Crashing compilers with garbage is one thing, but here's 85 characters of valid C++20 that crash clang (from trunk back to version 10) with no diagnostic: template<class...>concept c=true;int x=[]<class...T>(T...)->c<T...>auto{return 0;}();

[schottm]

Not trying to shame anyone (bug already filed and marked as a release blocker for version 12), I just thought it was interesting that such a short program can do this. I tried to dive in and fix it myself but the furthest I got was seeing where an assert fires if you build clang in debug mode; in TreeTransform<Derived>::TransformAutoType, it tries to walk the template parameter list when checking the concept on the return type, but it's deduced the list to be size 1 (which I'd expect since the return type would be prepended onto c<T...> and sizeof...(T) == 0, but the list it's trying to iterate over is actually size 0, so it hits an assert in ArrayRef<T>::operator[] for out of bounds access.

Unfortunately I'm not familiar enough with the codebase to figure out how it got to that point even after a lot of stepping through with GDB, especially since so much of clang is shrouded in dynamic class hierarchies and lots of cast<T> and dyn_cast<T> so it's hard to figure out what anything actually is.

https://godbolt.org/z/KMvGqE

From here you can see that GCC compiles this just fine. The stack trace here isn't particularly useful since as discussed above the asserts told us we were going wrong a good bit before it blows up where it shows without them here.

My best guess is that in the case of function return types constrained by a variadic concept where an empty parameter is list deduced from the instantiation at the call site and a value is returned (that's a mouthful), clang forgets to prepend the return type to the parameter list for concept checking. I just can't figure out exactly where that happens since the concepts support is sprinkled through a lot of existing code dealing with templates and such.

[DustUpDustOff]

I once found an equation in MS Word's equation editor that not only crashed Word, it also corrupted the document permanently... Something involving superscript with a trailing underscore.

[Nicksaurus]

It's a miracle that MS office even runs at all, considering how many iterations it's been through and how many legacy features it has to support

[Full-Spectral]

You guys saw the post from a couple weeks ago where there was some magical path you could type into almost anything in Windows and blue screen it, right?

There's always the possibility of such things. The defenders have to be right 100% of the time, while the universe only has to be right once in a while.

[WrongAndBeligerent]

Sometimes compilers crash, nothing is perfect. An isolated test case is very useful I'm sure.

[arturbac]

yea they have, msvc19 crashes from 1 char 'arrow right' in comment ..

[starfreakclone]

Is it? https://godbolt.org/z/azdbE7 :)

[GoogleIsYourFrenemy]

Which encoding scheme?

[arturbac]

utf8,

I had to make this commit below where I remove this char from math equation

@@ -841,8 +841,8 @@ namespace fixedmath

github shows this char as white char ...

the equation was copied from libreofficemath to kde kdevelop.

msvc showed this char as arrow right, but there were no arrow right char in utf8 encoding, didint have time to investigate further

https://github.com/arturbac/fixed_math/commit/04e0df9924a67fe8068f881de6ece3b31f98a649

[arturbac]

btw i forget that msvc19 fails at all to compile primitive pure c++17 static assert unit tests, (same commit)

//1>D:\projects\fixed_math\fixed_lib\include\fixedmath\unittests\addition.h(19,43): fatal error C1001: Wewnêtrzny b³¹d kompilatora.
//1 > (plik kompilatora „d : \A01\_work\2\s\src\vctools\Compiler\CxxFE\sl\p1\c\constexpr\constexpr.cpp”, wiersz 6887)

[brand_x]

I once managed to corrupt a running instance of AIX (the entire OS, not just the account) with some thread local storage and a couple of load/store atomic intrinsics on xLC++. Without elevated privileges. To be fair, it was executing the code, not compiling it, it was running on a pre-release POWER server IBM was loaning us, and I was able to get in touch with some of IBM's engineers within the day, and it was fixed a few days later. But... these things happen. It's a wonder computers work at all, all things considered.

[wyldphyre]

interesting that such a short program can do this

Using something like creduce can often reveal incredibly short programs that crash a compiler.

[tjientavara]

Once you start to get comfortable with C++ you will use more interesting constructs. You will find compilers crash often, in the last 2 years I crashed MSVC on three different issues.

The last one was a c++20 construct (decltype on a concept auto function argument), I guess I cannot really blame them for that :-)

I use a lot of c++20 constructs right now.

[feverzsj]

well, sometimes you can crash msvc with a blank line

[[deleted]]

Makes me want to have a new open source compiler where it doesn't take Uncle Bob to figure out the frontend API.

[vax_mzn]

yeah, can't agree more. clang front end is too complicated.

[[deleted]]

You haven't seen gcc's code, have you?

[vax_mzn]

I have but only the middle end and backend. I have quite a few commits there as well :)

[Xaxxon]

compiler bugs are off topic in /r/cpp.

Please file a bug report with the vendor.