What is this error message telling me?

[Jonathan M Davis via Digitalmars-d-learn]

On Tuesday, April 11, 2017 14:51:44 Anonymous via Digitalmars-d-learn wrote:
> I was watching a dconf presentation from last year and wanted to
> try this out: https://github.com/luismarques/parnas72. It doesn't
> compile / run as it is and the problem seems to be in the
> function below.
>
> import std.algorithm;
> import std.range;
> import std.uni;
> /// Performs [["foo", "bar"], ["baz"]] -> ["baz", "foo bar"]
> auto alphabetized(Range)(Range range)
> {
>      return range
>          .map!(line => line.joiner(" "))
>          .array
>          .sort!((a, b) => icmp(a, b) < 0);
> }
>
> void main()
> {
>   auto a = alphabetized([["foo", "bar"], ["baz"]]);
> }
>
>
> More specifically, icmp doesn't seem to be allowed as the
> predicate for sort:
>
> Here's the error message I get:
>
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\uni.d(7082): Error:
> function 'std.algorithm.searching.skipOver!(Result,
> dstring).skipOver' is not nothrow
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\uni.d(7055): Error:
> nothrow function 'std.uni.fullCasedCmp!(Result).fullCasedCmp' may
> throw
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\uni.d(7136): Error:
> template instance std.uni.fullCasedCmp!(Result) error
> instantiating
> test.d(14):        instantiated from here: icmp!(Result, Result)
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm\sorting.d(1851):
>   instantiated from here: __lambda3!(Result, Result) test.d(14):
> instantiated from here: sort!((a, b) =>
> icmp(a, b) < 0, cast(SwapStrategy)0, Result[])
> test.d(19):        instantiated from here:
> alphabetized!(string[][])
> C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm\sorting.d(1863):
> Error: static assert  "Invalid predicate passed to sort: __lambda3"
> test.d(14):        instantiated from here: sort!((a, b) =>
> icmp(a, b) < 0, cast(SwapStrategy)0, Result[])
> test.d(19):        instantiated from here:
> alphabetized!(string[][])
>
>
> My question is, how do I begin to understand error messages like
> the above? I looked at the signature for sort and icmp and don't
> get what the problem is.

Well, it's telling you that skipOver isn't nothrow, which only matters if
it's being called from a nothrow function. It's then telling you that
fullCasedCmp can't compile, because it's marked as nothrow, and it's calling
a function that isn't nothrow (and thus could throw) - since it's calling
skipOver. In turn, it's telling you that it can't instantiate fullCasedCmp
(since it couldn't compile it, because it's nothrow but is calling a
function that can throw). It's then telling you that fullCaseCmp was
instantiated inside of icmp and that icmp was instantiated inside of a
lambda that's at line 14 of test.d. At that point, you see a line in your
code, so you know where in your code things went wrong, but it continues on,
indicating that that means that sort won't compile and then that
alphabetized won't compile.

So, the problem here at the root of all of this is that when icmp is
compiled with the ranges that you gave it, skipOver is inferred to _not_ be
nothrow, whereas fullCasedCmp which is calling skipOver and in turn is
called from icmp is marked as nothrow. So, the ranges that you're passing to
icmp have functions which are called in skipOver which are not nothrow
(probably front and popFront).

This is a bug with fullCasedCmp (which is a private function calle by icmp).
Someone decided to mark it as nothrow when it should _not_ have been marked
as nothrow. It's also marked as @trusted and pure - both of which are also
wrong. The code is incorrectly assuming that the ranges that will be passed
in have pure and nothrow functions and that they're not doing anything
@system which can't be assumed to be @trusted. That will be true for some
ranges but not others. In general, marking templated functions with
attributes is a problem for exactly this reason. They must correctly apply
to _all_ valid template arguments, or they shouldn't be there.

So, this should be reported as a bug in icmp.

https://issues.dlang.org/

- Jonathan M Davis