A scalar op-assign on a global or class variable (`$i += v`) is the raw C operator: no overflow check, `**=` does not compile, and the rhs runs before the slot is read

[elektronaut]

Environment

spinel:  da7617b5 (master, 2026-09-24)
CRuby:   ruby 4.0.7 (2026-09-15 revision 229531a6cf) +PRISM [arm64-darwin23]
cc:      Apple clang version 21.0.0 (clang-2100.3.34.2)

Problem

A scalar op-assign on a global or a class variable ($i += v, -=, *=, **=, <<= and so on) is emitted as the raw C operator ref OP= v. That has four effects:

• Overflow isn't checked. In the default --int-overflow=raise mode (docs/int-overflow.md), a local raises RangeError on overflow. A global prints nil or a wrapped value instead.
• **= doesn't compile, and neither does an op-assign on a Bignum global.
• The slot is read after the right-hand side runs, so a right-hand side that reassigns it changes the result.
• Ivars share part of this: bitwise ops and Float on an ivar keep the raw operator, and the slot read is a sibling C argument of the right-hand side, so C leaves their order unspecified.

The GlobalVariableOperatorWriteNode and ClassVariableOperatorWriteNode arms in emit_stmt (src/codegen_stmt.c), and the matching value-position arms in src/codegen_expr.c, handle String + and Array slots only; every scalar falls through to the raw operator. The local's op-assign already has checked Integer, Bignum and Float arms. #4875 fixed the same ordering for Array slots.

Reproducer

$i = 2**62
$i += 2**62
p $i

Expected: RangeError, the way i = 2**62; i += 2**62 raises on the same build (spinel's default mode). CRuby promotes, to 9223372036854775808.

Actual (spinel): builds, prints

nil

The order:

def bump
  $j = 100
  3
end
$j = 10
$j += bump
p $j   # CRuby 13; spinel 103

Scope

238 generated programs: $i / @@i / @i / local × += -= *= /= %= **= <<= |= and Float += × statement and value × with and without overflow × a side-effecting right-hand side inline and through a method.

Rows	spinel
global, no overflow (54)	28 wrong, silently; 6 C errors
class variable, no overflow (54)	28 wrong, silently; 6 C errors
ivar, no overflow (54)	12 wrong: |=, <<= and Float with a side-effecting right-hand side
global / class variable, overflow (20)	16 wrong (nil, 0, wrapped), 4 C errors (**=)
ivar, overflow (10)	2 wrong (<<= 64 prints an empty line)
local, overflow (10)	RangeError (correct in this mode)

This isn't a regression. Checked on da7617b.

Suggested regression test

test/scalar_op_assign_reads_slot_first.rb for the order rows, plus global and class-variable RangeError rows in test/int_overflow_op_assign.rb.