Don't keel over on trivial Fnptr subtyping

crates/flux-infer/src/infer.rs

@@ -550,6 +550,10 @@ impl Sub {
                 }
                 Ok(())
             }
+            (BaseTy::FnPtr(sig_a), BaseTy::FnPtr(sig_b)) => {
+                assert_eq!(sig_a, sig_b);
+                Ok(())
+            }
             _ => Err(query_bug!("incompatible base types: `{a:?}` - `{b:?}`"))?,
         }
     }

tests/tests/pos/surface/fnptr00.rs

@@ -0,0 +1,18 @@
+#![allow(unused)]
+
+type Binop = fn(i32, i32) -> i32;
+
+struct Foo {
+    f: Binop,
+}
+
+// TODO: should actually support the below, not just leave it as `trusted`
+#[flux::trusted]
+fn bar(b: Binop) -> i32 {
+    b(3, 4)
+}
+
+fn test00(foo: Foo) -> i32 {
+    let x = bar(foo.f);
+    x
+}