another related case that deserved a test

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

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`.