Fix #9267: Avoid accessing inexistent next phase

  * Only compute `elimErasedCtx` when error is emitted
  * Fall back to erasure context if it is the last phase

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/Bridges.scala

@@ -16,7 +16,9 @@ class Bridges(root: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
 
   assert(ctx.phase == ctx.erasurePhase.next)
   private val preErasureCtx = ctx.withPhase(ctx.erasurePhase)
-  private val elimErasedCtx = ctx.withPhase(ctx.elimErasedValueTypePhase.next)
+  private def elimErasedCtx =
+    if ctx.elimErasedValueTypePhase.exists then ctx.withPhase(ctx.elimErasedValueTypePhase.next)
+    else ctx // can happen with -Ystop-after:erasure
 
   private class BridgesCursor(implicit ctx: Context) extends OverridingPairs.Cursor(root) {
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -65,6 +65,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/run/i5606.scala", defaultOptions.and("-Yretain-trees")),
       compileFile("tests/pos-custom-args/i5498-postfixOps.scala", defaultOptions withoutLanguageFeature "postfixOps"),
       compileFile("tests/pos-custom-args/i8875.scala", defaultOptions.and("-Xprint:getters")),
+      compileFile("tests/pos-custom-args/i9267.scala", defaultOptions.and("-Ystop-after:erasure")),
     ).checkCompile()
   }
 

tests/pos-custom-args/i9267.scala

@@ -0,0 +1 @@
+class A