Add when context manager to group common constraints in packages (#24650)

This PR adds a context manager that permit to group the common part of a `when=` argument and add that to the context:
```python
class Gcc(AutotoolsPackage):
    with when('+nvptx'):
        depends_on('cuda')
        conflicts('@:6', msg='NVPTX only supported in gcc 7 and above')
        conflicts('languages=ada')
        conflicts('languages=brig')
        conflicts('languages=go')
```
The above snippet is equivalent to:
```python
class Gcc(AutotoolsPackage):
    depends_on('cuda', when='+nvptx')
    conflicts('@:6', when='+nvptx', msg='NVPTX only supported in gcc 7 and above')
    conflicts('languages=ada', when='+nvptx')
    conflicts('languages=brig', when='+nvptx')
    conflicts('languages=go', when='+nvptx')
```
which needs a repetition of the `when='+nvptx'` argument. The context manager might help improving readability and permits to group together directives related to the same semantic aspect (e.g. all the directives needed to model the behavior of `gcc` when `+nvptx` is active). 

Modifications:

- [x] Added a `when` context manager to be used with package directives
- [x] Add unit tests and documentation for the new feature
- [x] Modified `cp2k` and `gcc` to show the use of the context manager

lib/spack/docs/packaging_guide.rst

@@ -1257,7 +1257,7 @@ Variants
 Many software packages can be configured to enable optional
 features, which often come at the expense of additional dependencies or
 longer build times. To be flexible enough and support a wide variety of
-use cases, Spack permits to expose to the end-user the ability to choose
+use cases, Spack allows you to expose to the end-user the ability to choose
 which features should be activated in a package at the time it is installed.
 The mechanism to be employed is the :py:func:`spack.directives.variant` directive.
 
@@ -2775,6 +2775,57 @@ packages be built with MVAPICH and GCC.
 
 See the :ref:`concretization-preferences` section for more details.
 
+
+.. _group_when_spec:
+
+----------------------------
+Common ``when=`` constraints
+----------------------------
+
+In case a package needs many directives to share the whole ``when=``
+argument, or just part of it, Spack allows you to group the common part
+under a context manager:
+
+.. code-block:: python
+
+   class Gcc(AutotoolsPackage):
+
+       with when('+nvptx'):
+           depends_on('cuda')
+           conflicts('@:6', msg='NVPTX only supported in gcc 7 and above')
+           conflicts('languages=ada')
+           conflicts('languages=brig')
+           conflicts('languages=go')
+
+The snippet above is equivalent to the more verbose:
+
+.. code-block:: python
+
+   class Gcc(AutotoolsPackage):
+
+       depends_on('cuda', when='+nvptx')
+       conflicts('@:6', when='+nvptx', msg='NVPTX only supported in gcc 7 and above')
+       conflicts('languages=ada', when='+nvptx')
+       conflicts('languages=brig', when='+nvptx')
+       conflicts('languages=go', when='+nvptx')
+
+Constraints stemming from the context are added to what is explicitly present in the
+``when=`` argument of a directive, so:
+
+.. code-block:: python
+
+   with when('+elpa'):
+       depends_on('elpa+openmp', when='+openmp')
+
+is equivalent to:
+
+.. code-block:: python
+
+   depends_on('elpa+openmp', when='+openmp+elpa')
+
+Constraints from nested context managers are also added together, but they are rarely
+needed or recommended.
+
 .. _install-method:
 
 ------------------

lib/spack/spack/directives.py

@@ -27,24 +27,22 @@ class OpenMpi(Package):
   * ``version``
 
 """
-
 import functools
 import os.path
 import re
 import sys
+from typing import List, Set  # novm
 
-from six import string_types
+import six
-from typing import Set, List  # novm
 
 import llnl.util.lang
 import llnl.util.tty.color
-
 import spack.error
 import spack.patch
 import spack.spec
 import spack.url
 import spack.variant
-from spack.dependency import Dependency, default_deptype, canonical_deptype
+from spack.dependency import Dependency, canonical_deptype, default_deptype
 from spack.fetch_strategy import from_kwargs
 from spack.resource import Resource
 from spack.version import Version, VersionChecksumError
@@ -114,6 +112,7 @@ class DirectiveMeta(type):
     # Set of all known directives
     _directive_names = set()  # type: Set[str]
     _directives_to_be_executed = []  # type: List[str]
+    _when_constraints_from_context = []  # type: List[str]
 
     def __new__(cls, name, bases, attr_dict):
         # Initialize the attribute containing the list of directives
@@ -167,6 +166,16 @@ def __init__(cls, name, bases, attr_dict):
 
         super(DirectiveMeta, cls).__init__(name, bases, attr_dict)
 
