Add test gen for multimorphic capture

crates/compiler/load_internal/src/file.rs

@@ -2599,9 +2599,20 @@ fn finish_specialization(
         .into_inner()
         .into_module_ids();
 
+    let mut all_ident_ids = state.constrained_ident_ids;
+    let multimorphic_idents = state
+        .multimorphic_names
+        .try_unwrap_names()
+        .expect("There were still outstanding Arc references to multimorphic_names");
+    let old_idents = all_ident_ids.insert(ModuleId::MULTIMORPHIC, multimorphic_idents);
+    debug_assert!(
+        old_idents.is_none() || old_idents.unwrap().is_empty(),
+        "duplicate multimorphic idents"
+    );
+
     let interns = Interns {
         module_ids,
-        all_ident_ids: state.constrained_ident_ids,
+        all_ident_ids,
     };
 
     let State {

crates/compiler/mono/src/layout.rs

@@ -852,6 +852,16 @@ impl MultimorphicNames {
     fn insert<'b>(&mut self, name: Symbol, captures_layouts: &'b [Layout<'b>]) -> Symbol {
         self.0.lock().unwrap().insert(name, captures_layouts)
     }
+
+    /// Assumes there is only one clone still alive.
+    /// If there is more than one clone alive, `self` is returned.
+    pub fn try_unwrap_names(self) -> Result<IdentIds, Self> {
+        let mutex = Arc::try_unwrap(self.0).map_err(Self)?;
+        let table = mutex
+            .into_inner()
+            .expect("how can there be another lock if we consumed the only ref?");
+        Ok(table.ident_ids)
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]

crates/compiler/test_gen/src/gen_primitives.rs

@@ -3498,3 +3498,30 @@ fn polymorphic_lambda_captures_polymorphic_value() {
         u64
     )
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm"))]
+fn multimorphic_lambda_set_u64_vs_u8_capture() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            capture : _ -> ({} -> Str)
+            capture = \val ->
+                \{} ->
+                    Num.toStr val
+
+            x : [True, False]
+            x = True
+
+            fun =
+                when x is
+                    True -> capture 123u64
+                    False -> capture 18u8
+
+            fun {}
+            "#
+        ),
+        RocStr::from("123"),
+        RocStr
+    )
+}