fix: resolve all stress test bugs

- Record equality: add Record case to values_equal in interpreter
- Invalid escapes: error on unknown escape sequences in lexer
- Unknown effects: validate effect names in check_function with suggestions
- Circular types: add DFS cycle detection in check_type_cycles
- Parser: require | for enum variants, enabling proper type alias syntax

All 265 tests pass.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/STRESS_TEST_FINDINGS.md

@@ -2,48 +2,43 @@
 
 This document summarizes issues found during systematic stress testing of the Lux parser, typechecker, and REPL.
 
-## Critical Bugs
+## Critical Bugs (All Fixed)
 
-### 1. Record Equality Returns False for Equal Records
+### 1. ~~Record Equality Returns False for Equal Records~~ (FIXED)
 ```lux
 let r1 = { x: 1, y: 2 }
 let r2 = { x: 1, y: 2 }
-r1 == r2  // Returns false! Should be true
+r1 == r2  // Now returns true
 ```
-**Impact**: High - breaks expected semantics for record comparison
-**Location**: Likely in interpreter's equality logic for `Value::Record`
+**Fix**: Added Record case to `Interpreter::values_equal` in `src/interpreter.rs`
 
-### 2. Invalid Escape Sequences Silently Accepted
+### 2. ~~Invalid Escape Sequences Silently Accepted~~ (FIXED)
 ```lux
-"\z\q\w"  // Returns "zqw" - backslashes silently dropped
-"\x41"    // Returns "x41" - hex escapes not supported but no error
-"\u0041"  // Returns "u0041" - unicode escapes not supported but no error
+"\z"  // Now produces error: "Invalid escape sequence: \z"
 ```
-**Impact**: High - users expect invalid escapes to error
-**Fix**: Parser should reject unknown escape sequences
+**Fix**: Modified lexer in `src/lexer.rs` to error on unknown escape sequences
 
-### 3. Unknown Effects Silently Accepted in Declarations
+### 3. ~~Unknown Effects Silently Accepted in Declarations~~ (FIXED)
 ```lux
-fn test(): Unit with {CompletelyFakeEffect} = ()  // No error!
+fn test(): Unit with {CompletelyFakeEffect} = ()  // Now produces error with suggestions
 ```
-**Impact**: Medium - typos in effect names not caught until call site
-**Fix**: Validate effect names during function declaration checking
+**Fix**: Added effect validation in `TypeChecker::check_function` in `src/typechecker.rs`
 
-### 4. No Forward References for Mutual Recursion
+### 4. ~~No Forward References for Mutual Recursion~~ (FIXED/DOCUMENTED)
 ```lux
-fn isEven(n: Int): Bool = if n == 0 then true else isOdd(n - 1)  // Error: isOdd undefined
+fn isEven(n: Int): Bool = if n == 0 then true else isOdd(n - 1)
 fn isOdd(n: Int): Bool = if n == 0 then false else isEven(n - 1)
+// Works in files (two-pass type checking), REPL limitation documented
 ```
-**Impact**: Medium - common pattern not supported
-**Fix**: Two-pass type checking or explicit forward declarations
+**Note**: Works when loaded from files due to two-pass type checking. REPL processes line-by-line, so forward references don't work there.
 
-### 5. Circular Type Definitions Silently Fail
+### 5. ~~Circular Type Definitions Silently Fail~~ (FIXED)
 ```lux
 type A = B
 type B = A
-// No output, no error
+// Now produces error: "Circular type definition detected: A -> B -> A"
 ```
-**Impact**: Medium - should produce clear error about circular definition
+**Fix**: Added `check_type_cycles` method with DFS cycle detection in `src/typechecker.rs`. Also fixed parser to require `|` for enum variants, allowing `type A = B` to be parsed as a type alias.
 
 ## Parser Issues
 

src/interpreter.rs

@@ -2810,6 +2810,11 @@ impl Interpreter {
             (Value::Tuple(a), Value::Tuple(b)) => {
                 a.len() == b.len() && a.iter().zip(b).all(|(x, y)| self.values_equal(x, y))
             }
+            (Value::Record(a), Value::Record(b)) => {
+                a.len() == b.len() && a.iter().all(|(k, v)| {
+                    b.get(k).map(|bv| self.values_equal(v, bv)).unwrap_or(false)
+                })
+            }
             (
                 Value::Constructor {
                     name: n1,
@@ -2824,6 +2829,7 @@ impl Interpreter {
                     && f1.len() == f2.len()
                     && f1.iter().zip(f2).all(|(x, y)| self.values_equal(x, y))
             }
+            (Value::Json(a), Value::Json(b)) => a == b,
             _ => false,
         }
     }

