Match if expression is const, to get the proper error message

[ryuukk]

Consider:

struct Stuff {

    int opApply(scope int delegate(const (Stuff)*) dg) 
    {
        return 0;
    }

};
extern(C) void main()
{
    const(Stuff)* stuff;
    foreach(const (Stuff)* it;*stuff)
    {}
}

Before:

_.d(13): Error: cannot uniquely infer `foreach` argument types

After:

_.d(13): Error: mutable method `_.Stuff.opApply` is not callable using a `const` object
_.d(4):        Consider adding `const` or `inout` here

Fixes bugzilla Issue 19114 - cannot uniquely infer foreach argument types is not a descriptive message
Fixes bugzilla Issue 24353 - Misleading error for foreach when opApply has wrong qualifier

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Testing this PR locally

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dub run digger -- build "master + dmd#16086"

[thewilsonator]

you will need to update test cases

[dkorpel]

Please add "Fix bugzilla issue 19114" and "Fix bugzilla issue 24353" to the commit message so the bot links the issues.

[ryuukk]

Is the fix correct at least?

[RazvanN7]

The findBestOpApplyMatch iterates through all of the overloads of opApply and selects the best match (if there is one) or errors in case you have more than one match. In case there is no match, the function returns null. This patch modifies the code so that if the instance type does not match the opApply instance type and the instance type is const then the considered opApply is chosen as the best match. This is wrong for multiple reasons:

• you are simply making const a particular case here; what happens with immutable, shared, etc.?
• you are considering a function that is not a match to be a match; this might break other parts of the compiler that call this function.
• this is not going to work properly when you have multiple opApply's that fail for similar reasons; you are just going to pick the last failing as the best match.
• consider the case when having both a const and a mutable opApply; you will obtain a wrong ambiguity error.

I think that the only solution here is to:

• call findBestOpApplyMatch.
• if there is a single opApply defined and it's not a match, then try calling it anyway (resolveFuncCall) to obtain a proper error message.
• if you have multiple opApplys and none of them are a match, then the current error message is fine.

So, the fix is not in this function.

[RazvanN7]

This is not the correct fix.

[RazvanN7]

@ryuukk are you willing to implement the proper solution?

[ryuukk]

Not right now, I have too much to do on my current project

[dkorpel]

When you want to continue, please re-open / open a new PR.