[Issue 8992] New: __traits(compileError)

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http://d.puremagic.com/issues/show_bug.cgi?id=8992

           Summary: __traits(compileError)
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrej.mitrovich at gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich at gmail.com> 2012-11-10 06:07:14 PST ---
Currently __traits(compiles) simply checks whether an expression is compilable
and returns true or false.

But it would be extremely useful if we had another trait which instead of
returning true/false, it returned the error message (if any).

Consider the case of the msgpack serialization library. It currently does a
check on whether a special serialization method is invokable on an object,
e.g.:

/** Somewhere in the "Packer" class of the library */
T object;
static if (__traits(compiles, { T t; t.toMsgpack(this_); })) {
{
}
else  // use library serialization routine
{
}

The problem here is if `toMsgpack` fails to instantiate (the library expects it
to be a templated function), you won't notice it because the library just picks
an internal routine for serialization as an alternative.

Here's a limited form of improvement we can already do right now (OT: I'm
making a pull for this):

static if (hasToMsgpack!T)  // first check if templated method exists
{
    static if (__traits(compiles, { T t; t.toMsgpack(this); })) {
        object.toMsgpack(this, withFieldName_);
    } else {
        static assert(0, "Couldn't invoke method 'toMsgpack' on type '" ~
Unqual!T.stringof ~ "'");  // print error on failed instantiation
    }
} else {  // internal routine
}

So now if your templated `toMsgpack` routine fails to instantiate you will get
an informative error:

msgpack.d(822): Error: static assert  "Couldn't invoke method 'toMsgpack' on
type 'Struct'"

But, imagine if we had a way to extract the error message too. Then we could
improve things even more:

static if (hasToMsgpack!T)  // first check if templated method exists
{
    static if (__traits(compiles, { T t; t.toMsgpack(this); })) {
        object.toMsgpack(this, withFieldName_);
    } else {
        enum string errorMsg = __traits(errorMsg, { T t; t.toMsgpack(this); });
        static assert(0, "Error instantiating 'toMsgpack' on type '" ~
Unqual!T.stringof ~ "'\n" ~ errorMsg);
    }
} else {
}

So let's say you've had a wrongly written template:

struct Struct
{
    void toMsgpack()(int)
    {
    }
}

The error could then be:
static assert  "Error instantiating 'toMsgpack' on type 'Struct'
Error: function t.toMsgpack (int _param_0) is not callable using argument types
(Packer)"

It's a major improvement over a generic message that you would have to
explicitly write in a static assert.

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