[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.

2 files changed, 20 insertions, 0 deletions

diff --git a/test/files/pos/t9392/client_2.scala b/test/files/pos/t9392/client_2.scala
new file mode 100644
index 0000000000..6b706fea12
--- /dev/null
+++ b/test/files/pos/t9392/client_2.scala
@@ -0,0 +1,4 @@
+class Client {
+  Macro()
+}
+
diff --git a/test/files/pos/t9392/macro_1.scala b/test/files/pos/t9392/macro_1.scala
new file mode 100644
index 0000000000..3f67ac17b2
--- /dev/null
+++ b/test/files/pos/t9392/macro_1.scala
@@ -0,0 +1,16 @@
+import language.experimental.macros
+
+
+object Macro {
+
+  import reflect.macros.blackbox.Context
+  def impl(c: Context)(): c.Tree = {
+    import c.universe._
+    val tree = q"""class C; new C"""
+    val tree1 = c.typecheck(tree)
+    val tpe = tree1.tpe
+    val tree2 = c.typecheck(c.untypecheck(tree1))
+    q"""$tree2.asInstanceOf[$tpe]"""
+  }
+  def apply(): Any = macro impl
+}