Fix Issue 18104: improve example showing that aliases cannot be used …

[ghost91-]

…for expressions

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18104	trivial	Alias example compiles where it states that it should be illegal

[JinShil]

alias b = s.i seems redundant and doesn't illustrate anything new. I suggest maybe adding a member static int j; to S and use that instead to illustrate the idea, or something like that.

[ghost91-]

You mean something like this?

struct S
{
    int i; 
    static int j;
}
S s;

alias a = s.i;   // ok, s.i is a symbol
alias b = S.j;   // ok, S.j also is a symbol
alias c = a + b; // illegal, a + b is an expression
a = 2;           // sets s.i to 2
b = 4;           // sets S.j to 4

[aG0aep6G]

a = 2; // sets s.i to 2

Doesn't work like that. An alias of s.i is the same as one of S.i. It doesn't carry this with it. It can't be used to access s.i.

[ghost91-]

Oh yeah, of course 🤦‍♂️ I suppose I was bit too quick here. Do you think, we should just omit the non static case in the example, or show that accessing in the non static case is not actually possible:

struct S
{
    int i; 
    static int j;
}
S s;

alias a = s.i;   // ok, s.i is a symbol
alias b = S.j;   // ok, S.j also is a symbol
alias c = a + b; // illegal, a + b is an expression
a = 2;           // error, a is aliased to S.i, but does not carry a this pointer with it
b = 4;           // sets S.j to 4

[ghost91-]

Or should we simplify the example even more? After all, it is about alias not working on expressions. Structs are not necessary to show this:

int a;

alias b = a;     // ok, a is a symbol
alias c = a + a; // illegal, a + a is an expression

[JinShil]

I recommend this:

struct S 
{
    static int i;
    static int j;
}

alias a = S.i; // OK, `S.i` is a symbol
alias b = S.j; // OK. `S.j` is also a symbol
alias c = a + b; // illegal, `a + b` is an expression
a = 2;         // sets `S.i` to `2`
b = 4;         // sets `S.j` to `4`

This purpose of this example is simply to illustrate the one can't alias an expression. Trying to show that S.i and s.i are equivalent is irrelevant, and actually that feature is currently being debated.

[ghost91-]

Sounds good to me, I will change things accordingly this evening