fix: Reject multi-element sequences without internal captures

crates/plotnik-compiler/src/analyze/type_check/infer.rs

@@ -581,9 +581,9 @@ impl<'a, 'd> InferenceVisitor<'a, 'd> {
         let flow = match quantifier {
             QuantifierKind::Optional => self.make_flow_optional(inner_info.flow),
             QuantifierKind::ZeroOrMore | QuantifierKind::OneOrMore => {
-                if !is_row_capture {
-                    self.check_strict_dimensionality(quant, &inner_info);
-                }
+                // Always check multi-element sequences (row capture doesn't help)
+                // Only skip internal capture check when is_row_capture
+                self.check_strict_dimensionality(quant, &inner_info, is_row_capture);
                 self.make_flow_array(inner_info.flow, &inner, quantifier.is_non_empty())
             }
         };
@@ -681,8 +681,46 @@ impl<'a, 'd> InferenceVisitor<'a, 'd> {
     }
 
     /// Check strict dimensionality rule for * and + quantifiers.
-    /// Captures inside a quantifier are forbidden unless marked as a row capture.
-    fn check_strict_dimensionality(&mut self, quant: &QuantifiedExpr, inner_info: &TermInfo) {
+    ///
+    /// Two checks:
+    /// 1. Multi-element patterns (Arity::Many) without captures can't be scalar arrays
+    ///    (applies regardless of is_row_capture - row capture doesn't help here)
+    /// 2. Internal captures require a row capture on the quantifier
+    ///    (skipped when is_row_capture=true)
+    fn check_strict_dimensionality(
+        &mut self,
+        quant: &QuantifiedExpr,
+        inner_info: &TermInfo,
+        is_row_capture: bool,
+    ) {
+        let op = quant
+            .operator()
+            .map(|t| t.text().to_string())
+            .unwrap_or_else(|| "*".to_string());
+
+        // Check 1: Multi-element patterns without captures can't be scalar arrays
+        // This check applies even with row capture - you can't meaningfully capture
+        // multiple nodes per iteration as a scalar
+        if inner_info.arity == Arity::Many && inner_info.flow.is_void() {
+            self.diag
+                .report(
+                    self.source_id,
+                    DiagnosticKind::MultiElementScalarCapture,
+                    quant.text_range(),
+                )
+                .message(format!(
+                    "sequence with `{}` matches multiple nodes but has no internal captures",
+                    op
+                ))
+                .emit();
+            return;
+        }
+
+        // Check 2: Internal captures require row capture (skip if already a row capture)
+        if is_row_capture {
+            return;
+        }
+
         let TypeFlow::Bubble(type_id) = &inner_info.flow else {
             return;
         };
@@ -695,11 +733,6 @@ impl<'a, 'd> InferenceVisitor<'a, 'd> {
             return;
         }
 
-        let op = quant
-            .operator()
-            .map(|t| t.text().to_string())
-            .unwrap_or_else(|| "*".to_string());
-
         let capture_names: Vec<_> = fields
             .keys()
             .map(|s| format!("`@{}`", self.interner.resolve(*s)))

crates/plotnik-compiler/src/analyze/type_check/type_check_tests.rs

@@ -664,6 +664,39 @@ fn error_named_node_with_capture_quantified() {
     ");
 }
 
+#[test]
+fn error_multi_element_sequence_no_captures() {
+    let input = "Bad = {(identifier) (number)}* @items";
+
+    let res = Query::expect_invalid(input);
+
+    insta::assert_snapshot!(res, @r"
+    error: sequence with `*` matches multiple nodes but has no internal captures
+      |
+    1 | Bad = {(identifier) (number)}* @items
+      |       ^^^^^^^^^^^^^^^^^^^^^^^^
+      |
+    help: add internal captures: `{(a) @a (b) @b}* @items`
+    ");
+}
+
+#[test]
+fn error_multi_element_alternation_branch() {
+    // Alternation where one branch is multi-element
+    let input = "Bad = [(identifier) {(number) (string)}]* @items";
+
+    let res = Query::expect_invalid(input);
+
+    insta::assert_snapshot!(res, @r"
+    error: sequence with `*` matches multiple nodes but has no internal captures
+      |
+    1 | Bad = [(identifier) {(number) (string)}]* @items
+      |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+      |
+    help: add internal captures: `{(a) @a (b) @b}* @items`
+    ");
+}
+
 #[test]
 fn recursive_type_with_alternation() {
     let input = indoc! {r#"

