Fix #8337: Flag F-bounded opaque types as errors

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -2596,8 +2596,8 @@ object Types {
     /** Update the value of the lazyref, discarding the compute function `refFn`
      *  Can be called only as long as the ref is still undefined.
      */
-    def update(tp: Type) =
-      assert(myRef == null)
+    def update(tp: Type)(using ctx: Context) =
+      assert(myRef == null || ctx.reporter.errorsReported)
       myRef = tp
       computed = true
       refFn = null

docs/docs/reference/other-new-features/opaques-details.md

@@ -20,7 +20,7 @@ The general form of a (monomorphic) opaque type alias is
 ```scala
 opaque type T >: L <: U = R
 ```
-where the lower bound `L` and the upper bound `U` may be missing, in which case they are assumed to be `scala.Nothing` and `scala.Any`, respectively. If bounds are given, it is checked that the right hand side `R` conforms to them, i.e. `L <: R` and `R <: U`.
+where the lower bound `L` and the upper bound `U` may be missing, in which case they are assumed to be `scala.Nothing` and `scala.Any`, respectively. If bounds are given, it is checked that the right hand side `R` conforms to them, i.e. `L <: R` and `R <: U`. F-bounds are not supported for opaque types: `T` is not allowed to appear in `L` or `U`.
 
 Inside the scope of the alias definition, the alias is transparent: `T` is treated
 as a normal alias of `R`. Outside its scope, the alias is treated as the abstract type

tests/neg/i8337.scala

@@ -0,0 +1,6 @@
+trait Foo[F <: Foo[F]]
+class Bar extends Foo[Bar]
+
+object Q {    // error: recursion limit exceeded
+  opaque type X <: Foo[X] = Bar // error: out of bounds
+}