resolve: Do not consider traits from ambiguous imports to be in scope

compiler/rustc_resolve/src/lib.rs

@@ -703,7 +703,7 @@ impl<'ra> Module<'ra> {
         if traits.is_none() {
             let mut collected_traits = Vec::new();
             self.for_each_child(resolver, |r, name, ns, binding| {
-                if ns != TypeNS {
+                if ns != TypeNS || binding.is_ambiguity_recursive() {
                     return;
                 }
                 if let Res::Def(DefKind::Trait | DefKind::TraitAlias, def_id) = binding.res() {

tests/ui/imports/ambiguous-trait-in-scope.rs

@@ -0,0 +1,30 @@
+mod m1 {
+    pub trait Trait {
+        fn method1(&self) {}
+    }
+    impl Trait for u8 {}
+}
+mod m2 {
+    pub trait Trait {
+        fn method2(&self) {}
+    }
+    impl Trait for u8 {}
+}
+
+fn test1() {
+    // Create an ambiguous import for `Trait` in one order
+    use m1::*;
+    use m2::*;
+    0u8.method1(); //~ ERROR no method named `method1` found for type `u8` in the current scope
+    0u8.method2(); //~ ERROR no method named `method2` found for type `u8` in the current scope
+}
+
+fn test2() {
+    // Create an ambiguous import for `Trait` in another order
+    use m2::*;
+    use m1::*;
+    0u8.method1(); //~ ERROR no method named `method1` found for type `u8` in the current scope
+    0u8.method2(); //~ ERROR no method named `method2` found for type `u8` in the current scope
+}
+
+fn main() {}

tests/ui/imports/ambiguous-trait-in-scope.stderr

@@ -0,0 +1,83 @@
+error[E0599]: no method named `method1` found for type `u8` in the current scope
+  --> $DIR/ambiguous-trait-in-scope.rs:18:9
+   |
+LL |         fn method1(&self) {}
+   |            ------- the method is available for `u8` here
+...
+LL |     0u8.method1();
+   |         ^^^^^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `Trait` which provides `method1` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use m1::Trait;
+   |
+help: there is a method `method2` with a similar name
+   |
+LL -     0u8.method1();
+LL +     0u8.method2();
+   |
+
+error[E0599]: no method named `method2` found for type `u8` in the current scope
+  --> $DIR/ambiguous-trait-in-scope.rs:19:9
+   |
+LL |         fn method2(&self) {}
+   |            ------- the method is available for `u8` here
+...
+LL |     0u8.method2();
+   |         ^^^^^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `Trait` which provides `method2` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use m2::Trait;
+   |
+help: there is a method `method1` with a similar name
+   |
+LL -     0u8.method2();
+LL +     0u8.method1();
+   |
+
+error[E0599]: no method named `method1` found for type `u8` in the current scope
+  --> $DIR/ambiguous-trait-in-scope.rs:26:9
+   |
+LL |         fn method1(&self) {}
+   |            ------- the method is available for `u8` here
+...
+LL |     0u8.method1();
+   |         ^^^^^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `Trait` which provides `method1` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use m1::Trait;
+   |
+help: there is a method `method2` with a similar name
+   |
+LL -     0u8.method1();
+LL +     0u8.method2();
+   |
+
+error[E0599]: no method named `method2` found for type `u8` in the current scope
+  --> $DIR/ambiguous-trait-in-scope.rs:27:9
+   |
+LL |         fn method2(&self) {}
+   |            ------- the method is available for `u8` here
+...
+LL |     0u8.method2();
+   |         ^^^^^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `Trait` which provides `method2` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use m2::Trait;
+   |
+help: there is a method `method1` with a similar name
+   |
+LL -     0u8.method2();
+LL +     0u8.method1();
+   |
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0599`.