crates/plotnik-compiler/src/diagnostics/message.rs

@@ -77,6 +77,7 @@ pub enum DiagnosticKind {
     MultiCaptureQuantifierNoName,
     UnusedBranchLabels,
     StrictDimensionalityViolation,
+    MultiElementScalarCapture,
     UncapturedOutputWithCaptures,
     AmbiguousUncapturedOutputs,
     DuplicateCaptureInScope,
@@ -181,6 +182,9 @@ impl DiagnosticKind {
             Self::AmbiguousUncapturedOutputs => {
                 Some("capture each expression explicitly: `(X) @x (Y) @y`")
             }
+            Self::MultiElementScalarCapture => {
+                Some("add internal captures: `{(a) @a (b) @b}* @items`")
+            }
             _ => None,
         }
     }
@@ -253,6 +257,9 @@ impl DiagnosticKind {
             Self::StrictDimensionalityViolation => {
                 "quantifier with captures requires a struct capture"
             }
+            Self::MultiElementScalarCapture => {
+                "cannot capture multi-element pattern as scalar array"
+            }
             Self::UncapturedOutputWithCaptures => {
                 "output-producing expression requires capture when siblings have captures"
             }
@@ -301,6 +308,7 @@ impl DiagnosticKind {
 
             // Type inference errors with context
             Self::StrictDimensionalityViolation => "{}".to_string(),
+            Self::MultiElementScalarCapture => "{}".to_string(),
             Self::DuplicateCaptureInScope => {
                 "capture `@{}` already defined in this scope".to_string()
             }

crates/plotnik-compiler/src/emit/emit_tests.rs

@@ -279,8 +279,9 @@ fn sequences_nested() {
 
 #[test]
 fn sequences_in_quantifier() {
+    // Sequence with internal captures - valid for struct array
     snap!(indoc! {r#"
-        Test = (array {(identifier) (number)}* @items)
+        Test = (array {(identifier) @id (number) @num}* @items)
     "#});
 }
 

crates/plotnik-compiler/src/emit/snapshots/plotnik_compiler__emit__emit_tests__sequences_in_quantifier.snap

@@ -1,29 +1,38 @@
 ---
 source: crates/plotnik-compiler/src/emit/emit_tests.rs
 ---
-Test = (array {(identifier) (number)}* @items)
+Test = (array {(identifier) @id (number) @num}* @items)
 ---
 [strings]
 S0 "Beauty will save the world"
-S1 "items"
-S2 "Test"
-S3 "array"
-S4 "identifier"
-S5 "number"
+S1 "id"
+S2 "num"
+S3 "items"
+S4 "Test"
+S5 "array"
+S6 "identifier"
+S7 "number"
 
 [type_defs]
 T0 = <Node>
-T1 = ArrayStar(T0)  ; <Node>*
-T2 = Struct  M0:1  ; { items }
+T1 = Struct  M0:2  ; { id, num }
+T2 = ArrayStar(T1)  ; T1*
+T3 = Struct  M2:1  ; { items }
+T4 = Struct  M3:1  ; { id }
+T5 = Struct  M4:1  ; { num }
 
 [type_members]
-M0: S1 → T1  ; items: T1
+M0: S1 → T0  ; id: <Node>
+M1: S2 → T0  ; num: <Node>
+M2: S3 → T2  ; items: T2
+M3: S1 → T0  ; id: <Node>
+M4: S2 → T0  ; num: <Node>
 
