Check for missing RHS of higher-kinded opaque types

Author: griggt

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1236,11 +1236,15 @@ object desugar {
   }
 
   def checkOpaqueAlias(tree: MemberDef)(using Context): MemberDef =
-    if !tree.mods.is(Opaque) then tree
-    else tree match
-      case TypeDef(_, bounds: TypeBoundsTree) if bounds.alias.isEmpty =>
+    def check(rhs: Tree): MemberDef = rhs match
+      case bounds: TypeBoundsTree if bounds.alias.isEmpty =>
         report.error(i"opaque type must have a right-hand side", tree.srcPos)
         tree.withMods(tree.mods.withoutFlags(Opaque))
+      case LambdaTypeTree(_, body) => check(body)
+      case _ => tree
+    if !tree.mods.is(Opaque) then tree
+    else tree match
+      case TypeDef(_, rhs) => check(rhs)
       case _ => tree
 
   /** Check that modifiers are legal for the definition `tree`.

tests/neg/i6055.check

@@ -0,0 +1,16 @@
+-- Error: tests/neg/i6055.scala:1:12 -----------------------------------------------------------------------------------
+1 |opaque type i0           // error: opaque type must have a right-hand side
+  |^^^^^^^^^^^^^^
+  |opaque type must have a right-hand side
+-- Error: tests/neg/i6055.scala:2:12 -----------------------------------------------------------------------------------
+2 |opaque type i2 <: Int    // error: opaque type must have a right-hand side
+  |^^^^^^^^^^^^^^^^^^^^^
+  |opaque type must have a right-hand side
+-- Error: tests/neg/i6055.scala:4:12 -----------------------------------------------------------------------------------
+4 |opaque type i1[_]        // error: opaque type must have a right-hand side
+  |^^^^^^^^^^^^^^^^^
+  |opaque type must have a right-hand side
+-- Error: tests/neg/i6055.scala:5:12 -----------------------------------------------------------------------------------
+5 |opaque type x[_] <: Int  // error: opaque type must have a right-hand side
+  |^^^^^^^^^^^^^^^^^^^^^^^
+  |opaque type must have a right-hand side

tests/neg/i6055.scala

@@ -1,5 +1,5 @@
-opaque type i0           // error: opaque type must be an alias type
-opaque type i2 <: Int    // error: opaque type must be an alias type
+opaque type i0           // error: opaque type must have a right-hand side
+opaque type i2 <: Int    // error: opaque type must have a right-hand side
 
-opaque type i1[_]        // error: opaque type must be an alias type
-opaque type x[_] <: Int  // error: opaque type must be an alias type
+opaque type i1[_]        // error: opaque type must have a right-hand side
+opaque type x[_] <: Int  // error: opaque type must have a right-hand side