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If /etc/dnf/dnf.conf contains the setting best=1 then dnf builddep will fail
for a package
BuildRequires:	perl-generators
will fail with the error
Error: 
 Problem: cannot install the best candidate for the job
  - package perl-generators-1.10-7.el8+1703+3db27f7b.noarch is excluded
rather than installing the perl-generators (assuming there aren't any other dependency issues).
This was happening when testing a mock rebuild of cyrus-imapd-3.0.7-15.el8.src.rpm but I reproduced it with a dummy package built from the spec file below to avoid dependency issues.

Name:		testpkg
Version:	0
Release:	1%{?dist}
Summary:	Test Package
License:	BSD
Source0:	test.txt

BuildRequires:	perl-generators

%description

%prep
%setup -q

%build
%configure
make %{?_smp_mflags}

%install
%make_install

%files
%doc

%changelog

I forgot to mention that I can't reproduce this in Fedora 29 so it looks like it has been fixed there.

Please can you provide a version of dnf, libdnf?

Please can you confirm that the issue is reproducible with dnf-4.0.9?

The problem is for the RHEL8 public beta rhel-8.0-beta-1 so
dnf-4.0.4-1.el8.noarch
libdnf-0.22.0-2.el8.x86_64
libsolv-0.6.35-3.el8.x86_64
which I believe are the latest publicly available versions.
I was not able to reproduce it on Fedora 29 which has
dnf-4.0.9-2.fc29.noarch
libdnf-0.22.3-1.fc29.x86_64
libsolv-0.7.2-1.fc29.x86_64
so the bug may well be fixed if there are later private RHEL8 builds of these packages.

The problem is valid. DNF selector using provide= ignores exclude. The issue cannot be easily fixed, but we have to.

*** This bug has been marked as a duplicate of bug 1677583 ***