 [type_names]
-N0: S2 → T2  ; Test
+N0: S4 → T3  ; Test
 
 [entrypoints]
-Test = 06 :: T2
+Test = 06 :: T3
 
 [transitions]
 _ObjWrap:
@@ -35,13 +44,15 @@ _ObjWrap:
 Test:
   06   !   (array)                          08
   07  ...  
-  08   ε   [Arr]                            18, 11
+  08   ε   [Arr]                            22, 12
   10                                        ▶
-  11   ε   [EndArr Set(M0)]                 10
-  13   ▷   (number) [Node Push]             22, 24
-  15   !   (identifier)                     13
-  16   ▷   _                                15, 16, 24
-  18   ▽   _                                15, 16, 24
-  20   ▷   _                                15, 20, 24
-  22   ▷   _                                15, 20, 24
-  24   △   [EndArr Set(M0)] _               10
+  11   ε                                    28
+  12   ε   [EndArr Set(M2)]                 10
+  14   ε   [EndObj Push]                    26, 11
+  16   ▷   (number) [Node Set(M1)]          14
+  18   !   [Obj] (identifier) [Node Set(M0)]  16
+  20   ▷   _                                18, 20, 28
+  22   ▽   _                                18, 20, 28
+  24   ▷   _                                18, 24, 28
+  26   ▷   _                                18, 24, 28
+  28   △   [EndArr Set(M2)] _               10

crates/plotnik-compiler/src/parser/tests/grammar/sequences_tests.rs

@@ -177,11 +177,14 @@ fn sequence_with_captures() {
 
 #[test]
 fn sequence_with_quantifier() {
+    // Note: This tests parser behavior only. The pattern `{(a) (b)}+` is
+    // semantically invalid (multi-element sequence without captures), but
+    // we're only testing that the parser produces the correct CST.
     let input = indoc! {r#"
     Q = {(a) (b)}+
     "#};
 
-    let res = Query::expect_valid_cst(input);
+    let res = Query::parse_cst(input);
 
     insta::assert_snapshot!(res, @r#"
     Root
@@ -306,11 +309,14 @@ fn sequence_with_alternation() {
 
 #[test]
 fn sequence_comma_separated_expression() {
+    // Note: This tests parser behavior only. The inner pattern `{"," (number)}*`
+    // is semantically invalid (multi-element sequence without captures), but
+    // we're only testing that the parser produces the correct CST.
     let input = indoc! {r#"
     Q = {(number) {"," (number)}*}
     "#};
 
-    let res = Query::expect_valid_cst(input);
+    let res = Query::parse_cst(input);
 
     insta::assert_snapshot!(res, @r#"
     Root

crates/plotnik-compiler/src/query/query_tests.rs

@@ -36,6 +36,47 @@ impl QueryAnalyzed {
         query
     }
 
+    /// Parse and validate syntax only (no semantic analysis).
+    /// Use this for pure parser/grammar tests.
+    #[track_caller]
+    fn parse_syntax_only(src: &str) -> Self {
+        use crate::diagnostics::DiagnosticKind::*;
+        let source_map = SourceMap::one_liner(src);
+        let query = QueryBuilder::new(source_map).parse().unwrap().analyze();
+        // Only check for parse errors (not semantic errors)
+        let diag = query.diagnostics();
+        let has_parse_error = diag.raw().iter().any(|d| {
+            matches!(
+                d.kind,
+                UnclosedTree
+                    | UnclosedSequence
+                    | UnclosedAlternation
+                    | UnclosedRegex
+                    | ExpectedExpression
+                    | ExpectedTypeName
+                    | ExpectedCaptureName
+                    | ExpectedFieldName
+                    | ExpectedSubtype
+                    | ExpectedPredicateValue
+                    | EmptyTree
+                    | EmptyAnonymousNode
+                    | EmptySequence
+                    | EmptyAlternation
+                    | BareIdentifier
+                    | InvalidSeparator
+                    | UnexpectedToken
+            )
+        });
+
+        if has_parse_error {
+            panic!(
+                "Expected valid syntax, got parse error:\n{}",
+                query.dump_diagnostics()
+            );
+        }
+        query
+    }
+
     #[track_caller]
     pub fn expect(src: &str) -> Self {
         let source_map = SourceMap::one_liner(src);
@@ -52,6 +93,12 @@ impl QueryAnalyzed {
         Self::parse_and_validate(src).dump_cst()
     }
 
+    /// Parse-only CST dump (for pure parser tests, no semantic validation).
+    #[track_caller]
+    pub fn parse_cst(src: &str) -> String {
+        Self::parse_syntax_only(src).dump_cst()
+    }
+
     #[track_caller]
     pub fn expect_valid_cst_full(src: &str) -> String {
         Self::parse_and_validate(src).dump_cst_full()