diff --git a/compiler/rustc_passes/src/eii.rs b/compiler/rustc_passes/src/eii.rs
index 691576e6a05fc..ab3f9f0d21825 100644
--- a/compiler/rustc_passes/src/eii.rs
+++ b/compiler/rustc_passes/src/eii.rs
@@ -116,7 +116,8 @@ pub(crate) fn check_externally_implementable_items<'tcx>(tcx: TyCtxt<'tcx>, ():
         }
 
         if default_impls.len() > 1 {
-            panic!("multiple not supported right now");
+            let decl_span = tcx.def_ident_span(decl_did).unwrap();
+            tcx.dcx().span_delayed_bug(decl_span, "multiple not supported right now");
         }
 
         let (local_impl, is_default) =
diff --git a/tests/ui/eii/default/multiple-default-impls-same-name.rs b/tests/ui/eii/default/multiple-default-impls-same-name.rs
new file mode 100644
index 0000000000000..080e0a46b7557
--- /dev/null
+++ b/tests/ui/eii/default/multiple-default-impls-same-name.rs
@@ -0,0 +1,19 @@
+#![crate_type = "lib"]
+#![feature(extern_item_impls)]
+// `eii` expands to, among other things, `macro eii() {}`.
+// If we have two eiis named the same thing, we have a duplicate definition
+// for that macro. The compiler happily continues compiling on duplicate
+// definitions though, to emit as many diagnostics as possible.
+// However, in the case of eiis, this can break the assumption that every
+// eii has only one default implementation, since the default for both eiis will
+// name resolve to the same eii definiton (since the other definition was duplicate)
+// This test tests for the previously-ICE that occurred when this assumption
+// (of 1 default) was broken which was reported in #149982.
+
+#[eii(eii1)]
+fn a() {}
+
+#[eii(eii1)]
+//~^ ERROR the name `eii1` is defined multiple times
+fn a() {}
+//~^ ERROR the name `a` is defined multiple times
diff --git a/tests/ui/eii/default/multiple-default-impls-same-name.stderr b/tests/ui/eii/default/multiple-default-impls-same-name.stderr
new file mode 100644
index 0000000000000..20544fcfeeba5
--- /dev/null
+++ b/tests/ui/eii/default/multiple-default-impls-same-name.stderr
@@ -0,0 +1,25 @@
+error[E0428]: the name `a` is defined multiple times
+  --> $DIR/multiple-default-impls-same-name.rs:18:1
+   |
+LL | fn a() {}
+   | ------ previous definition of the value `a` here
+...
+LL | fn a() {}
+   | ^^^^^^ `a` redefined here
+   |
+   = note: `a` must be defined only once in the value namespace of this module
+
+error[E0428]: the name `eii1` is defined multiple times
+  --> $DIR/multiple-default-impls-same-name.rs:16:1
+   |
+LL | #[eii(eii1)]
+   | ------------ previous definition of the macro `eii1` here
+...
+LL | #[eii(eii1)]
+   | ^^^^^^^^^^^^ `eii1` redefined here
+   |
+   = note: `eii1` must be defined only once in the macro namespace of this module
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0428`.
diff --git a/tests/ui/eii/default/multiple-default-impls.rs b/tests/ui/eii/default/multiple-default-impls.rs
new file mode 100644
index 0000000000000..8d2ad0c1e1378
--- /dev/null
+++ b/tests/ui/eii/default/multiple-default-impls.rs
@@ -0,0 +1,18 @@
+#![crate_type = "lib"]
+#![feature(extern_item_impls)]
+// `eii` expands to, among other things, `macro eii() {}`.
+// If we have two eiis named the same thing, we have a duplicate definition
+// for that macro. The compiler happily continues compiling on duplicate
+// definitions though, to emit as many diagnostics as possible.
+// However, in the case of eiis, this can break the assumption that every
+// eii has only one default implementation, since the default for both eiis will
+// name resolve to the same eii definiton (since the other definition was duplicate)
+// This test tests for the previously-ICE that occurred when this assumption
+// (of 1 default) was broken which was reported in #149982.
+
+#[eii(eii1)]
+fn a() {}
+
+#[eii(eii1)]
+//~^ ERROR the name `eii1` is defined multiple times
+fn b() {}
diff --git a/tests/ui/eii/default/multiple-default-impls.stderr b/tests/ui/eii/default/multiple-default-impls.stderr
new file mode 100644
index 0000000000000..b270b2346cd50
--- /dev/null
+++ b/tests/ui/eii/default/multiple-default-impls.stderr
@@ -0,0 +1,14 @@
+error[E0428]: the name `eii1` is defined multiple times
+  --> $DIR/multiple-default-impls.rs:16:1
+   |
+LL | #[eii(eii1)]
+   | ------------ previous definition of the macro `eii1` here
+...
+LL | #[eii(eii1)]
+   | ^^^^^^^^^^^^ `eii1` redefined here
+   |
+   = note: `eii1` must be defined only once in the macro namespace of this module
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0428`.