src/lexer.rs

@@ -484,13 +484,21 @@ impl<'a> Lexer<'a> {
                             current_literal.push('}');
                         }
                         _ => {
+                            let escape_start = self.pos;
                             let escaped = match self.advance() {
                                 Some('n') => '\n',
                                 Some('r') => '\r',
                                 Some('t') => '\t',
                                 Some('\\') => '\\',
                                 Some('"') => '"',
-                                Some(c) => c,
+                                Some('0') => '\0',
+                                Some('\'') => '\'',
+                                Some(c) => {
+                                    return Err(LexError {
+                                        message: format!("Invalid escape sequence: \\{}", c),
+                                        span: Span::new(escape_start - 1, self.pos),
+                                    });
+                                }
                                 None => {
                                     return Err(LexError {
                                         message: "Unterminated string".into(),

src/main.rs

@@ -1813,6 +1813,46 @@ c")"#;
         assert_eq!(eval(r#"let x = typeOf("hello")"#).unwrap(), r#""String""#);
     }
 
+    // Bug fix tests
+    #[test]
+    fn test_record_equality() {
+        assert_eq!(eval("let x = { a: 1, b: 2 } == { a: 1, b: 2 }").unwrap(), "true");
+        assert_eq!(eval("let x = { a: 1 } == { a: 2 }").unwrap(), "false");
+        assert_eq!(eval("let x = { a: 1, b: 2 } == { a: 1, b: 3 }").unwrap(), "false");
+    }
+
+    #[test]
+    fn test_invalid_escape_sequence() {
+        let result = eval(r#"let x = "\z""#);
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("Invalid escape sequence"));
+    }
+
+    #[test]
+    fn test_unknown_effect_error() {
+        let result = eval("fn test(): Unit with {FakeEffect} = ()");
+        assert!(result.is_err());
+        assert!(result.unwrap_err().contains("Unknown effect"));
+    }
+
+    #[test]
+    fn test_circular_type_definitions() {
+        let result = eval("type A = B\ntype B = A");
+        assert!(result.is_err(), "Should detect circular type definitions");
+        let err = result.unwrap_err();
+        assert!(err.contains("Circular"), "Error should mention 'Circular': {}", err);
+    }
+
+    #[test]
+    fn test_mutual_recursion() {
+        let source = r#"
+            fn isEven(n: Int): Bool = if n == 0 then true else isOdd(n - 1)
+            fn isOdd(n: Int): Bool = if n == 0 then false else isEven(n - 1)
+            let result = isEven(10)
+        "#;
+        assert_eq!(eval(source).unwrap(), "true");
+    }
+
     // Pipe with stdlib tests
     #[test]
     fn test_pipe_with_list() {
@@ -2948,7 +2988,6 @@ c")"#;
     mod example_tests {
         use super::*;
         use std::fs;
-        use std::path::Path;
 
         fn check_file(path: &str) -> Result<(), String> {
             let source = fs::read_to_string(path)

src/parser.rs

@@ -478,11 +478,11 @@ impl Parser {
             self.advance();
             self.skip_newlines();
 
-            if self.check(TokenKind::Pipe) || self.peek_is_variant() {
-                // Enum type
+            if self.check(TokenKind::Pipe) {
+                // Enum type - requires leading | for variants
                 (TypeDef::Enum(self.parse_variants()?), Vec::new())
             } else {
-                // Type alias
+                // Type alias - any type expression
                 (TypeDef::Alias(self.parse_type()?), Vec::new())
             }
         } else {

src/typechecker.rs

@@ -532,6 +532,9 @@ impl TypeChecker {
             self.collect_declaration(decl);
         }
 
+        // Check for circular type definitions
+        self.check_type_cycles(program);
+
         // Second pass: type check all declarations
         for decl in &program.declarations {
             self.check_declaration(decl);
@@ -544,6 +547,130 @@ impl TypeChecker {
         }
     }
 
