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/reservedExpressionSyntax.kt

@@ -0,0 +1,33 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+package test
+
+object ClassMemberMarker
+
+class a<T> {
+    fun foo() = ClassMemberMarker
+}
+
+class b<T1, T2> {
+    fun foo() = ClassMemberMarker
+}
+
+fun Int.foo() {}
+
+class Test {
+    val <T> List<T>.a: Int get() = size
+    val <T> List<T>.b: Int? get() = size
+
+    fun <T> List<T>.testCallable1(): () -> Unit = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>a<T><!>::foo
+    fun <T> List<T>.testCallable2(): () -> Unit = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>b<!>?::<!UNSAFE_CALL!>foo<!>
+    fun <T> List<T>.testCallable3(): () -> Unit = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>b<T, Any><!>::<!UNSAFE_CALL!>foo<!>
+    fun <T> List<T>.testCallable4(): () -> Unit = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>b<T><!>?::<!UNSAFE_CALL!>foo<!>
+
+    fun <T> List<T>.testClassLiteral1() = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>a<T><!>::class
+    fun <T> List<T>.testClassLiteral2() = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>b<!>?::class
+    fun <T> List<T>.testClassLiteral3() = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>b<T, Any><!>::class
+
+    fun <T> List<T>.testUnresolved1() = <!UNRESOLVED_REFERENCE!>unresolved<!><T>::<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>foo<!>
+    fun <T> List<T>.testUnresolved2() = <!RESERVED_SYNTAX_IN_CALLABLE_REFERENCE_LHS!>a<<!UNRESOLVED_REFERENCE!>unresolved<!>><!>::foo
+    fun <T> List<T>.testUnresolved3() = a<<!SYNTAX!><!>>::foo
+    fun <T> List<T>.testUnresolved4() = <!UNRESOLVED_REFERENCE!>unresolved<!>?::<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>foo<!>
+}

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

@@ -0,0 +1,48 @@
+package
+
+package test {
+    public fun kotlin.Int.foo(): kotlin.Unit
+
+    public object ClassMemberMarker {
+        private constructor ClassMemberMarker()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public final class Test {
+        public constructor Test()
+        public final val </*0*/ T> kotlin.collections.List<T>.a: kotlin.Int
+        public final val </*0*/ T> kotlin.collections.List<T>.b: kotlin.Int?
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+        public final fun </*0*/ T> kotlin.collections.List<T>.testCallable1(): () -> kotlin.Unit
+        public final fun </*0*/ T> kotlin.collections.List<T>.testCallable2(): () -> kotlin.Unit
+        public final fun </*0*/ T> kotlin.collections.List<T>.testCallable3(): () -> kotlin.Unit
+        public final fun </*0*/ T> kotlin.collections.List<T>.testCallable4(): () -> kotlin.Unit
+        public final fun </*0*/ T> kotlin.collections.List<T>.testClassLiteral1(): kotlin.reflect.KClass<kotlin.Int>
+        public final fun </*0*/ T> kotlin.collections.List<T>.testClassLiteral2(): kotlin.reflect.KClass<kotlin.Int?>
+        public final fun </*0*/ T> kotlin.collections.List<T>.testClassLiteral3(): kotlin.reflect.KClass<kotlin.Int?>
+        public final fun </*0*/ T> kotlin.collections.List<T>.testUnresolved1(): [ERROR : Error function type]
+        public final fun </*0*/ T> kotlin.collections.List<T>.testUnresolved2(): kotlin.reflect.KFunction0<kotlin.Unit>
+        public final fun </*0*/ T> kotlin.collections.List<T>.testUnresolved3(): kotlin.reflect.KFunction0<kotlin.Unit>
+        public final fun </*0*/ T> kotlin.collections.List<T>.testUnresolved4(): [ERROR : Error function type]
+    }
+
+    public final class a</*0*/ T> {
+        public constructor a</*0*/ T>()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final fun foo(): test.ClassMemberMarker
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+
+    public final class b</*0*/ T1, /*1*/ T2> {
+        public constructor b</*0*/ T1, /*1*/ T2>()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public final fun foo(): test.ClassMemberMarker
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}

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<!>

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
+}