Forbid defining inline traits inside inline traits

When inlining the body of an inline trait, trying to inline another inline
trait creates typing issues.

For example, with the following code:
```
inline trait A[T]:
  inline trait InnerA[U]:
      val x: (T, U) = ???

class B extends A[Int]:
  class InnerB extends InnerA[String]
```
when the Inlining phase tries to inline InnerA inside of InnerB, it
should try to change A#T to B#T but does not know that A exists in a
higher scope.

An idea to solve this would be to inline everything from A inside of B
before inlining InnerA inside of InnerB, but this adds quite a bit of
complexity.
Another one would be to keep track, when inlining a trait, of all the
type parameters like we do for term parameters. This could be done more
easily.

This will be left as future work for the time being.

Author: timotheeandres

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2675,6 +2675,12 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       val body1 = addAccessorDefs(cls, typedStats(impl.body, dummy)(using ctx.inClassContext(self1.symbol))._1)
 
       if !ctx.isAfterTyper && cls.isInlineTrait then
+        body1.map(_.symbol).filter(_.isInlineTrait).foreach(innerInlTrait =>
+          report.error(
+            em"Implementation restriction: an inline trait cannot be defined inside of another inline trait",
+            innerInlTrait.srcPos
+          )
+        )
         val membersToInline = body1.filter(member => Inlines.isInlineableFromInlineTrait(cls, member))
         val wrappedMembersToInline = Block(membersToInline, unitLiteral).withSpan(cdef.span)
         PrepareInlineable.registerInlineInfo(cls, wrappedMembersToInline)

tests/neg/inline-trait-body-trait-inline.scala

@@ -0,0 +1,7 @@
+inline trait A[T]:
+  inline trait InnerA[U]: // error
+    val x: (T, U) = ???
+
+class B extends A[Int]:
+  class InnerB extends InnerA[String]
+  def f: (Int, String) = InnerB().x