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digitalmars.D.bugs - [Issue 16059] New: needed better error messages for failed overloads


          Issue ID: 16059
           Summary: needed better error messages for failed overloads
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: poliklosio.dlang gmail.com

I think I may have an idea for a usable solution to improve error messages.

Example of an error I got from dmd:

C:\D\dmd2\windows\bin\..\..\src\phobos\std\conv.d(298,24): Error: template
std.conv.toImpl cannot deduce function from argument types
!(ubyte[])(RangeT!(Array!ubyte)), candidates are:
std.conv.toImpl(T, S)(S value) if (isImplicitlyConvertible!(S, T) &&
!isEnumStrToStr!(S, T) && !isNullToStr!(S, T))
std.conv.toImpl(T, S)(ref S s) if (isRawStaticArray!S)
std.conv.toImpl(T, S)(S value) if (!isImplicitlyConvertible!(S, T) &&
is(typeof(S.init.opCast!T()) : T) && !isExactSomeString!T &&
std.conv.toImpl(T, S)(S value) if (!isImplicitlyConvertible!(S, T) && is(T ==
struct) && is(typeof(T(value))))
std.conv.toImpl(T, S)(S value) if (!isImplicitlyConvertible!(S, T) && is(T ==
class) && is(typeof(new T(value))))
C:\D\dmd2\windows\bin\..\..\src\phobos\std\conv.d(298,24):        ... (9 more,
-v to show) ...
src\app.d(36,36): Error: template instance
std.conv.to!(ubyte[]).to!(RangeT!(Array!ubyte)) error instantiating
dmd failed with exit code 1.

There are multiple problems with this error message:
- The information that the source of the problem was the call to a "to"
function is not at the top.
- It is missing some very important information, in particular the general idea
of what types are accepted by the "to" function. It only shows the way the
constrains are *implemented*, it doesn't tell directly what the author wanted
to achieve. When one writes 20 overloads, its usually because there is some
commmon property among the accepted types which cannot be easily expressed with
the type system.
- It doesn't show any useful speculation as to what mith be the cause of the
error in this particular use case.

There was a forum thread started by Andrei about other improvements to messages
http://forum.dlang.org/post/nfllhq$19tk$1 digitalmars.com
but it was focused on generating better messages from the constraints
themselves, which is, IMHO, an inferior solution if not a lost cause.

I would like to propose another solution which may improve experience
regardless of the number of overloads, architecture of constrains and all the
other technical issues, by laveraging the expressive power of English language.

If the library writer could write something like this (example for the
std.conv.to case):

pragma(on_overload_resolution_error, "toImpl", "
You used the \"to\" function incorrectly which failed at the point of toImpl
template instantiation.
The \"to\" function is meant for simple convertions which typically preserve
the converted value, for example from double -> int, or double[] -> int[].
Typical mistakes when calling the \"to\" function involve: Calling on range
types rather than arrays...

This tells the compiler to display the "You used the ..." message at the top of
the appropriate error message, in case there is an overload resolution error
when using the toImpl function.

May 22 2016