Fix ICE in default impl error reporting

compiler/rustc_middle/src/ty/error.rs

@@ -430,7 +430,9 @@ impl<'tcx> TyCtxt<'tcx> {
                     (ty::Projection(_), ty::Projection(_)) => {
                         diag.note("an associated type was expected, but a different one was found");
                     }
-                    (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p)) => {
+                    (ty::Param(p), ty::Projection(proj)) | (ty::Projection(proj), ty::Param(p))
+                        if self.def_kind(proj.item_def_id) != DefKind::ImplTraitPlaceholder =>
+                    {
                         let generics = self.generics_of(body_owner_def_id);
                         let p_span = self.def_span(generics.type_param(p, self).def_id);
                         if !sp.contains(p_span) {

src/test/ui/impl-trait/in-trait/specialization-broken.rs

@@ -0,0 +1,26 @@
+// FIXME(compiler-errors): I'm not exactly sure if this is expected to pass or not.
+// But we fixed an ICE anyways.
+
+#![feature(specialization)]
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+trait Foo {
+    fn bar(&self) -> impl Sized;
+}
+
+default impl<U> Foo for U
+where
+    U: Copy,
+{
+    fn bar(&self) -> U {
+        //~^ ERROR method `bar` has an incompatible type for trait
+        *self
+    }
+}
+
+impl Foo for i32 {}
+
+fn main() {
+    1i32.bar();
+}

src/test/ui/impl-trait/in-trait/specialization-broken.stderr

@@ -0,0 +1,23 @@
+error[E0053]: method `bar` has an incompatible type for trait
+  --> $DIR/specialization-broken.rs:16:22
+   |
+LL | default impl<U> Foo for U
+   |              - this type parameter
+...
+LL |     fn bar(&self) -> U {
+   |                      ^
+   |                      |
+   |                      expected associated type, found type parameter `U`
+   |                      help: change the output type to match the trait: `impl Sized`
+   |
+note: type in trait
+  --> $DIR/specialization-broken.rs:9:22
+   |
+LL |     fn bar(&self) -> impl Sized;
+   |                      ^^^^^^^^^^
+   = note: expected fn pointer `fn(&U) -> impl Sized`
+              found fn pointer `fn(&U) -> U`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0053`.