Resolve assoc fns & consts defined on fn ptr tys

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -692,6 +692,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             | ty::Slice(_)
             | ty::RawPtr(_)
             | ty::Ref(..)
+            | ty::FnPtr(_)
             | ty::Never
             | ty::Tuple(..) => self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty),
             _ => {}

tests/ui/associated-item/associated-item-on-function-pointer.rs

@@ -0,0 +1,34 @@
+// Check that we successfully resolve associated functions and constants defined on
+// function pointer types. Regression test for issue #108270.
+
+// check-pass
+
+#![feature(rustc_attrs)]
+#![rustc_coherence_is_core]
+
+impl fn() {
+    const ARITY: usize = 0;
+
+    fn handle() {}
+
+    fn apply(self) {
+        self()
+    }
+}
+
+impl for<'src> fn(&'src str) -> bool {
+    fn execute(self, source: &'static str) -> bool {
+        self(source)
+    }
+}
+
+fn main() {
+    let _: usize = <fn()>::ARITY;
+    <fn()>::handle();
+
+    let f: fn() = main;
+    f.apply();
+
+    let predicate: fn(&str) -> bool = |source| !source.is_empty();
+    let _ = predicate.execute("...");
+}