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.kt

@@ -0,0 +1,7 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+package test
+
+fun nullableFun(): Int? = null
+fun Int.foo() {}
+
+val test1 = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>nullableFun()<!>?::<!UNSAFE_CALL!>foo<!>