+    @staticmethod
+    def push_to_context(when_spec):
+        """Add a spec to the context constraints."""
+        DirectiveMeta._when_constraints_from_context.append(when_spec)
+
+    @staticmethod
+    def pop_from_context():
+        """Pop the last constraint from the context"""
+        return DirectiveMeta._when_constraints_from_context.pop()
+
     @staticmethod
     def directive(dicts=None):
         """Decorator for Spack directives.
@@ -205,15 +214,16 @@ class Foo(Package):
         This is just a modular way to add storage attributes to the
         Package class, and it's how Spack gets information from the
         packages to the core.
-
         """
         global __all__
 
-        if isinstance(dicts, string_types):
+        if isinstance(dicts, six.string_types):
             dicts = (dicts, )
+
         if not isinstance(dicts, Sequence):
             message = "dicts arg must be list, tuple, or string. Found {0}"
             raise TypeError(message.format(type(dicts)))
+
         # Add the dictionary names if not already there
         DirectiveMeta._directive_names |= set(dicts)
 
@@ -223,6 +233,23 @@ def _decorator(decorated_function):
 
             @functools.wraps(decorated_function)
             def _wrapper(*args, **kwargs):
+                # Inject when arguments from the context
+                if DirectiveMeta._when_constraints_from_context:
+                    # Check that directives not yet supporting the when= argument
+                    # are not used inside the context manager
+                    if decorated_function.__name__ in ('version', 'variant'):
+                        msg = ('directive "{0}" cannot be used within a "when"'
+                               ' context since it does not support a "when=" '
+                               'argument')
+                        msg = msg.format(decorated_function.__name__)
+                        raise DirectiveError(msg)
+
+                    when_spec_from_context = ' '.join(
+                        DirectiveMeta._when_constraints_from_context
+                    )
+                    when_spec = kwargs.get('when', '') + ' ' + when_spec_from_context
+                    kwargs['when'] = when_spec
+
                 # If any of the arguments are executors returned by a
                 # directive passed as an argument, don't execute them
                 # lazily. Instead, let the called directive handle them.
@@ -331,7 +358,7 @@ def _depends_on(pkg, spec, when=None, type=default_deptype, patches=None):
         patches = [patches]
 
     # auto-call patch() directive on any strings in patch list
-    patches = [patch(p) if isinstance(p, string_types) else p
+    patches = [patch(p) if isinstance(p, six.string_types) else p
                for p in patches]
     assert all(callable(p) for p in patches)
 

lib/spack/spack/multimethod.py

@@ -24,14 +24,12 @@
 depending on the scenario, regular old conditionals might be clearer,
 so package authors should use their judgement.
 """
-
 import functools
 import inspect
 
-from llnl.util.lang import caller_locals
+import spack.directives
-
-import spack.architecture
 import spack.error
+from llnl.util.lang import caller_locals
 from spack.spec import Spec
 
 
@@ -156,72 +154,80 @@ def __call__(self, package_self, *args, **kwargs):
 
 
 class when(object):
-    """This annotation lets packages declare multiple versions of
-       methods like install() that depend on the package's spec.
-       For example:
-
-       .. code-block:: python
-
-          class SomePackage(Package):
-              ...
-
-              def install(self, prefix):
-                  # Do default install
-
-              @when('target=x86_64:')
-              def install(self, prefix):
-                  # This will be executed instead of the default install if
-                  # the package's target is in the x86_64 family.
-
-              @when('target=ppc64:')
-              def install(self, prefix):
-                  # This will be executed if the package's target is in
-                  # the ppc64 family
-
-       This allows each package to have a default version of install() AND
-       specialized versions for particular platforms.  The version that is
-       called depends on the architecutre of the instantiated package.
-
-       Note that this works for methods other than install, as well.  So,
-       if you only have part of the install that is platform specific, you
-       could do this:
-
-       .. code-block:: python
-
-          class SomePackage(Package):
-              ...
-              # virtual dependence on MPI.
-              # could resolve to mpich, mpich2, OpenMPI
-              depends_on('mpi')
-
-              def setup(self):
-                  # do nothing in the default case
-                  pass
-
-              @when('^openmpi')
-              def setup(self):
-                  # do something special when this is built with OpenMPI for
-                  # its MPI implementations.
-
-
-              def install(self, prefix):
-                  # Do common install stuff
-                  self.setup()
-                  # Do more common install stuff
-
-       Note that the default version of decorated methods must
-       *always* come first.  Otherwise it will override all of the
-       platform-specific versions.  There's not much we can do to get
-       around this because of the way decorators work.
-    """
-
     def __init__(self, condition):
+        """Can be used both as a decorator, for multimethods, or as a context
+        manager to group ``when=`` arguments together.
+
+        Examples are given in the docstrings below.
+
+        Args:
+            condition (str): condition to be met
+        """
         if isinstance(condition, bool):
             self.spec = Spec() if condition else None
         else:
             self.spec = Spec(condition)
 
