Rollup merge of #109896 - Nilstrieb:integers-are-not-fn-ptrs-remember-this-dear-transmuter, r=compiler-errors

Never consider int and float vars for `FnPtr` candidates

This solves a regression where `0.0.cmp()` was ambiguous when a custom trait with a `cmp` method was in scope.

For integers it shouldn't be a problem in practice so I wasn't able to add a test.

I'm not sure whether there could be more issues hidden in the shadows as mentioned in the issue, but this should at least fix the problematic regression immediately.

fixes #109892

r? oli-obk

tests/ui/fn/fn-ptr-trait-int-float-infer-var.rs

@@ -0,0 +1,10 @@
+// check-pass
+trait MyCmp {
+    fn cmp(&self) {}
+}
+impl MyCmp for f32 {}
+
+fn main() {
+    // Ensure that `impl<F: FnPtr> Ord for F` is never considered for int and float infer vars.
+    0.0.cmp();
+}