+    /// Check for circular type alias definitions
+    fn check_type_cycles(&mut self, program: &Program) {
+        use std::collections::HashSet;
+
+        // Build a map of type alias dependencies
+        let mut alias_deps: HashMap<String, Vec<String>> = HashMap::new();
+
+        for decl in &program.declarations {
+            if let Declaration::Type(type_decl) = decl {
+                if let ast::TypeDef::Alias(type_expr) = &type_decl.definition {
+                    let deps = self.collect_type_references(type_expr);
+                    alias_deps.insert(type_decl.name.name.clone(), deps);
+                }
+            }
+        }
+
+        // Check for cycles using DFS
+        fn has_cycle(
+            name: &str,
+            alias_deps: &HashMap<String, Vec<String>>,
+            visiting: &mut HashSet<String>,
+            visited: &mut HashSet<String>,
+            cycle_path: &mut Vec<String>,
+        ) -> bool {
+            if visiting.contains(name) {
+                cycle_path.push(name.to_string());
+                return true;
+            }
+            if visited.contains(name) {
+                return false;
+            }
+
+            visiting.insert(name.to_string());
+            cycle_path.push(name.to_string());
+
+            if let Some(deps) = alias_deps.get(name) {
+                for dep in deps {
+                    if alias_deps.contains_key(dep) {
+                        if has_cycle(dep, alias_deps, visiting, visited, cycle_path) {
+                            return true;
+                        }
+                    }
+                }
+            }
+
+            visiting.remove(name);
+            cycle_path.pop();
+            visited.insert(name.to_string());
+            false
+        }
+
+        let mut visited = HashSet::new();
+
+        for type_name in alias_deps.keys() {
+            let mut visiting = HashSet::new();
+            let mut cycle_path = Vec::new();
+
+            if has_cycle(type_name, &alias_deps, &mut visiting, &mut visited, &mut cycle_path) {
+                // Find the span for this type declaration
+                let span = program.declarations.iter().find_map(|d| {
+                    if let Declaration::Type(t) = d {
+                        if t.name.name == *type_name {
+                            return Some(t.span);
+                        }
+                    }
+                    None
+                }).unwrap_or(Span::default());
+
+                self.errors.push(TypeError {
+                    message: format!(
+                        "Circular type definition detected: {}",
+                        cycle_path.join(" -> ")
+                    ),
+                    span,
+                });
+            }
+        }
+    }
+
+    /// Collect all type names referenced in a type expression
+    fn collect_type_references(&self, type_expr: &TypeExpr) -> Vec<String> {
+        let mut refs = Vec::new();
+        self.collect_type_refs_helper(type_expr, &mut refs);
+        refs
+    }
+
+    fn collect_type_refs_helper(&self, type_expr: &TypeExpr, refs: &mut Vec<String>) {
+        match type_expr {
+            TypeExpr::Named(ident) => {
+                // Only include user-defined types, not builtins
+                match ident.name.as_str() {
+                    "Int" | "Float" | "Bool" | "String" | "Char" | "Unit" | "_" => {}
+                    name => refs.push(name.to_string()),
+                }
+            }
+            TypeExpr::App(constructor, args) => {
+                self.collect_type_refs_helper(constructor, refs);
+                for arg in args {
+                    self.collect_type_refs_helper(arg, refs);
+                }
+            }
+            TypeExpr::Function { params, return_type, .. } => {
+                for p in params {
+                    self.collect_type_refs_helper(p, refs);
+                }
+                self.collect_type_refs_helper(return_type, refs);
+            }
+            TypeExpr::Tuple(elements) => {
+                for e in elements {
+                    self.collect_type_refs_helper(e, refs);
+                }
+            }
+            TypeExpr::Record(fields) => {
+                for f in fields {
+                    self.collect_type_refs_helper(&f.typ, refs);
+                }
+            }
+            TypeExpr::Unit => {}
+            TypeExpr::Versioned { base, .. } => {
+                self.collect_type_refs_helper(base, refs);
+            }
+        }
+    }
+
     /// Type check a program with module support
     pub fn check_program_with_modules(
         &mut self,
@@ -790,6 +917,28 @@ impl TypeChecker {
     }
 
     fn check_function(&mut self, func: &FunctionDecl) {
+        // Validate that all declared effects exist
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test"];
+        for effect in &func.effects {
+            let is_builtin = builtin_effects.contains(&effect.name.as_str());
+            let is_defined = self.env.lookup_effect(&effect.name).is_some();
+            if !is_builtin && !is_defined {
+                // Find similar effect names for suggestion
+                let mut available: Vec<&str> = builtin_effects.to_vec();
+                available.extend(self.env.effects.keys().map(|s| s.as_str()));
+                let suggestions = find_similar_names(&effect.name, available, 2);
+
+                let mut message = format!("Unknown effect: {}", effect.name);
+                if let Some(hint) = format_did_you_mean(&suggestions) {
+                    message.push_str(&format!(". {}", hint));
+                }
+                self.errors.push(TypeError {
+                    message,
+                    span: effect.span,
+                });
+            }
+        }
+
         // Set up the environment with parameters
         let mut local_env = self.env.clone();
         for param in &func.params {