inference test for #104649

src/test/ui/inference/issue-104649.rs

@@ -0,0 +1,32 @@
+type Result<T, E = Error> = ::std::result::Result<T, E>;
+struct Error;
+
+trait ForEach {
+    type Input;
+    fn for_each<F, U>(self, f: F)
+    where
+        F: FnOnce(Self::Input) -> U;
+}
+
+impl<T> ForEach for A<T> {
+    type Input = T;
+    fn for_each<F, U>(self, f: F)
+    where
+        F: FnOnce(Self::Input) -> U,
+    {
+        todo!()
+    }
+}
+
+struct A<T>(T);
+
+fn main() {
+    let a = A(Result::Ok(Result::Ok(()))); //~ ERROR type annotations needed
+    a.for_each(|a: Result<_>| {
+        let f = || match a {
+            Ok(Ok(a)) => {}
+            Ok(Err(a)) => {}
+            Err(a) => {}
+        };
+    });
+}

src/test/ui/inference/issue-104649.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed for `A<std::result::Result<std::result::Result<(), E>, Error>>`
+  --> $DIR/issue-104649.rs:24:9
+   |
+LL |     let a = A(Result::Ok(Result::Ok(())));
+   |         ^
+   |
+help: consider giving `a` an explicit type, where the type for type parameter `E` is specified
+   |
+LL |     let a: A<std::result::Result<std::result::Result<(), E>, Error>> = A(Result::Ok(Result::Ok(())));
+   |          +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.