From 66bdfb80a94e11a4d26ad2fb99a60f3ddb673f9f Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sat, 3 Sep 2022 22:05:40 +0200
Subject: [PATCH] Fix isSubType for static objects filling in type projections

Fixes #15931
---
 .../dotty/tools/dotc/core/TypeComparer.scala  |  3 +--
 .../src/dotty/tools/dotc/core/Types.scala     |  6 +++--
 .../tools/dotc/transform/TreeChecker.scala    |  2 +-
 tests/pos/i15931.scala                        | 24 +++++++++++++++++++
 tests/{neg => pos}/i8338.scala                |  0
 5 files changed, 30 insertions(+), 5 deletions(-)
 create mode 100644 tests/pos/i15931.scala
 rename tests/{neg => pos}/i8338.scala (100%)

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 78fbea352bf3..2454133dc05a 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -311,8 +311,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 thirdTryNamed(tp2)
               else
                 (  (tp1.name eq tp2.name)
-                && tp1.isMemberRef
-                && tp2.isMemberRef
+                && tp2.isPrefixDependentMemberRef
                 && isSubPrefix(tp1.prefix, tp2.prefix)
                 && tp1.signature == tp2.signature
                 && !(sym1.isClass && sym2.isClass)  // class types don't subtype each other
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 0df3fa368d5a..92408ed28e76 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -2496,8 +2496,10 @@ object Types {
       symd.maybeOwner.membersNeedAsSeenFrom(prefix) && !symd.is(NonMember)
       || prefix.isInstanceOf[Types.ThisType] && symd.is(Opaque) // see pos/i11277.scala for a test where this matters
 
-    /** Is this a reference to a class or object member? */
-    def isMemberRef(using Context): Boolean = designator match {
+    /** Is this a reference to a class or object member with an info that might depend
+     *  on the prefix?
+     */
+    def isPrefixDependentMemberRef(using Context): Boolean = designator match {
       case sym: Symbol => infoDependsOnPrefix(sym, prefix)
       case _ => true
     }
diff --git a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
index 5791d0c7d119..d1cd4e8729a2 100644
--- a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -385,7 +385,7 @@ class TreeChecker extends Phase with SymTransformer {
 
       val sym = tree.symbol
       val symIsFixed = tpe match {
-        case tpe: TermRef => ctx.erasedTypes || !tpe.isMemberRef
+        case tpe: TermRef => ctx.erasedTypes || !tpe.isPrefixDependentMemberRef
         case _ => false
       }
       if (sym.exists && !sym.is(Private) &&
diff --git a/tests/pos/i15931.scala b/tests/pos/i15931.scala
new file mode 100644
index 000000000000..cd57094576eb
--- /dev/null
+++ b/tests/pos/i15931.scala
@@ -0,0 +1,24 @@
+sealed trait TP:
+  type C
+  type P
+
+final class Foo extends TP:
+  class C
+  enum P:
+    case A, B
+
+object Bar extends TP:
+  class C
+  enum P:
+    case A, B, C
+
+// Works
+def test =
+  summon[Foo#P <:< TP#P]
+  val a: TP#P = Foo().P.A
+
+  // These fail
+  val b: TP#P = Bar.P.A: Bar.P
+  summon[Bar.type#P <:< TP#P]
+  summon[Bar.P <:< TP#P]
+  val c: TP#C = ??? : Bar.C
diff --git a/tests/neg/i8338.scala b/tests/pos/i8338.scala
similarity index 100%
rename from tests/neg/i8338.scala
rename to tests/pos/i8338.scala