fix: Fix invalid state due to cached IR (#18262)

crates/polars-plan/src/plans/conversion/dsl_to_ir.rs

@@ -794,8 +794,11 @@ pub fn to_alp_impl(
             IR::Sink { input, payload }
         },
         DslPlan::IR { node, dsl, version } => {
-            return if let (true, Some(node)) = (version == lp_arena.version(), node) {
-                Ok(node)
+            return if node.is_some()
+                && version == lp_arena.version()
+                && convert.used_arenas.insert(version)
+            {
+                Ok(node.unwrap())
             } else {
                 to_alp_impl(owned(dsl), expr_arena, lp_arena, convert)
             }

crates/polars-plan/src/plans/conversion/stack_opt.rs

@@ -7,6 +7,12 @@ pub(super) struct ConversionOptimizer {
     scratch: Vec<Node>,
     simplify: Option<SimplifyExprRule>,
     coerce: Option<TypeCoercionRule>,
+    // IR's can be cached in the DSL.
+    // But if they are used multiple times in DSL (e.g. concat/join)
+    // then it can occur that we take a slot multiple times.
+    // So we keep track of the arena versions used and allow only
+    // one unique IR cache to be reused.
+    pub(super) used_arenas: PlHashSet<u32>,
 }
 
 impl ConversionOptimizer {
@@ -27,6 +33,7 @@ impl ConversionOptimizer {
             scratch: Vec::with_capacity(8),
             simplify,
             coerce,
+            used_arenas: Default::default(),
         }
     }
 

py-polars/tests/unit/test_schema.py

@@ -65,3 +65,9 @@ def test_schema_in_map_elements_returns_scalar() -> None:
 
     assert (q.collect_schema()) == schema
     assert q.collect().schema == schema
+
+
+def test_ir_cache_unique_18198() -> None:
+    lf = pl.LazyFrame({"a": [1]})
+    lf.collect_schema()
+    assert pl.concat([lf, lf]).collect().to_dict(as_series=False) == {"a": [1, 1]}