By my understanding of scope, the first example should be fine.
Your understanding of scope is fine. This is not a scoping error. It is an inconsistent use of simple name error.
int i = 10; // error, ‘i’ already exists
That is not the error that is reported. The error that is reported is “a local variable named i cannot be declared in this scope because it would give a different meaning to i which is already used in a child scope to denote something else“
The error message is telling you what the error is; read the error message again. It nowhere says that there is a conflict between the declarations; it says that the error is because that changes the meaning of the simple name. The error is not the redeclaration; it is perfectly legal to have two things in two different scopes that have the same name, even if those scopes nest. What is not legal is to have one simple name mean two different things in nested local variable declarations spaces.
You would get the error “a local variable named i is already defined in this scope” if instead you did something like
int i = 10;
int i = 10;
Surely ‘i’ is either in scope or not.
Sure — but so what? Whether a given i is in scope or not is irrelevant. For example:
Perfectly legal. The outer i is in scope throughout M. There is no problem at all with declaring a local i that shadows the outer scope. What would be a problem is if you said
int x = i;
foreach(char i in ...
Because now you’ve used i to mean two different things in two nested local variable declaration spaces — a loop variable and a field. That’s confusing and error-prone, so we make it illegal.
Is there something non-obvious about scope I don’t understand which means the compiler genuinely can’t resolve this?
I don’t understand the question. Obviously the compiler is able to completely analyze the program; if the compiler could not resolve the meaning of each usage of i then how could it report the error message? The compiler is completely able to determine that you’ve used ‘i’ to mean two different things in the same local variable declaration space, and reports the error accordingly.