Location

https://github.com/apple/swift-book/blob/main/TSPL.docc/ReferenceManual/Expressions.md?plain=1#L64

Description

An in-out-expression is defined as being & identifier.
This is incorrect, and can be seen on line 51 that it's meant to be & expression. (here: https://github.com/apple/swift-book/blob/main/TSPL.docc/ReferenceManual/Expressions.md?plain=1#L51)
I'm assuming this is just a simple error that was overlooked but I will leave a demonstration below of why the version with identifier isn't correct.

Given this example program:

enum A {}

struct MyStruct {
	var a: A
}

func myFunc(_: inout A) {}

func myOtherFunc(s: inout MyStruct) {
	myFunc(&s.a)
}

This is compliable code in swiftc.

Our point of concern is the use of an in-out expression as a parameter in the call to myFunc
There is no possible derivation of this subsection of the code given the production for in-out-expression with identifier rather than expression.
Let us look at some possible derivation paths for myFunc(&s.a)

Possible derivation path: (terminals in bold)

1. -> function-call-expression
2. -> postfix-expression function-call-argument-clause
3. -> primary-expression function-call-argument-clause
4. -> identifier generic-argument-clause? function-call-argument-clause
5. -> myFunc generic-argument-clause? function-call-argument-clause
6. -> myFunc function-call-argument-clause
7. -> myFunc ( function-call-argument-list )
8. -> myFunc ( function-call-argument )
9. -> myFunc ( expression )
10. -> myFunc ( try-operator? await-operator? prefix-expression infix-expressions? )
11. -> myFunc ( await-operator? prefix-expression infix-expressions? )
12. -> myFunc ( prefix-expression infix-expressions? )
13. -> myFunc ( prefix-operator? postfix-expression infix-expressions? )
14. -> myFunc ( postfix-expression infix-expressions? )
15. -> myFunc ( explicit-member-expression infix-expressions? )
16. -> myFunc ( postfix-expression . identifier generic-argument-clause? infix-expressions? )
17. -> myFunc ( primary-expression . identifier generic-argument-clause? infix-expressions? )
18. -> myFunc ( identifier generic-argument-clause? . identifier generic-argument-clause? infix-expressions? )
19. -> myFunc ( ??? generic-argument-clause? . identifier generic-argument-clause? infix-expressions? )

???- cannot reach a production that will get us &s

Let's look at another possible derivation path.
Picking up from step 12 in the above derivation:
12) -> myFunc ( prefix-expression infix-expressions? )
13) -> myFunc ( in-out-expression infix-expressions? )
14) -> myFunc ( & identifier infix-expressions? )
15) -> myFunc ( & ??? infix-expressions? )

???- cannot reach a production that will get us &s.a

Correction

Line 64 should read in-out-expression → & expression