add check for uninhabited types along side never

Author: Kivooeo

compiler/rustc_mir_build/src/builder/mod.rs

@@ -839,7 +839,26 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.infcx.typing_env(self.param_env),
             );
 
-            if !ty_is_inhabited {
+            // check if the function's return type is inhabited
+            // this was added here because of this regression
+            // https://github.com/rust-lang/rust/issues/149571
+            let output_is_inhabited =
+                if matches!(self.tcx.def_kind(self.def_id), DefKind::Fn | DefKind::AssocFn) {
+                    self.tcx
+                        .fn_sig(self.def_id)
+                        .instantiate_identity()
+                        .skip_binder()
+                        .output()
+                        .is_inhabited_from(
+                            self.tcx,
+                            self.parent_module,
+                            self.infcx.typing_env(self.param_env),
+                        )
+                } else {
+                    true
+                };
+
+            if !ty_is_inhabited && output_is_inhabited {
                 // Unreachable code warnings are already emitted during type checking.
                 // However, during type checking, full type information is being
                 // calculated but not yet available, so the check for diverging

tests/ui/uninhabited/uninhabited-unreachable-warning-149571.rs

@@ -0,0 +1,10 @@
+#![deny(unreachable_code)]
+//@ run-pass
+
+use std::convert::Infallible;
+
+pub fn foo(f: impl FnOnce() -> Infallible) -> Infallible {
+    f()
+}
+
+fn main() {}