     def __call__(self, method):
+        """This annotation lets packages declare multiple versions of
+        methods like install() that depend on the package's spec.
+
+        For example:
+
+           .. code-block:: python
+
+              class SomePackage(Package):
+                  ...
+
+                  def install(self, prefix):
+                      # Do default install
+
+                  @when('target=x86_64:')
+                  def install(self, prefix):
+                      # This will be executed instead of the default install if
+                      # the package's target is in the x86_64 family.
+
+                  @when('target=ppc64:')
+                  def install(self, prefix):
+                      # This will be executed if the package's target is in
+                      # the ppc64 family
+
+           This allows each package to have a default version of install() AND
+           specialized versions for particular platforms.  The version that is
+           called depends on the architecutre of the instantiated package.
+
+           Note that this works for methods other than install, as well.  So,
+           if you only have part of the install that is platform specific, you
+           could do this:
+
+           .. code-block:: python
+
+              class SomePackage(Package):
+                  ...
+                  # virtual dependence on MPI.
+                  # could resolve to mpich, mpich2, OpenMPI
+                  depends_on('mpi')
+
+                  def setup(self):
+                      # do nothing in the default case
+                      pass
+
+                  @when('^openmpi')
+                  def setup(self):
+                      # do something special when this is built with OpenMPI for
+                      # its MPI implementations.
+
+
+                  def install(self, prefix):
+                      # Do common install stuff
+                      self.setup()
+                      # Do more common install stuff
+
+           Note that the default version of decorated methods must
+           *always* come first.  Otherwise it will override all of the
+           platform-specific versions.  There's not much we can do to get
+           around this because of the way decorators work.
+        """
         # In Python 2, Get the first definition of the method in the
         # calling scope by looking at the caller's locals. In Python 3,
         # we handle this using MultiMethodMeta.__prepare__.
@@ -238,6 +244,32 @@ def __call__(self, method):
 
         return original_method
 
+    def __enter__(self):
+        """Inject the constraint spec into the `when=` argument of directives
+        in the context.
+
+        This context manager allows you to write:
+
+            with when('+nvptx'):
+                conflicts('@:6', msg='NVPTX only supported from gcc 7')
+                conflicts('languages=ada')
+                conflicts('languages=brig')
+
+        instead of writing:
+
+             conflicts('@:6', when='+nvptx', msg='NVPTX only supported from gcc 7')
+             conflicts('languages=ada', when='+nvptx')
+             conflicts('languages=brig', when='+nvptx')
+
+        Context managers can be nested (but this is not recommended for readability)
+        and add their constraint to whatever may be already present in the directive
+        `when=` argument.
+        """
+        spack.directives.DirectiveMeta.push_to_context(str(self.spec))
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        spack.directives.DirectiveMeta.pop_from_context()
+
 
 class MultiMethodError(spack.error.SpackError):
     """Superclass for multimethod dispatch errors"""

lib/spack/spack/test/directives.py

@@ -32,3 +32,13 @@ def test_true_directives_exist(mock_packages):
 
     assert cls.patches
     assert Spec() in cls.patches
+
+
+def test_constraints_from_context(mock_packages):
+    pkg_cls = spack.repo.path.get_pkg_class('with-constraint-met')
+
+    assert pkg_cls.dependencies
+    assert Spec('@1.0') in pkg_cls.dependencies['b']
+
+    assert pkg_cls.conflicts
+    assert (Spec('@1.0'), None) in pkg_cls.conflicts['%gcc']

var/spack/repos/builtin.mock/packages/with-constraint-met/package.py

@@ -0,0 +1,20 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class WithConstraintMet(Package):
+    """Package that tests True when specs on directives."""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/example-1.0.tar.gz"
+
+    version('2.0', '0123456789abcdef0123456789abcdef')
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    with when('@1.0'):
+        depends_on('b')
+        conflicts('%gcc')

var/spack/repos/builtin/packages/cp2k/package.py

@@ -2,10 +2,9 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
+import copy
 import os
 import os.path
-import copy
 
 import spack.util.environment
 
@@ -68,7 +67,7 @@ class Cp2k(MakefilePackage, CudaPackage):
     variant('lmax',
             description='Maximum supported angular momentum (HFX and others)',
             default='5',
-            values=tuple(map(str, HFX_LMAX_RANGE)),
+            values=[str(x) for x in HFX_LMAX_RANGE],
             multi=False)
 
     depends_on('python', type='build')
@@ -78,71 +77,88 @@ class Cp2k(MakefilePackage, CudaPackage):
     depends_on('lapack')
     depends_on('fftw-api@3')
 
