ContextBounds(bounds, cxBounds) =>
+              for cbound <- cxBounds do  report.error(IllegalContextBounds(), cbound.srcPos)
+              bounds
           TypeDef(name, lambdaAbstract(hkparams, bounds)).withMods(mods)
         }
       }

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

@@ -225,6 +225,9 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case FormatInterpolationErrorID // errorNumber: 209
   case ValueClassCannotExtendAliasOfAnyValID // errorNumber: 210
   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

@@ -3255,3 +3255,11 @@ class MatchIsNotPartialFunction(using Context) extends SyntaxMsg(MatchIsNotParti
        |
        |Efficient operations will use `applyOrElse` to avoid computing the match twice,
        |but the `apply` body would be executed "per element" in the example."""
+
+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

@@ -1,12 +1,12 @@
--- [E040] Syntax Error: tests/neg/i22552.scala:3:10 --------------------------------------------------------------------
+-- [E214] Syntax Error: tests/neg/i22552.scala:3:10 --------------------------------------------------------------------
 3 |  type A[X: TC]                     // error
-  |          ^
-  |          ']' expected, but ':' found
--- [E040] Syntax Error: tests/neg/i22552.scala:4:13 --------------------------------------------------------------------
+  |          ^^^^
+  |          Context bounds are not allowed in this position
+-- [E214] Syntax Error: tests/neg/i22552.scala:4:13 --------------------------------------------------------------------
 4 |  type C = [X: TC] =>> List[X]      // error
-  |             ^
-  |             ']' expected, but ':' found
--- [E040] Syntax Error: tests/neg/i22552.scala:5:13 --------------------------------------------------------------------
+  |             ^^^^
+  |             Context bounds are not allowed in this position
+-- [E214] Syntax Error: tests/neg/i22552.scala:5:13 --------------------------------------------------------------------
 5 |  type D = [X: TC] => () => List[X] // error
-  |             ^
-  |             ']' expected, but ':' found
+  |             ^^^^
+  |             Context bounds are not allowed in this position

tests/neg/i22660.check

@@ -0,0 +1,4 @@
+-- [E214] Syntax Error: tests/neg/i22660.scala:2:10 --------------------------------------------------------------------
+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