diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 776c8ad528c..cc877e2fd30 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -1969,7 +1969,7 @@ fn visit_ty(&mut self, ty: &'a Ty) {
                     None
                 }
             })
-            .map(|res| res.expect_full_res())
+            .and_then(|res| res.full_res())
             .filter(|res| {
                 // Permit the types that unambiguously always
                 // result in the same type constructor being used
diff --git a/src/test/ui/resolve/issue-102946.rs b/src/test/ui/resolve/issue-102946.rs
new file mode 100644
index 00000000000..c6feca6f32f
--- /dev/null
+++ b/src/test/ui/resolve/issue-102946.rs
@@ -0,0 +1,7 @@
+impl Error for str::Utf8Error {
+    //~^ ERROR cannot find trait `Error` in this scope
+    //~| ERROR ambiguous associated type
+    fn description(&self)  {}
+}
+
+fn main() {}
diff --git a/src/test/ui/resolve/issue-102946.stderr b/src/test/ui/resolve/issue-102946.stderr
new file mode 100644
index 00000000000..65be0258e6d
--- /dev/null
+++ b/src/test/ui/resolve/issue-102946.stderr
@@ -0,0 +1,26 @@
+error[E0405]: cannot find trait `Error` in this scope
+  --> $DIR/issue-102946.rs:1:6
+   |
+LL | impl Error for str::Utf8Error {
+   |      ^^^^^ not found in this scope
+   |
+help: consider importing this trait
+   |
+LL | use std::error::Error;
+   |
+
+error[E0223]: ambiguous associated type
+  --> $DIR/issue-102946.rs:1:16
+   |
+LL | impl Error for str::Utf8Error {
+   |                ^^^^^^^^^^^^^^
+   |
+help: you are looking for the module in `std`, not the primitive type
+   |
+LL | impl Error for std::str::Utf8Error {
+   |                +++++
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0223, E0405.
+For more information about an error, try `rustc --explain E0223`.