unable to concat strings in for loop "operator += not defined on left operand type string"

[ttytm]

Describe the bug

mut arr := ['a', 'b', 'c']
for mut c in arr {
	c += 'd'
}

Reproduction Steps

above

Expected Behavior

Working code. Same as arr[i] += 'str'

Current Behavior

error: operator `+=` not defined on left operand type `string`
    1 | mut arr := ['a', 'b', 'c']
    2 | for mut c in arr {
    3 |     c += 'd'
      |     ^
    4 | }

Possible Solution

No response

Additional Information/Context

No response

V version

v0.4.5

Environment details (OS name and version, etc.)

Linux x64

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