Syntax forms 'a<T>::foo', 'a?::foo' have special cases resolved for future releases.

'a<T>::foo' is reserved if 'a' is a simple name and can be resolved as an expression
(this can be extended to 'a.b.c<T>::foo' case, although that is rather hard to implement using PSI).

 'a?::foo' is reserved if 'a' can be resolved as an expression.

compiler/testData/diagnostics/tests/callableReference/bound/reservedExpressionSyntax2.txt

@@ -0,0 +1,7 @@
+package
+
+package test {
+    public val test1: kotlin.reflect.KFunction0<kotlin.Unit>
+    public fun nullableFun(): kotlin.Int?
+    public fun kotlin.Int.foo(): kotlin.Unit
+}