[backport] SI-9392 Avoid crash in GenBCode for incoherent trees

A macro in shapeless was generating a tree of the form:

```
{
	class C#2
	new C#2
}.setType(C#1)
```

This happened due to an error in the macro; it used untypecheck
to try to fix the owner-chain consistency problem, but kept a
reference to the previous version of the block-local class symbol
`C` and used this in the resulting tree.

This commit detects the particular situation we encountered, and
avoids the crash by not creating the `NestedInfo` for the
`BType` corresponding to `C#1`. The code comment discusses why I
think this is safe, and suggests a refactoring that would mean
we only ever try to construct `NestedInfo` when we are going to
need them.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
index cf29c8090b..7b238e56eb 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
@@ -351,6 +351,14 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
     val isTopLevel = innerClassSym.rawowner.isPackageClass
     // impl classes are considered top-level, see comment in BTypes
     if (isTopLevel || considerAsTopLevelImplementationArtifact(innerClassSym)) None
+    else if (innerClassSym.rawowner.isTerm)
+      // SI-9392 An errant macro might leave a reference to a local class symbol that no longer exists in the tree,
+      //         this avoids an assertion error in that case. AFAICT, we don't actually need the `NestedInfo` for all BTypes,
+      //         only for ones that describe classes defined in the trees that reach the backend, so this is safe enough.
+      //
+      //         TODO Can we avoid creating `NestedInfo` for each type that is referred to, and instead only create if for
+      //         symbols of ClassDefs?
+      None
     else {
       // See comment in BTypes, when is a class marked static in the InnerClass table.
       val isStaticNestedClass = isOriginallyStaticOwner(innerClassSym.originalOwner)