SI-9133 Harden against infinite loop in NoSymbol.owner

The available evidence gathered in an IDE hang suggests that
while editing erronenous code, a call to `Erasure#javaSig` by the
IDE's structure builder triggered the `ExplicitOuter` info transformer
on a symbol with some sort of incoherent owner chain, which led to
an infinite loop in `NoSymbol#outerClass`.

This commit hardens that method to work in the same manner as a call
to `NoSymbol.owner`: log the error under -Xdev or -Ydebug and return
return `NoSymbol` to soldier on without crashing / hanging.

I haven't formulated a theory about how we might have ended up with
the corrupt owner chain.

2 files changed, 10 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/internal/Symbols.scala b/src/reflect/scala/reflect/internal/Symbols.scala
index e91bfadc85..d5fc52abbf 100644
--- a/src/reflect/scala/reflect/internal/Symbols.scala
+++ b/src/reflect/scala/reflect/internal/Symbols.scala
@@ -2054,7 +2054,11 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
      *  where it is the outer class of the enclosing class.
      */
     final def outerClass: Symbol =
-      if (owner.isClass) owner
+      if (this == NoSymbol) {
+        // ideally we shouldn't get here, but its better to harden against this than suffer the infinite loop in SI-9133
+        devWarningDumpStack("NoSymbol.outerClass", 15)
+        NoSymbol
+      } else if (owner.isClass) owner
       else if (isClassLocalToConstructor) owner.enclClass.outerClass
       else owner.outerClass
 
diff --git a/test/junit/scala/tools/nsc/symtab/SymbolTableTest.scala b/test/junit/scala/tools/nsc/symtab/SymbolTableTest.scala
index 11e955a4bb..895ad9d683 100644
--- a/test/junit/scala/tools/nsc/symtab/SymbolTableTest.scala
+++ b/test/junit/scala/tools/nsc/symtab/SymbolTableTest.scala
@@ -44,4 +44,9 @@ class SymbolTableTest {
     assertFalse("Foo should be a superclass of Foo", fooSymbol.tpe <:< barSymbol.tpe)
   }
 
+  @Test
+  def noSymbolOuterClass_t9133: Unit = {
+    import symbolTable._
+    assert(NoSymbol.outerClass == NoSymbol)
+  }
 }