Tweak improveReadOnly

odersky · odersky · commit d33a39da9d43 · 2025-06-10T15:50:23.000+02:00
Widen to cap.rd only if parent type does not extend Mutable.
diff --git a/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala b/compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala
@@ -1571,7 +1571,7 @@ class CheckCaptures extends Recheck, SymTransformer:
       case actual @ CapturingType(parent, refs)
       if parent.derivesFrom(defn.Caps_Mutable)
           && expected.isValueType
-          && !expected.isMutableType
+          && !expected.derivesFromMutable
           && !expected.isSingleton
           && !expected.isBoxedCapturing =>
         actual.derivedCapturingType(parent, refs.readOnly)
diff --git a/tests/neg-custom-args/captures/scope-extrude-mut.check b/tests/neg-custom-args/captures/scope-extrude-mut.check
@@ -1,9 +1,10 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrude-mut.scala:9:8 ------------------------------
 9 |    a = a1  // error
   |        ^^
-  |        Found:    A^{a1.rd}
-  |        Required: A^
+  |        Found:    (a1 : A^)
+  |        Required: A^²
   |
-  |        where:    ^ refers to a fresh root capability in the type of variable a
+  |        where:    ^  refers to a fresh root capability created in value a1 when constructing mutable A
+  |                  ^² refers to a fresh root capability in the type of variable a
   |
   | longer explanation available when compiling with `-explain`

Original file line number	Diff line number	Diff line change
`@@ -1,9 +1,10 @@`
`1`	`1`	`-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/scope-extrude-mut.scala:9:8 ------------------------------`
`2`	`2`	`9 \| a = a1 // error`
`3`	`3`	`\| ^^`
`4`		`- \| Found: A^{a1.rd}`
`5`		`- \| Required: A^`
	`4`	`+ \| Found: (a1 : A^)`
	`5`	`+ \| Required: A^²`
`6`	`6`	`\|`
`7`		`- \| where: ^ refers to a fresh root capability in the type of variable a`
	`7`	`+ \| where: ^ refers to a fresh root capability created in value a1 when constructing mutable A`
	`8`	`+ \| ^² refers to a fresh root capability in the type of variable a`
`8`	`9`	`\|`
`9`	`10`	\| longer explanation available when compiling with `-explain`