Support extensions on singleton objects

docs/whats_new.md

@@ -9,6 +9,7 @@ For a programmer-focused migration summary across 1.5.x, see `docs/whats_new_1_5
 - `1.5.4` is the stabilization release for the 1.5 feature set.
 - The 1.5 line now brings together richer ranges and loops, interpolation, math modules, immutable and observable collections, richer `lyngio`, and much better CLI/IDE support.
 - `1.5.4` specifically fixes user-visible issues around decimal arithmetic, mixed numeric flows, list behavior, and observable list hooks.
+- `1.5.4` also fixes extension-member registration for named singleton `object` declarations, so `fun X.foo()` and `val X.bar` now work as expected.
 - The docs, homepage samples, and release metadata now point at the current stable version.
 
 ## User Highlights Across 1.5.x
@@ -303,6 +304,23 @@ object Config {
 Config.show()
 ```
 
+Named singleton objects can also be used as extension receivers:
+
+```lyng
+object X {
+    fun base() = "base"
+}
+
+fun X.decorate(value): String {
+    this.base() + ":" + value.toString()
+}
+
+val X.tag get() = this.base() + ":tag"
+
+assertEquals("base:42", X.decorate(42))
+assertEquals("base:tag", X.tag)
+```
+
 ### Nested Declarations and Lifted Enums
 You can now declare classes, objects, enums, and type aliases inside another class. These nested declarations live in the class namespace (no outer instance capture) and are accessed with a qualifier.