-    # require libxsmm-1.11+ since 1.10 can leak file descriptors in Fortran
+    with when('smm=libxsmm'):
-    depends_on('libxsmm@1.11:~header-only', when='smm=libxsmm')
+        # require libxsmm-1.11+ since 1.10 can leak file descriptors in Fortran
-    # use pkg-config (support added in libxsmm-1.10) to link to libxsmm
+        depends_on('libxsmm@1.11:~header-only')
-    depends_on('pkgconfig', type='build', when='smm=libxsmm')
+        # use pkg-config (support added in libxsmm-1.10) to link to libxsmm
-    # ... and in CP2K 7.0+ for linking to libint2
+        depends_on('pkgconfig', type='build')
-    depends_on('pkgconfig', type='build', when='+libint@7.0:')
+        # please set variants: smm=blas by configuring packages.yaml or install
-    depends_on('pkgconfig', type='build', when='+libxc@7.0:')
+        # cp2k with option smm=blas on aarch64
+        conflicts('target=aarch64:', msg='libxsmm is not available on arm')
 
-    # libint & libxc are always statically linked
+    with when('+libint'):
-    depends_on('libint@1.1.4:1.2', when='+libint@3.0:6.9', type='build')
+        # ... and in CP2K 7.0+ for linking to libint2
-    for lmax in HFX_LMAX_RANGE:
+        depends_on('pkgconfig', type='build', when='@7.0:')
-        # libint2 can be linked dynamically again
+        # libint & libxc are always statically linked
-        depends_on('libint@2.6.0:+fortran tune=cp2k-lmax-{0}'.format(lmax),
+        depends_on('libint@1.1.4:1.2', when='@3.0:6.9')
-                   when='+libint@7.0: lmax={0}'.format(lmax))
+        for lmax in HFX_LMAX_RANGE:
+            # libint2 can be linked dynamically again
+            depends_on('libint@2.6.0:+fortran tune=cp2k-lmax-{0}'.format(lmax),
+                       when='@7.0: lmax={0}'.format(lmax))
 
-    depends_on('libxc@2.2.2:3.99.0', when='+libxc@:5.5999', type='build')
+    with when('+libxc'):
-    depends_on('libxc@4.0.3:4.99.0', when='+libxc@6.0:6.9', type='build')
+        depends_on('pkgconfig', type='build', when='@7.0:')
-    depends_on('libxc@4.0.3:4.99.0', when='+libxc@7.0:8.1')
+        depends_on('libxc@2.2.2:3.99.0', when='@:5.5999', type='build')
-    depends_on('libxc@5.1.3:5.1.99', when='+libxc@8.2:')
+        depends_on('libxc@4.0.3:4.99.0', when='@6.0:6.9', type='build')
+        depends_on('libxc@4.0.3:4.99.0', when='@7.0:8.1')
+        depends_on('libxc@5.1.3:5.1.99', when='@8.2:')
 
