fix(flow): handle lesser array length guards

Narrow array lengths through complementary less-than branches. Preserve
condition flow through explicit empty else blocks and clamp exclusive
bounds at i64::MAX to avoid overflowing during inference.

Fixes #1207
Assisted-by: Codex

Author: lewis6991

crates/emmylua_code_analysis/src/compilation/analyzer/flow/bind_analyze/engine.rs

@@ -161,7 +161,6 @@ enum Continuation {
         post_if: FlowId,
         current: FlowId,
         else_block: Option<LuaBlock>,
-        has_else_clause: bool,
     },
     /// if statement: finalize after the else block completes
     IfFinal { post_if: FlowId, else_label: FlowId },
@@ -764,7 +763,6 @@ impl<'a, 'b> BindEngine<'a, 'b> {
         let then_label = self.binder.create_branch_label();
         let clauses = if_stat.get_else_if_clause_list().collect::<Vec<_>>();
         let else_clause = if_stat.get_else_clause();
-        let has_else_clause = else_clause.is_some();
         let else_block = else_clause.and_then(|clause| clause.get_block());
 
         self.stack.push(Continuation::IfBranchDone {
@@ -774,7 +772,6 @@ impl<'a, 'b> BindEngine<'a, 'b> {
             post_if: post_if_label,
             current,
             else_block,
-            has_else_clause,
         });
         if let Some(then_block) = if_stat.get_block() {
             self.stack
@@ -1114,7 +1111,6 @@ impl<'a, 'b> BindEngine<'a, 'b> {
                 post_if,
                 current,
                 else_block,
-                has_else_clause,
             } => {
                 self.binder.add_antecedent(post_if, value);
                 if idx >= clauses.len() {
@@ -1128,9 +1124,7 @@ impl<'a, 'b> BindEngine<'a, 'b> {
                             Step::Task(Task::Block(block, else_label))
                         }
                         None => {
-                            if !has_else_clause {
-                                self.binder.add_antecedent(post_if, else_label);
-                            }
+                            self.binder.add_antecedent(post_if, else_label);
                             Step::Done(finalize_if(self.binder, post_if, else_label))
                         }
                     }
@@ -1146,7 +1140,6 @@ impl<'a, 'b> BindEngine<'a, 'b> {
                         post_if,
                         current,
                         else_block,
-                        has_else_clause,
                     });
                     if let Some(block) = clause.get_block() {
                         self.stack.push(Continuation::ThenBlock { block });

crates/emmylua_code_analysis/src/compilation/test/flow.rs

@@ -3155,6 +3155,57 @@ _2 = a[1]
         ));
     }
 
+    #[test]
+    fn test_issue_1207_lesser_array_length_guards() {
+        let mut ws = VirtualWorkspace::new();
+        assert!(ws.has_no_diagnostic(
+            DiagnosticCode::AssignTypeMismatch,
+            r#"
+local a --- @type string[]
+if #a <= 1 then error() end
+
+--- @type string
+_ = a[2]
+
+local b --- @type string[]
+if #b < 2 then error() end
+
+--- @type string
+_ = b[2]
+            "#
+        ));
+    }
+
+    #[test]
+    fn test_issue_1207_array_length_bound_does_not_overflow() {
+        let mut ws = VirtualWorkspace::new();
+        assert!(ws.has_no_diagnostic(
+            DiagnosticCode::AssignTypeMismatch,
+            r#"
+local a --- @type string[]
+if #a <= 9223372036854775807 then error() end
+
+--- @type string
+_ = a[1]
+            "#
+        ));
+    }
+
+    #[test]
+    fn test_issue_1207_empty_else_preserves_array_length_guard() {
+        let mut ws = VirtualWorkspace::new();
+        assert!(ws.has_no_diagnostic(
+            DiagnosticCode::AssignTypeMismatch,
+            r#"
+local a --- @type string[]
+if not (#a > 1) then error() else end
+
+--- @type string
+_ = a[2]
+            "#
+        ));
+    }
+
     #[test]
     fn test_return_cast_with_fallback() {
         let mut ws = VirtualWorkspace::new();

crates/emmylua_code_analysis/src/semantic/infer/narrow/condition_flow/binary_flow.rs

@@ -63,25 +63,46 @@ pub fn get_type_at_binary_expr(
             right_expr,
             condition_flow.invert(),
         ),
-        BinaryOperator::OpGt => try_get_at_gt_or_ge_expr(
+        BinaryOperator::OpGt => try_get_at_array_len_expr(
             db,
             cache,
             var_ref_id,
             flow_node,
             left_expr,
             right_expr,
             condition_flow,
-            true,
+            1,
         ),
-        BinaryOperator::OpGe => try_get_at_gt_or_ge_expr(
+        BinaryOperator::OpGe => try_get_at_array_len_expr(
             db,
             cache,
             var_ref_id,
             flow_node,
             left_expr,
             right_expr,
             condition_flow,
-            false,
+            0,
+        ),
+        // The false branches are equivalent to `>=` and `>` respectively.
+        BinaryOperator::OpLt => try_get_at_array_len_expr(
+            db,
+            cache,
+            var_ref_id,
+            flow_node,
+            left_expr,
+            right_expr,
+            condition_flow.invert(),
+            0,
+        ),
+        BinaryOperator::OpLe => try_get_at_array_len_expr(
+            db,
+            cache,
+            var_ref_id,
+            flow_node,
+            left_expr,
+            right_expr,
+            condition_flow.invert(),
+            1,
         ),
         BinaryOperator::OpNilCoalescing => {
             try_get_at_nil_coalescing(db, cache, var_ref_id, left_expr, condition_flow)
@@ -177,15 +198,15 @@ fn try_get_at_eq_or_neq_expr(
 }
 
 #[allow(clippy::too_many_arguments)]
-fn try_get_at_gt_or_ge_expr(
+fn try_get_at_array_len_expr(
     db: &DbIndex,
     cache: &mut LuaInferCache,
     var_ref_id: &VarRefId,
     flow_node: &FlowNode,
     left_expr: LuaExpr,
     right_expr: LuaExpr,
     condition_flow: InferConditionFlow,
-    gt: bool,
+    max_adjustment: i64,
 ) -> Result<ConditionFlowAction, InferFailReason> {
     match left_expr {
         LuaExpr::UnaryExpr(unary_expr) => {
@@ -217,7 +238,7 @@ fn try_get_at_gt_or_ge_expr(
                 expr: right_expr,
                 resume: ExprTypeContinuation::ArrayLen {
                     subquery_condition_flow: condition_flow,
-                    max_adjustment: if gt { 1 } else { 0 },
+                    max_adjustment,
                 },
             })
         }

crates/emmylua_code_analysis/src/semantic/infer/narrow/condition_flow/mod.rs

@@ -293,10 +293,10 @@ impl PendingConditionNarrow {
                     LuaType::Array(array_type),
                     LuaType::IntegerConst(i) | LuaType::DocIntegerConst(i),
                 ) if matches!(condition_flow, InferConditionFlow::TrueCondition) => {
-                    let new_array_type = LuaArrayType::new(
-                        array_type.get_base().clone(),
-                        LuaArrayLen::Max(*i + *max_adjustment),
-                    );
+                    // Array bounds are i64, so clamp an exclusive bound above i64::MAX.
+                    let max_len = i.saturating_add(*max_adjustment);
+                    let new_array_type =
+                        LuaArrayType::new(array_type.get_base().clone(), LuaArrayLen::Max(max_len));
                     LuaType::Array(new_array_type.into())
                 }
                 _ => antecedent_type,