https://issues.dlang.org/show_bug.cgi?id=17226

Jack Stouffer <jack@jackstouffer.com> changed:

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                 CC|                            |jack@jackstouffer.com

--- Comment #1 from Jack Stouffer <jack@jackstouffer.com> ---
Full example of catching the exception and still going

import std.string;
import std.stdio;

void foo(int i) {
    // In this case a %s is forgotten but it could be any other trivial error.
    assert(i == 42, format("Bad parameter:", i));
}

void main() {
    try {
        foo(43);
    } catch (Exception) {
        writeln("something threw");
    }

    writeln("This still runs despite being in undefined state");
}

Yeah, this is really, really bad.

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https://issues.dlang.org/show_bug.cgi?id=17226

Jack Stouffer <jack@jackstouffer.com> changed:

           What    |Removed                     |Added
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           Hardware|x86_64                      |All
                 OS|Linux                       |All

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https://issues.dlang.org/show_bug.cgi?id=17226

jiki@red.email.ne.jp changed:

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           Keywords|                            |EH
                 CC|                            |jiki@red.email.ne.jp

--- Comment #2 from jiki@red.email.ne.jp ---
Ah, I'd just been wondering why an assert message eats its location
information.
Come on, come on :)

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https://issues.dlang.org/show_bug.cgi?id=17226

RazvanN <razvan.nitu1305@gmail.com> changed:

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                 CC|                            |razvan.nitu1305@gmail.com

--- Comment #3 from RazvanN <razvan.nitu1305@gmail.com> ---
Not sure if this is a dmd error. I think this might be druntime related :-?

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https://issues.dlang.org/show_bug.cgi?id=17226

Heromyth <bitworld@qq.com> changed:

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--- Comment #4 from Heromyth <bitworld@qq.com> ---
We also encountered this problem. We can't find out which statement is wrong, just get an error message.

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https://issues.dlang.org/show_bug.cgi?id=17226

--- Comment #5 from RazvanN <razvan.nitu1305@gmail.com> ---
Associated discussion about this: https://forum.dlang.org/thread/qwixdanceeupdefyqwnr@forum.dlang.org

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https://issues.dlang.org/show_bug.cgi?id=17226

hsteoh@qfbox.info changed:

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--- Comment #6 from hsteoh@qfbox.info ---
Just use compile-time format string checking:

          assert(myAssumption, format!"%s doesn't work!"(blah));

This produces a compile error:

          string s = format!"blah %d blah"(123.45);

Problem solved.

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https://issues.dlang.org/show_bug.cgi?id=17226

Paul Backus <snarwin+bugzilla@gmail.com> changed:

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--- Comment #7 from Paul Backus <snarwin+bugzilla@gmail.com> ---
To be specific, the issue here is that according to the language spec:

> The first AssignExpression must evaluate to true. If it does not, an Assert Failure has occurred and the program enters an Invalid State.
>
> [...]
>
> Undefined Behavior: Once in an Invalid State the behavior of the continuing execution of the program is undefined.

i.e., once the condition has been evaluated to false, continuing to execute is undefined *regardless* of what happens in the evaluation of the message.

Perhaps the simplest way to fix this is to have assert(condition, message) evaluate the message *first*, so that Ali's example has behavior equivalent to the following code:

---
import std.format;

void foo(int i) {
    auto __msg = format("Bad parameter:", i);
    assert(i == 42, __msg);
}

void main() {
    foo(43);
}
---

This way, if the message expression throws, the assert's condition is never evaluated, and the program does not enter an invalid state.

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https://issues.dlang.org/show_bug.cgi?id=17226

kinke <kinke@gmx.net> changed:

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--- Comment #8 from kinke <kinke@gmx.net> ---
(In reply to Paul Backus from comment #7)
> Perhaps the simplest way to fix this is to have assert(condition, message) evaluate the message *first*

No. Changing the evaluation order would have a deep impact:

* Side effects of evaluating the msg first might change the assert condition's
outcome.
* You don't want a potentially expensive msg expression to be evaluated
unconditionally, i.e., including the regular case where the assertion holds.
* The msg expression may depend on the state after a failed condition - e.g.,
`assert(x.trySomething(), x.getLastErrorMsg())` (horrible, I know...).

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https://issues.dlang.org/show_bug.cgi?id=17226

--- Comment #9 from Ali Cehreli <acehreli@yahoo.com> ---
I think the "invalid state" ship has sailed here because a D program by default dumps backtrace to stderr. Can that always work? No, but I think we accept it as best-effort.

I think the same applies to the assert message: A best-effort to show something meaningful. May work but it should not hide the actual issue.

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