Use InferCtxt::probe to properly detect ambiguous candidates

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2235,7 +2235,8 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let mut applicable_candidates: Vec<_> = candidates
             .iter()
             .filter_map(|&(impl_, (assoc_item, def_scope))| {
-                let ocx = ObligationCtxt::new(&infcx);
+                infcx.probe(|_| {
+                    let ocx = ObligationCtxt::new_in_snapshot(&infcx);
 
                     let impl_ty = tcx.type_of(impl_);
                     let impl_substs = self.fresh_item_substs(impl_, &infcx);
@@ -2252,8 +2253,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
                     let impl_bounds = ocx.normalize(&cause, param_env, impl_bounds);
 
-                let impl_obligations =
-                    traits::predicates_for_generics(|_, _| cause.clone(), param_env, impl_bounds);
+                    let impl_obligations = traits::predicates_for_generics(
+                        |_, _| cause.clone(),
+                        param_env,
+                        impl_bounds,
+                    );
 
                     ocx.register_obligations(impl_obligations);
 
@@ -2265,6 +2269,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
                     Some((assoc_item, def_scope))
                 })
+            })
             .collect();
 
         if applicable_candidates.len() > 1 {

tests/ui/associated-inherent-types/ambiguity.rs

@@ -0,0 +1,16 @@
+#![feature(inherent_associated_types)]
+#![allow(incomplete_features)]
+
+struct Wrapper<T>(T);
+
+impl Wrapper<i32> {
+    type Foo = i32;
+}
+
+impl Wrapper<()> {
+    type Foo = ();
+}
+
+fn main() {
+    let _: Wrapper<_>::Foo = (); //~ ERROR multiple applicable items in scope
+}

tests/ui/associated-inherent-types/ambiguity.stderr

@@ -0,0 +1,20 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/ambiguity.rs:15:24
+   |
+LL |     let _: Wrapper<_>::Foo = ();
+   |                        ^^^ multiple `Foo` found
+   |
+note: candidate #1 is defined in an impl for the type `Wrapper<i32>`
+  --> $DIR/ambiguity.rs:7:5
+   |
+LL |     type Foo = i32;
+   |     ^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<()>`
+  --> $DIR/ambiguity.rs:11:5
+   |
+LL |     type Foo = ();
+   |     ^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0034`.