Address review: revert := deep-copy change, keep reference-assign semantics

Reverts the arithmetic deep-copy added to := in chaiscript_eval.hpp,
restoring its original reference-assign behavior as requested.
The optimizer fix (heap-allocated loop counter) remains and is
sufficient to prevent the #635 segfault.

Updated tests to use correct expected values for reference-assign
semantics and added more thorough coverage: nested loops, multiple
async calls, and value-assign comparison.

Requested by @lefticus in PR #671 review.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/chaiscript/language/chaiscript_eval.hpp

@@ -473,11 +473,7 @@ namespace chaiscript {
           }
         } else if (this->text == ":=") {
           if (params[0].is_undef() || Boxed_Value::type_match(params[0], params[1])) {
-            if (params[1].get_type_info().is_arithmetic() && params[0].get_type_info().is_arithmetic()) {
-              params[0].assign(Boxed_Number::clone(params[1]));
-            } else {
-              params[0].assign(params[1]);
-            }
+            params[0].assign(params[1]);
             params[0].reset_return_value();
           } else {
             throw exception::eval_error("Mismatched types in equation");

unittests/assign_no_aliasing.chai

@@ -1,7 +1,41 @@
-// Verify that := performs a value copy, not an alias
-var a = 10
-var b = 20
-a := b
-++b
-assert_equal(20, a)
-assert_equal(21, b)
+// Issue #635: optimized for loop with := must not produce dangling values
+// := is reference-assign, so ret aliases i and sees the loop exit value
+
+// Basic: capture loop variable via :=
+var ret = 0
+for (var i = 0; i < 100; ++i) {
+  ret := i
+}
+assert_equal(100, ret)
+
+// Return from function containing optimized for loop
+def loop_result() {
+  var ret = 0
+  for (var i = 0; i < 200; ++i) {
+    ret := i
+  }
+  return ret
+}
+assert_equal(200, loop_result())
+
+// Multiple calls return consistent results
+assert_equal(loop_result(), loop_result())
+
+// Nested optimized for loops
+var outer_val = 0
+var inner_val = 0
+for (var i = 0; i < 10; ++i) {
+  for (var j = 0; j < 10; ++j) {
+    inner_val := j
+  }
+  outer_val := i
+}
+assert_equal(10, outer_val)
+assert_equal(10, inner_val)
+
+// Value-assign (=) captures last body-iteration value, not exit value
+var ret2 = 0
+for (var i = 0; i < 100; ++i) {
+  ret2 = i
+}
+assert_equal(99, ret2)

unittests/async_return_value.chai

@@ -1,3 +1,6 @@
+// Issue #635: async + optimized for loop must not segfault from dangling pointer
+// := is reference-assign, so ret aliases i and sees the loop exit value
+
 var func = fun(){
   var ret = 0;
   for (var i = 0; i < 1000; ++i) {
@@ -9,5 +12,14 @@ var func = fun(){
 var&fut1 = async(func);
 var fut2 = async(func);
 
-assert_equal(999, fut1.get())
-assert_equal(999, fut2.get())
+assert_equal(1000, fut1.get())
+assert_equal(1000, fut2.get())
+
+// Multiple concurrent async calls to stress the scenario
+var results = []
+for (var n = 0; n < 5; ++n) {
+  results.push_back(async(func))
+}
+for (var n = 0; n < 5; ++n) {
+  assert_equal(1000, results[n].get())
+}