Disallow context bounds in type lambdas

Author: jan-pieter

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

@@ -1798,8 +1798,22 @@ object Parsers {
         val start = in.offset
         val tparams = typeParamClause(ParamOwner.Type)
         if in.token == TLARROW then
-          atSpan(start, in.skipToken()):
-            LambdaTypeTree(tparams, toplevelTyp())
+          val hasContextBounds = tparams.exists(_.rhs match {
+            case x: ContextBounds => true
+            case _ => false
+          })
+          if hasContextBounds then
+            // Filter illegal context bounds and report syntax error
+            atSpan(start, in.skipToken()):
+              LambdaTypeTree(tparams.map {
+                case TypeDef(name, rhs: ContextBounds) =>
+                  syntaxError(em"context bounds are not allowed in type lambdas", rhs.span)
+                  TypeDef(name, TypeBoundsTree(EmptyTree, EmptyTree))
+                case other => other
+              }, toplevelTyp())
+          else
+            atSpan(start, in.skipToken()):
+              LambdaTypeTree(tparams, toplevelTyp())
         else if in.token == ARROW || isPureArrow(nme.PUREARROW) then
           val arrowOffset = in.skipToken()
           val body = toplevelTyp(nestedIntoOK(in.token))
@@ -3475,7 +3489,7 @@ object Parsers {
      *
      *  HkTypeParamClause ::=  ‘[’ HkTypeParam {‘,’ HkTypeParam} ‘]’
      *  HkTypeParam       ::=  {Annotation} [‘+’ | ‘-’]
-     *                         (id | ‘_’) [HkTypePamClause] TypeBounds
+     *                         (id | ‘_’) [HkTypeParamClause] TypeBounds
      */
     def typeParamClause(paramOwner: ParamOwner): List[TypeDef] = inBracketsWithCommas {
 

tests/neg/i22552.check

@@ -0,0 +1,8 @@
+-- [E040] Syntax Error: tests/neg/i22552.scala:3:10 --------------------------------------------------------------------
+3 |  type A[X: TC]                     // error
+  |          ^
+  |          ']' expected, but ':' found
+-- Error: tests/neg/i22552.scala:4:15 ----------------------------------------------------------------------------------
+4 |  type C = [X: TC] =>> List[X]      // error
+  |               ^^
+  |               context bounds are not allowed in type lambdas

tests/neg/i22552.scala

@@ -0,0 +1,5 @@
+trait Foo:
+  type TC[T]
+  type A[X: TC]                     // error
+  type C = [X: TC] =>> List[X]      // error
+  type D = [X: TC] => () => List[X] // allowed context bound