From e88947266b1eac1c8797c01768a81ddb781bbdc6 Mon Sep 17 00:00:00 2001
From: Wojciech Mazur <wmazur@virtuslab.com>
Date: Tue, 3 Dec 2024 22:45:07 +0100
Subject: [PATCH 1/2] Survive inaccessible types when computing implicit scope

Also: Give a better error message later when encountering a missing type that
refers to a private member of a base class. The previous one was misleading since
it referred to a potentially missing class file, which is certainly not the case here.

Fixes #21543

[Cherry-picked 472555dca6866b4ae1fde35f3abe99579689b8eb][modified]
---
 compiler/src/dotty/tools/dotc/core/TypeErrors.scala |  2 ++
 compiler/src/dotty/tools/dotc/typer/Implicits.scala |  4 ++++
 tests/neg/i21543.scala                              | 13 +++++++++++++
 3 files changed, 19 insertions(+)
 create mode 100644 tests/neg/i21543.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeErrors.scala b/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
index 80dd412d0dbe..c7a9a232ed0d 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
@@ -73,6 +73,8 @@ class MissingType(val pre: Type, val name: Name)(using Context) extends TypeErro
       case _ if givenSelf.exists && givenSelf.member(name).exists =>
         i"""$name exists as a member of the self type $givenSelf of $cls
            |but it cannot be called on a receiver whose type does not extend $cls"""
+      case _ if pre.baseClasses.exists(_.findMember(name, pre, Private, EmptyFlags).exists) =>
+        i"$name is a private member in a base class"
       case _ =>
         missingClassFile
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index 3329055fd5d1..378a33b1da45 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -781,6 +781,10 @@ trait ImplicitRunInfo:
           override def stopAt = StopAt.Static
           private val seen = util.HashSet[Type]()
 
+          override def derivedTypeBounds(tp: TypeBounds, lo: Type, hi: Type): Type =
+            if lo.exists && hi.exists then super.derivedTypeBounds(tp, lo, hi)
+            else NoType // Survive inaccessible types, for instance in i21543.scala.
+
           def applyToUnderlying(t: TypeProxy) =
             if seen.contains(t) then
               WildcardType
diff --git a/tests/neg/i21543.scala b/tests/neg/i21543.scala
new file mode 100644
index 000000000000..98de8d3ec939
--- /dev/null
+++ b/tests/neg/i21543.scala
@@ -0,0 +1,13 @@
+object CompilerCrash {
+  trait Scope {
+    private type Event = String
+
+    case class Cmd(events: List[Event])
+  }
+
+  new Scope {
+    val commands = List(
+      Cmd(List("1", "2"))
+    )
+  }
+}
\ No newline at end of file

From 0484e65078127272209dcda9a5d46606ebb4ab6e Mon Sep 17 00:00:00 2001
From: Wojciech Mazur <wmazur@virtuslab.com>
Date: Tue, 3 Dec 2024 22:45:32 +0100
Subject: [PATCH 2/2] Fix check files

[Cherry-picked cd9a7c58afaab6d07bab91ebc059d98313a4713d][modified]
---
 tests/neg/i21543.check | 22 ++++++++++++++++++++++
 tests/neg/i21543.scala |  2 +-
 2 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i21543.check

diff --git a/tests/neg/i21543.check b/tests/neg/i21543.check
new file mode 100644
index 000000000000..9fa9a7779d7a
--- /dev/null
+++ b/tests/neg/i21543.check
@@ -0,0 +1,22 @@
+-- [E007] Type Mismatch Error: tests/neg/i21543.scala:10:15 ------------------------------------------------------------
+10 |      Cmd(List("1", "2")) // error // error
+   |               ^^^
+   |               Found:    ("1" : String)
+   |               Required: Event
+   |
+   |               Note that I could not resolve reference Event.
+   |               Event is a private member in a base class
+   |                                
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/i21543.scala:10:20 ------------------------------------------------------------
+10 |      Cmd(List("1", "2")) // error // error
+   |                    ^^^
+   |                    Found:    ("2" : String)
+   |                    Required: Event
+   |
+   |                    Note that I could not resolve reference Event.
+   |                    Event is a private member in a base class
+   |                                     
+   |
+   | longer explanation available when compiling with `-explain`
diff --git a/tests/neg/i21543.scala b/tests/neg/i21543.scala
index 98de8d3ec939..aaadce6d22b4 100644
--- a/tests/neg/i21543.scala
+++ b/tests/neg/i21543.scala
@@ -7,7 +7,7 @@ object CompilerCrash {
 
   new Scope {
     val commands = List(
-      Cmd(List("1", "2"))
+      Cmd(List("1", "2")) // error // error
     )
   }
 }
\ No newline at end of file