compound assignment

TODO.md

@@ -100,7 +100,7 @@
     - `while condition : i = i + 1 { ... }` - execute the update after each completed iteration
     - the condition and update may be parenthesized independently for visual clarity
     - update targets must already be declared and mutable; loops do not introduce implicit induction variables
-    - compound assignment (`+=`) remains deferred
+    - update clauses support ordinary and compound assignment
   - ranges (implemented; see section below)
   - for loops (implemented; operates on ranges, arrays, slices, and pointers-to-arrays). examples:
     - `for items |item| { ... }` - capture just the `item` value in the array/slice (uses copy semantics, i.e. gets a `T`)
@@ -112,7 +112,18 @@
     - `for 0..(len) |i| { ... }` or equivalently `for 0..=(len - 1) |i| { ... }` - calculating range bounds, expressions must be parenthesized
   - for all conditionals/guards, parentheses are optional but allowed for visual clarity
 
-6. compound assignment: `+=`, `-=`, `*=`, `/=`
+6. compound assignment: `+=`, `-=`, `*=`, `/=` (implemented)
+  - added the binary arithmetic operators `-`, `*`, `/` (previously only `+` existed); `*`/`/`
+    bind tighter than `+`/`-`, and prefix `-` (negation) is unchanged
+  - compound assignments preserve the target, operator, and right-hand side explicitly through
+    parsing and checking; lowering computes the target address once, then loads, applies the
+    operation, and stores through that address
+  - side-effecting index, field-base, and dereference expressions are evaluated once in
+    left-to-right order
+  - integer arithmetic traps on overflow (`Sub_Checked`/`Mul_Checked` via the LLVM
+    `.with.overflow` intrinsics) and integer `/` traps on divide-by-zero and `INT_MIN / -1`;
+    floats follow IEEE (`fadd`/`fsub`/`fmul`/`fdiv`, no trap)
+  - constant folding (global initializers) covers `-`, `*`, `/` alongside `+`
 
 7. enums (native and c interop) (see below)