Invalid ? suggestion on mismatched Ok(T)

Author: eopb

compiler/rustc_hir_typeck/src/demand.rs

@@ -983,6 +983,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         } else {
             return false;
         }
+        let e_ok = args_e.type_at(0);
+        let f_ok = args_f.type_at(0);
+        if !self.infcx.can_eq(self.param_env, f_ok, e_ok) {
+            return false;
+        }
         let e = args_e.type_at(1);
         let f = args_f.type_at(1);
         if self

tests/ui/type/type-check/issue-116967-cannot-coerce-returned-result.rs

@@ -0,0 +1,6 @@
+fn foo() -> Result<String, ()> {
+    let out: Result<(), ()> = Ok(());
+    out //~ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/type/type-check/issue-116967-cannot-coerce-returned-result.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-116967-cannot-coerce-returned-result.rs:3:5
+   |
+LL | fn foo() -> Result<String, ()> {
+   |             ------------------ expected `Result<String, ()>` because of return type
+LL |     let out: Result<(), ()> = Ok(());
+LL |     out
+   |     ^^^ expected `Result<String, ()>`, found `Result<(), ()>`
+   |
+   = note: expected enum `Result<String, _>`
+              found enum `Result<(), _>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.