2 files changed, 49 insertions, 0 deletions
diff --git a/tests/ui/generic-const-items/assoc-const-missing-type.rs b/tests/ui/generic-const-items/assoc-const-missing-type.rs
new file mode 100644
index 00000000000..80f282ea134
--- /dev/null
+++ b/tests/ui/generic-const-items/assoc-const-missing-type.rs
@@ -0,0 +1,17 @@
+// Ensure that we properly deal with missing/placeholder types inside GACs.
+// issue: rust-lang/rust#124833
+#![feature(generic_const_items)]
+#![allow(incomplete_features)]
+
+trait Trait {
+    const K<T>: T;
+}
+
+impl Trait for () {
+    const K<T> = ();
+    //~^ ERROR missing type for `const` item
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+}
+
+fn main() {}
diff --git a/tests/ui/generic-const-items/assoc-const-missing-type.stderr b/tests/ui/generic-const-items/assoc-const-missing-type.stderr
new file mode 100644
index 00000000000..158e9a068f6
--- /dev/null
+++ b/tests/ui/generic-const-items/assoc-const-missing-type.stderr
@@ -0,0 +1,32 @@
+error[E0308]: mismatched types
+  --> $DIR/assoc-const-missing-type.rs:11:18
+   |
+LL |     const K<T> = ();
+   |             -    ^^ expected type parameter `T`, found `()`
+   |             |
+   |             expected this type parameter
+   |
+   = note: expected type parameter `T`
+                   found unit type `()`
+
+error: missing type for `const` item
+  --> $DIR/assoc-const-missing-type.rs:11:15
+   |
+LL |     const K<T> = ();
+   |               ^ help: provide a type for the associated constant: `()`
+
+error[E0308]: mismatched types
+  --> $DIR/assoc-const-missing-type.rs:11:18
+   |
+LL |     const K<T> = ();
+   |             -    ^^ expected type parameter `T`, found `()`
+   |             |
+   |             expected this type parameter
+   |
+   = note: expected type parameter `T`
+                   found unit type `()`
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.