fix(checker): detect $env() calls at compile time

builtin/builtin_test.go

@@ -300,19 +300,13 @@ func TestBuiltin_errors(t *testing.T) {
 	}
 }
 
-// The get() builtin must return an error when called with
-// insufficient arguments at runtime, even if compile-time checks
-// are bypassed (regression test for OSS-Fuzz #479270603).
-func TestBuiltin_get_runtime_args_check(t *testing.T) {
+func TestBuiltin_env_not_callable(t *testing.T) {
 	code := `$env(''matches'i'?t:get().UTC())`
 	env := map[string]any{"t": 1}
 
-	program, err := expr.Compile(code, expr.Env(env))
-	require.NoError(t, err)
-
-	_, err = expr.Run(program, env)
+	_, err := expr.Compile(code, expr.Env(env))
 	require.Error(t, err)
-	assert.Contains(t, err.Error(), "invalid number of arguments")
+	assert.Contains(t, err.Error(), "is not callable")
 }
 
 func TestBuiltin_types(t *testing.T) {

checker/checker.go

@@ -651,6 +651,11 @@ func (v *Checker) callNode(node *ast.CallNode) Nature {
 		return *node.Nature()
 	}
 
+	// $env is not callable.
+	if id, ok := node.Callee.(*ast.IdentifierNode); ok && id.Value == "$env" {
+		return v.error(node, "%s is not callable", v.config.Env.String())
+	}
+
 	nt := v.visit(node.Callee)
 	if nt.IsUnknown(&v.config.NtCache) {
 		return Nature{}

checker/checker_test.go

@@ -681,6 +681,30 @@ invalid operation: > (mismatched types string and int) (1:30)
 invalid operation: + (mismatched types int and bool) (1:6)
  | 1; 2 + true; 3
  | .....^
+`,
+		},
+		{
+			`$env()`,
+			`
+mock.Env is not callable (1:1)
+ | $env()
+ | ^
+`,
+		},
+		{
+			`$env(1)`,
+			`
+mock.Env is not callable (1:1)
+ | $env(1)
+ | ^
+`,
+		},
+		{
+			`$env(abs())`,
+			`
+mock.Env is not callable (1:1)
+ | $env(abs())
+ | ^
 `,
 		},
 	}

vm/vm_test.go

@@ -1512,20 +1512,44 @@ func TestVM_StackUnderflow(t *testing.T) {
 	}
 }
 
-func TestVM_OpCall_InvalidNumberOfArguments(t *testing.T) {
-	// This test ensures that calling a function with wrong number of arguments
-	// produces a clear error message instead of a panic.
-	// Regression test for clusterfuzz issue with expression:
-	// $env(''matches'  '? :now().UTC(g).d)//
-
+func TestVM_EnvNotCallable(t *testing.T) {
+	// $env is the environment, not a function.
 	env := map[string]any{
 		"ok": true,
 	}
 
 	code := `$env('' matches ' '? : now().UTC(g))`
-	program, err := expr.Compile(code, expr.Env(env))
+	_, err := expr.Compile(code, expr.Env(env))
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "is not callable")
+}
+
+func TestVM_OpCall_InvalidNumberOfArguments(t *testing.T) {
+	// Test that the VM validates argument count at runtime.
+	// Compile without Env() so compiler generates OpCall without type info.
+	program, err := expr.Compile(`fn(1, 2)`)
 	require.NoError(t, err)
 
+	// Run with a function that has different arity
+	env := map[string]any{
+		"fn": func(a int) int { return a },
+	}
+
+	_, err = expr.Run(program, env)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "invalid number of arguments")
+}
+
+func TestVM_OpCall_InvalidNumberOfArguments_Variadic(t *testing.T) {
+	// Test variadic function with too few arguments.
+	program, err := expr.Compile(`fn()`)
+	require.NoError(t, err)
+
+	// Run with a variadic function that requires at least 1 argument
+	env := map[string]any{
+		"fn": func(first int, rest ...int) int { return first },
+	}
+
 	_, err = expr.Run(program, env)
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "invalid number of arguments")