add test + don't warn on Res::SelfTy

Author: lcnr

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -863,6 +863,9 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                                         // See the `need_type_info/type-alias.rs` test for
                                         // some examples.
                                     }
+                                    // There cannot be inference variables in the self type,
+                                    // so there's nothing for us to do here.
+                                    Res::SelfTy { .. } => {}
                                     _ => warn!(
                                         "unexpected path: def={:?} substs={:?} path={:?}",
                                         def, substs, path,

src/test/ui/inference/need_type_info/concrete-impl.rs

@@ -0,0 +1,16 @@
+trait Ambiguous<A> {
+    fn method() {}
+}
+
+struct One;
+struct Two;
+struct Struct;
+
+impl Ambiguous<One> for Struct {}
+impl Ambiguous<Two> for Struct {}
+
+fn main() {
+    <Struct as Ambiguous<_>>::method();
+    //~^ ERROR type annotations needed
+    //~| ERROR type annotations needed
+}

src/test/ui/inference/need_type_info/concrete-impl.stderr

@@ -0,0 +1,33 @@
+error[E0282]: type annotations needed
+  --> $DIR/concrete-impl.rs:13:5
+   |
+LL |     <Struct as Ambiguous<_>>::method();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `Self` declared on the trait `Ambiguous`
+   |
+help: consider specifying the generic argument
+   |
+LL |     <Struct as Ambiguous::<_>>::method();
+   |                         ~~~~~
+
+error[E0283]: type annotations needed
+  --> $DIR/concrete-impl.rs:13:5
+   |
+LL |     <Struct as Ambiguous<_>>::method();
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `Self` declared on the trait `Ambiguous`
+   |
+note: multiple `impl`s satisfying `Struct: Ambiguous<_>` found
+  --> $DIR/concrete-impl.rs:9:1
+   |
+LL | impl Ambiguous<One> for Struct {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | impl Ambiguous<Two> for Struct {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: consider specifying the generic argument
+   |
+LL |     <Struct as Ambiguous::<_>>::method();
+   |                         ~~~~~
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0282, E0283.
+For more information about an error, try `rustc --explain E0282`.

src/test/ui/inference/need_type_info/self-ty-in-path.rs

@@ -0,0 +1,13 @@
+// Test that we don't ICE when encountering a `Self` in a path.
+struct TestErr<T>(T);
+
+impl<T> TestErr<T> {
+    fn func_a<U>() {}
+
+    fn func_b() {
+        Self::func_a();
+        //~^ ERROR type annotations needed
+    }
+}
+
+fn main() {}

src/test/ui/inference/need_type_info/self-ty-in-path.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed
+  --> $DIR/self-ty-in-path.rs:8:9
+   |
+LL |         Self::func_a();
+   |         ^^^^^^^^^^^^ cannot infer type of the type parameter `U` declared on the associated function `func_a`
+   |
+help: consider specifying the generic argument
+   |
+LL |         Self::func_a::<U>();
+   |                     +++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.