Rollup merge of #126673 - oli-obk:dont_rely_on_err_reporting, r=compi…

…ler-errors

Ensure we don't accidentally succeed when we want to report an error

This also changes the `DefiningOpaqueTypes::No` to `Yes` without adding tests, as it is solely run on the error path to improve diagnostics. I was unable to provide a test that changes diagnostics, as all the tests I came up with ended up successfully constraining the opaque type and thus succeeding the coercion.

r? ```@compiler-errors```

cc #116652

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1245,11 +1245,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     expr,
                 );
 
-                return self
+                return Err(self
                     .commit_if_ok(|_| {
-                        self.at(cause, self.param_env).lub(DefineOpaqueTypes::No, prev_ty, new_ty)
+                        self.at(cause, self.param_env).lub(DefineOpaqueTypes::Yes, prev_ty, new_ty)
                     })
-                    .map(|ok| self.register_infer_ok_obligations(ok));
+                    .unwrap_err());
             }
         }
 
@@ -1259,10 +1259,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 if let Some(e) = first_error {
                     Err(e)
                 } else {
-                    self.commit_if_ok(|_| {
-                        self.at(cause, self.param_env).lub(DefineOpaqueTypes::No, prev_ty, new_ty)
-                    })
-                    .map(|ok| self.register_infer_ok_obligations(ok))
+                    Err(self
+                        .commit_if_ok(|_| {
+                            self.at(cause, self.param_env).lub(
+                                DefineOpaqueTypes::Yes,
+                                prev_ty,
+                                new_ty,
+                            )
+                        })
+                        .unwrap_err())
                 }
             }
             Ok(ok) => {