fix: Return proper error for bare lambda expressions (#5634)

Co-authored-by: Claude <[email protected]>

Author: max-sixty

prqlc/prqlc/src/semantic/resolver/types.rs

@@ -106,16 +106,21 @@ impl Resolver<'_> {
                 // similarly as normal table references, we want to be able to infer columns
                 // of this table, which means it needs to be defined somewhere
                 // in the module structure.
-                let frame = self.declare_table_for_literal(
-                    found
-                        .clone()
-                        .id
-                        // This is quite rare but possible with something like
-                        // `a -> b` at the moment.
-                        .ok_or_else(|| Error::new_bug(4280))?,
-                    None,
-                    found.alias.clone(),
-                );
+                let Some(id) = found.id else {
+                    // Expression has no id - this happens with bare lambdas like `x -> y`
+                    let found_desc = match &found.kind {
+                        ExprKind::Func(_) => "a function".to_string(),
+                        kind => format!("{kind:?}"),
+                    };
+                    return Err(Error::new(Reason::Expected {
+                        who: None,
+                        expected: "a table".to_string(),
+                        found: found_desc,
+                    })
+                    .with_span(found.span));
+                };
+
+                let frame = self.declare_table_for_literal(id, None, found.alias.clone());
 
                 // override the empty frame with frame of the new table
                 found.lineage = Some(frame)

prqlc/prqlc/tests/integration/bad_error_messages.rs

@@ -211,11 +211,17 @@ fn nested_groups() {
 
 #[test]
 fn a_arrow_b() {
-    // This is fairly low priority, given how idiosyncratic the query is. If
-    // we find other cases, we should increase the priority.
     assert_snapshot!(compile(r###"
     x -> y
-    "###).unwrap_err(), @"Error: internal compiler error; tracked at https://github.com/PRQL/prql/issues/4280");
+    "###).unwrap_err(), @r"
+    Error:
+       ╭─[ :2:5 ]
+       │
+     2 │     x -> y
+       │     ───┬──
+       │        ╰──── expected a table, but found a function
+    ───╯
+    ");
 }
 
 #[test]