From f66011b0f07e51ba1f1b7266f2cd7eb440c59d0d Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sat, 28 Oct 2023 18:50:18 +0200
Subject: [PATCH 1/3] Fix error detected in review of #18699

---
 .../src/dotty/tools/dotc/cc/CheckCaptures.scala  | 16 ++++++++++------
 tests/pos/i18699.scala                           |  7 +++++++
 2 files changed, 17 insertions(+), 6 deletions(-)
 create mode 100644 tests/pos/i18699.scala

diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
index c52e1aa6714e..fab0689b4df2 100644
--- a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
+++ b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -826,12 +826,16 @@ class CheckCaptures extends Recheck, SymTransformer:
      */
     def adaptUniversal(actual: Type, expected: Type, tree: Tree)(using Context): Type =
       if expected.captureSet.disallowsUniversal && actual.captureSet.isUniversal then
-        val localRoot = impliedRoot(tree)
-        CapturingType(
-            actual.stripCapturing,
-            localRoot.termRef.singletonCaptureSet,
-            actual.isBoxedCapturing)
-          .showing(i"adapt universal $actual vs $expected = $result", capt)
+        if actual.isInstanceOf[SingletonType] then
+          // capture set is only exposed when widening
+          adaptUniversal(actual.widen, expected, tree)
+        else
+          val localRoot = impliedRoot(tree)
+          CapturingType(
+              actual.stripCapturing,
+              localRoot.termRef.singletonCaptureSet,
+              actual.isBoxedCapturing)
+            .showing(i"adapt universal $actual vs $expected = $result", capt)
       else actual
 
     private inline val debugSuccesses = false
diff --git a/tests/pos/i18699.scala b/tests/pos/i18699.scala
new file mode 100644
index 000000000000..61b21b0d3ad4
--- /dev/null
+++ b/tests/pos/i18699.scala
@@ -0,0 +1,7 @@
+import language.experimental.captureChecking
+trait Cap:
+  def use: Int = 42
+
+def test2(cs: List[Cap^]): Unit =
+  val t0: Cap^{cap[test2]} = cs.head  // error
+  var t1: Cap^{cap[test2]} = cs.head  // error
\ No newline at end of file

From 78c2220bc2c57655cf82751967188b8c2df7d91c Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sun, 29 Oct 2023 16:02:57 +0100
Subject: [PATCH 2/3] Move test to avoid pickling issues

---
 tests/pos-custom-args/captures/i18699.scala | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 tests/pos-custom-args/captures/i18699.scala

diff --git a/tests/pos-custom-args/captures/i18699.scala b/tests/pos-custom-args/captures/i18699.scala
new file mode 100644
index 000000000000..61b21b0d3ad4
--- /dev/null
+++ b/tests/pos-custom-args/captures/i18699.scala
@@ -0,0 +1,7 @@
+import language.experimental.captureChecking
+trait Cap:
+  def use: Int = 42
+
+def test2(cs: List[Cap^]): Unit =
+  val t0: Cap^{cap[test2]} = cs.head  // error
+  var t1: Cap^{cap[test2]} = cs.head  // error
\ No newline at end of file

From 041f9ceaf4edf02dbf197d6e636eeed2f09a6693 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sun, 29 Oct 2023 17:55:09 +0100
Subject: [PATCH 3/3] Make printing of capture sets work also when testing
 Pickler

---
 .../src/dotty/tools/dotc/printing/RefinedPrinter.scala     | 4 +++-
 tests/pos-custom-args/captures/i18699.scala                | 7 -------
 2 files changed, 3 insertions(+), 8 deletions(-)
 delete mode 100644 tests/pos-custom-args/captures/i18699.scala

diff --git a/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala b/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
index 4f63bfb38691..bc8e2e9bc88e 100644
--- a/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
+++ b/compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala
@@ -640,7 +640,9 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
           try changePrec(GlobalPrec)(toText(arg) ~ "^" ~ toTextCaptureSet(captureSet))
           catch case ex: IllegalCaptureRef => toTextAnnot
         if annot.symbol.maybeOwner == defn.RetainsAnnot
-            && Feature.ccEnabled && Config.printCaptureSetsAsPrefix && !printDebug
+            && Feature.ccEnabled
+            && Config.printCaptureSetsAsPrefix && !printDebug
+            && Phases.checkCapturesPhase.exists // might be missing on -Ytest-pickler
         then toTextRetainsAnnot
         else toTextAnnot
       case EmptyTree =>
diff --git a/tests/pos-custom-args/captures/i18699.scala b/tests/pos-custom-args/captures/i18699.scala
deleted file mode 100644
index 61b21b0d3ad4..000000000000
--- a/tests/pos-custom-args/captures/i18699.scala
+++ /dev/null
@@ -1,7 +0,0 @@
-import language.experimental.captureChecking
-trait Cap:
-  def use: Int = 42
-
-def test2(cs: List[Cap^]): Unit =
-  val t0: Cap^{cap[test2]} = cs.head  // error
-  var t1: Cap^{cap[test2]} = cs.head  // error
\ No newline at end of file