[bug] Bad explanation on error message for unmatched dependency.

I have looked at the bug reports for this and i found https://github.com/conan-io/conan/issues/7864 , but I was unable to understand what to do.

I have a fairly small conanfile (excerpt from a bigger project):

from conans import ConanFile, CMake

class DiagramServerConan(ConanFile):
    settings = "os", "compiler", "build_type", "arch"

    generators = (
        "cmake_find_package",
        "cmake_paths",
    )

    requires = (
        "flex/2.6.4",
        "bison/3.7.6",
    )

this (correctly) tells me that I have an unmatched dependency;

ERROR: Conflict in bison/3.7.6:
    'bison/3.7.6' requires 'm4/1.4.19' while 'flex/2.6.4' requires 'm4/1.4.18'.
    To fix this conflict you need to override the package 'm4' in your root package.

So I add m4 in my root package:

from conans import ConanFile, CMake

class DiagramServerConan(ConanFile):
    settings = "os", "compiler", "build_type", "arch"

    generators = (
        "cmake_find_package",
        "cmake_paths",
    )

    requires = (
        "flex/2.6.4",
        "bison/3.7.6",
        "m4/1.4.19",
    )

and now I have a different error:

WARN: flex/2.6.4: requirement m4/1.4.18 overridden by your conanfile to m4/1.4.19 
ERROR: Conflict in flex/2.6.4:
    'flex/2.6.4' requires 'm4/1.4.18' while 'bison/3.7.6' requires 'm4/1.4.19'.
    To fix this conflict you need to override the package 'm4' in your root package.
➜  testconan2 

the wording on the error is strange, as adding the m4 to my requires is adding to my root package (if I’m not mistaken). I also tried to add on build_requires, to the same issue.

[tarcisiofischer]

Well, to be fair, Conan could try pointing a little more information in the error message, hoping to get the user in the right direction.

To be more specific, let’s go back to the original error:

ERROR: Conflict in flex/2.6.4:
    'flex/2.6.4' requires 'm4/1.4.18' while 'flex/2.6.4' requires 'm4/1.4.19'.
    To fix this conflict you need to override the package 'm4' in your root package.

It is impossible to know from which flex Conan is talking about due to the lack of context. I guess it assumes ‘conancenter’, but that’s just the common case - If the user tries to go to conancenter like I did, he won’t find anything useful. It’s just misleading… Or it assumes the user will know it, but it’s always good to double check than relying on my memory 😃

So I propose the error message could be something like this:

ERROR: Conflict in flex/2.6.4:
    'flex/2.6.4:HASH' (from 'local build') requires 'm4/1.4.18' while 'flex/2.6.4:HASH' (from 'conancenter') requires 'm4/1.4.19'.
    To fix this conflict you need to override the package 'm4' in your root package.

    If the cached package is unwanted, you can try removing it from the cache using
    $ conan remove flex/2.6.4:123456789ABC

There’s no need to fetch anything from the server to print this info, I believe. But I don’t know if I’m being too narrowed by this specific case. What do you think, @memsharded ?

[memsharded]

The message error should be improved, then. There was no mention about cache issues, just a bit of nonsense about impossible dependency graphs.

But this is not a cache issue. You can install packages in a blank cache, directly from the server, and still have conflicts. These conflicts happen because of diamonds in the graph:

• You create zlib/1.2.11 and zlib/1.2.12 packages, upload to the server
• You create boost/1.67 that depends on zlib/1.2.11, works great, upload to the server
• You create poco/1.11 that depends on zlib/1.2.12 works great, upload to the server
• In a blank client, with a blank cache, you setup a project that depends on boost/1.67 and poco/1.11. That will download the different things to the cache, evaluate the graph and trigger a version conflict between zlib/1.2.11 and zlib/1.2.12

That is not a cache issue at all. It is an impossible, conflicted graph, but it is not a failure about the cache.

Just in case, Conan commands will not go and check the servers on every command, if they can find things locally in the cache. This is a very desired and requested behavior by tons of users, because of performance. Users can opt-in to check latest things from the server with --update, and if that happens, Conan might download new versions or revisions of packages.