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is duplicated by:

• SR-7335 Mutating struct when accessing read only member through subscript

Issue Description:

Summary:
In Xcode 9.3 Beta 4, the following code emits the string "Bug! didSet {} called on read" to the console. No property read should trigger the didSet call.

Steps to Reproduce:
Paste this Swift code into the swift command-line REPL:
protocol ProtocolWithArray {
var arrayOfInt: [Int] { get set }
}

var questionSet: ProtocolWithArray? {
didSet {
print("Bug! didSet {} called on read.")
}
}

func fiddle() -> Int? {
return questionSet?.arrayOfInt[0]
}

print("Starting")
fiddle()
print("Done")

Expected Results (as seen with Xcode 9.2):

Welcome to Apple Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)

...

Starting
Done

Actual Results (as seen with Xcode 9.3 Beta 4):

Welcome to Apple Swift version 4.1 (swiftlang-902.0.43 clang-902.0.37.1).

...

Starting
Bug! didSet {} called on read.
Done