Rollup merge of rust-lang#83699 - JohnTitor:issue-68830, r=Dylan-DPC

Add a regression test for issue-68830

Closes rust-lang#68830

src/test/ui/specialization/issue-68830-spurious-diagnostics.rs

@@ -0,0 +1,23 @@
+// A regression test for #68830. This checks we don't emit
+// a verbose `conflicting implementations` error.
+
+#![feature(specialization)]
+#![allow(incomplete_features)]
+
+struct BadStruct {
+    err: MissingType //~ ERROR: cannot find type `MissingType` in this scope
+}
+
+trait MyTrait<T> {
+    fn foo();
+}
+
+impl<T, D> MyTrait<T> for D {
+    default fn foo() {}
+}
+
+impl<T> MyTrait<T> for BadStruct {
+    fn foo() {}
+}
+
+fn main() {}

src/test/ui/specialization/issue-68830-spurious-diagnostics.stderr

@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `MissingType` in this scope
+  --> $DIR/issue-68830-spurious-diagnostics.rs:8:10
+   |
+LL |     err: MissingType
+   |          ^^^^^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.