preparations for specializing closures

compiler/reporting/src/error/type.rs

@@ -924,6 +924,10 @@ fn add_category<'b>(
             alloc.private_tag_name(*name),
             alloc.text(" private tag application has the type:"),
         ]),
+        TagApply {
+            tag_name: TagName::Closure(_name),
+            args_count: _,
+        } => unreachable!("closure tags are for internal use only"),
 
         Record => alloc.concat(vec![this_is, alloc.text(" a record of type:")]),
 

compiler/reporting/src/report.rs

@@ -248,6 +248,7 @@ impl<'a> RocDocAllocator<'a> {
         match tn {
             TagName::Global(uppercase) => self.global_tag_name(uppercase),
             TagName::Private(symbol) => self.private_tag_name(symbol),
+            TagName::Closure(_symbol) => unreachable!("closure tags are internal only"),
         }
     }