From e2fdcb2da79b2a0782c6f7442e344b00ba7886bc Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 12 Jan 2021 09:44:35 +0100
Subject: [PATCH 1/2] Special case comparisons between LazyRefs and bottom/top
 types

Fixes #11064

When comparing a LazyRef with some other type, we need to force the LazyRef
which can potentially lead to a cycle (as observed in #11064). However, in the situations
```
Nothing <: LazyRef(...)
LazyRef(...) <: Any
```
we can skip that since the comparison is always true. This scheme fixes #11064, but it breaks
down in similar situations if additional bounds are introduced (see neg/i11064.scala).
So it is quite fragile. But it's probably the best we can do.

The example in #11064 looks harmless, but type-theoretically it's actually a pretty explosive
mix of F-bounded types and existential types (in Scala 2). In Scala 2, we need to force more
for F-bounded types and we need to approximate existential types by dependent types, so
not everything works the same way.

My long term advice would be: get rid of F-bounds. Model them with intersections at the
use site.
---
 compiler/src/dotty/tools/dotc/core/TypeComparer.scala | 7 ++++---
 tests/neg/i11064.scala                                | 9 +++++++++
 tests/pos/i11064.scala                                | 9 +++++++++
 3 files changed, 22 insertions(+), 3 deletions(-)
 create mode 100644 tests/neg/i11064.scala
 create mode 100644 tests/pos/i11064.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 7bb942e9ab6a..c1d0d2faaeb2 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -306,7 +306,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         }
         compareWild
       case tp2: LazyRef =>
-        !tp2.evaluating && recur(tp1, tp2.ref)
+        isBottom(tp1) || !tp2.evaluating && recur(tp1, tp2.ref)
       case tp2: AnnotatedType if !tp2.isRefining =>
         recur(tp1, tp2.parent)
       case tp2: ThisType =>
@@ -373,7 +373,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             if (recur(info1.alias, tp2)) return true
             if (tp1.prefix.isStable) return tryLiftedToThis1
           case _ =>
-            if (tp1 eq NothingType) return true
+            if (tp1 eq NothingType) || isBottom(tp1) then return true
         }
         thirdTry
       case tp1: TypeParamRef =>
@@ -420,7 +420,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         // If `tp1` is in train of being evaluated, don't force it
         // because that would cause an assertionError. Return false instead.
         // See i859.scala for an example where we hit this case.
-        !tp1.evaluating && recur(tp1.ref, tp2)
+        tp2.isRef(AnyClass, skipRefined = false)
+        || !tp1.evaluating && recur(tp1.ref, tp2)
       case tp1: AnnotatedType if !tp1.isRefining =>
         recur(tp1.parent, tp2)
       case AndType(tp11, tp12) =>
diff --git a/tests/neg/i11064.scala b/tests/neg/i11064.scala
new file mode 100644
index 000000000000..7cfab64d0e04
--- /dev/null
+++ b/tests/neg/i11064.scala
@@ -0,0 +1,9 @@
+trait TypedArray[T, Repr]
+
+trait Ops[T <: TypedArray[_, T]] {
+  def typedArray(): T
+}
+
+object Test {
+  def test(ops: Ops[_ <: TypedArray[_ <: AnyRef, _]]) = ops.typedArray()  // error: Recursion limit exceeded.
+}
\ No newline at end of file
diff --git a/tests/pos/i11064.scala b/tests/pos/i11064.scala
new file mode 100644
index 000000000000..286b3d8dcc07
--- /dev/null
+++ b/tests/pos/i11064.scala
@@ -0,0 +1,9 @@
+trait TypedArray[T, Repr]
+
+trait Ops[T <: TypedArray[_, T]] {
+  def typedArray(): T
+}
+
+object Test {
+  def test(ops: Ops[_ <: TypedArray[_, _]]) = ops.typedArray()
+}
\ No newline at end of file

From 4e009c6d2010639522bcb232af15f1aced93a0c9 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Tue, 12 Jan 2021 11:55:56 +0100
Subject: [PATCH 2/2] Fix tests

---
 .../{neg => neg-custom-args/allow-deep-subtypes}/i11064.scala | 0
 tests/neg/i6225.scala                                         | 4 ++--
 2 files changed, 2 insertions(+), 2 deletions(-)
 rename tests/{neg => neg-custom-args/allow-deep-subtypes}/i11064.scala (100%)

diff --git a/tests/neg/i11064.scala b/tests/neg-custom-args/allow-deep-subtypes/i11064.scala
similarity index 100%
rename from tests/neg/i11064.scala
rename to tests/neg-custom-args/allow-deep-subtypes/i11064.scala
diff --git a/tests/neg/i6225.scala b/tests/neg/i6225.scala
index 6d2f7943a394..148a484fd0f1 100644
--- a/tests/neg/i6225.scala
+++ b/tests/neg/i6225.scala
@@ -1,11 +1,11 @@
-object O1 {  // error: cannot be instantiated
+object O1 {
   type A[X] = X
   opaque type T = A // error: opaque type alias must be fully applied
 }
 
 object O2 {
   opaque type A[X] = X
-  object A { // error: cannot be instantiated
+  object A {
     opaque type T = A  // error: opaque type alias must be fully applied
   }
 }