-    depends_on('mpi@2:', when='+mpi')
+    with when('+mpi'):
-    depends_on('scalapack', when='+mpi')
+        depends_on('mpi@2:')
-    depends_on('cosma+scalapack', when='+cosma')
+        depends_on('scalapack')
-    depends_on('cosma+cuda+scalapack', when='+cosma+cuda')
+
-    depends_on('elpa@2011.12:2016.13+openmp', when='+openmp+elpa@:5.999')
+    with when('+cosma'):
-    depends_on('elpa@2011.12:2017.11+openmp', when='+openmp+elpa@6.0:6.999')
+        depends_on('cosma+scalapack')
-    depends_on('elpa@2018.05:+openmp', when='+openmp+elpa@7.0:')
+        depends_on('cosma+cuda', when='+cuda')
-    depends_on('elpa@2011.12:2016.13~openmp', when='~openmp+elpa@:5.999')
+        conflicts('~mpi')
-    depends_on('elpa@2011.12:2017.11~openmp', when='~openmp+elpa@6.0:6.999')
+        # COSMA support was introduced in 8+
-    depends_on('elpa@2018.05:~openmp', when='~openmp+elpa@7.0:')
+        conflicts('@:7.999')
-    depends_on('plumed+shared+mpi', when='+plumed+mpi')
+
-    depends_on('plumed+shared~mpi', when='+plumed~mpi')
+    with when('+elpa'):
+        conflicts('~mpi', msg='elpa requires MPI')
+        depends_on('elpa+openmp', when='+openmp')
+        depends_on('elpa~openmp', when='~openmp')
+        depends_on('elpa@2011.12:2016.13', when='@:5.999')
+        depends_on('elpa@2011.12:2017.11', when='@6.0:6.999')
+        depends_on('elpa@2018.05:', when='@7.0:')
+
+    with when('+plumed'):
+        depends_on('plumed+shared')
+        depends_on('plumed+mpi', when='+mpi')
+        depends_on('plumed~mpi', when='~mpi')
 
     # while we link statically against PEXSI, its own deps may be linked in
     # dynamically, therefore can't set this as pure build-type dependency.
-    depends_on('pexsi+fortran@0.9.0:0.9.999', when='+pexsi@:4.999')
+    with when('+pexsi'):
-    depends_on('pexsi+fortran@0.10.0:', when='+pexsi@5.0:')
+        conflicts('~mpi', msg='pexsi requires MPI')
+        depends_on('pexsi+fortran@0.9.0:0.9.999', when='@:4.999')
+        depends_on('pexsi+fortran@0.10.0:', when='@5.0:')
 
-    # only OpenMP should be consistenly used, all other common things
+    # only OpenMP should be consistently used, all other common things
     # like ELPA, SCALAPACK are independent and Spack will ensure that
-    # a consistent/compat. combination is pulled in to the dependency graph.
+    # a consistent/compatible combination is pulled into the dependency graph.
-    depends_on('sirius@:6.999+fortran+vdwxc+shared+openmp', when='@:7.999+sirius+openmp')
+    with when('+sirius'):
-    depends_on('sirius@:6.999+fortran+vdwxc+shared~openmp', when='@:7.999+sirius~openmp')
+        depends_on('sirius+fortran+vdwxc+shared')
+        depends_on('sirius+openmp', when='+openmp')
+        depends_on('sirius~openmp', when='~openmp')
+        depends_on('sirius@:6.999', when='@:7.999')
+        depends_on('sirius@7:', when='@8:')
+        conflicts('~mpi')
+        # sirius support was introduced in 7+
+        conflicts('@:6.999')
 
-    depends_on('sirius@7:+fortran+vdwxc+shared+openmp', when='@8:+sirius+openmp')
+    with when('+libvori'):
+        depends_on('libvori@201219:', when='@8.1', type='build')
+        depends_on('libvori@210412:', when='@8.2:', type='build')
+        # libvori support was introduced in 8+
+        conflicts('@:7.999')
 
     # the bundled libcusmm uses numpy in the parameter prediction (v7+)
     # which is written using Python 3
     depends_on('py-numpy', when='@7:+cuda', type='build')
     depends_on('python@3.6:', when='@7:+cuda', type='build')
 
-    depends_on('libvori@201219:', when='@8.1+libvori', type='build')
-    depends_on('libvori@210412:', when='@8.2:+libvori', type='build')
     depends_on('spglib', when='+spglib')
-
-    # PEXSI, ELPA, COSMA and SIRIUS depend on MPI
-    conflicts('~mpi', '+pexsi')
-    conflicts('~mpi', '+elpa')
-    conflicts('~mpi', '+sirius')
-    conflicts('~mpi', '+cosma')
-    conflicts('+sirius', '@:6.999')  # sirius support was introduced in 7+
-    conflicts('+cosma', '@:7.999')  # COSMA support was introduced in 8+
-
-    conflicts('+libvori', '@:7.999')  # libvori support was introduced in 8+
-
     conflicts('~cuda', '+cuda_fft')
     conflicts('~cuda', '+cuda_blas')
 
@@ -156,10 +172,6 @@ class Cp2k(MakefilePackage, CudaPackage):
     conflicts('%clang')
     conflicts('%nag')
 
-    # please set variants: smm=blas by configuring packages.yaml or install
-    # cp2k with option smm=blas on aarch64
-    conflicts('smm=libxsmm',  when='target=aarch64:', msg='libxsmm is not available on arm')
-
     conflicts('^fftw~openmp', when='+openmp')
     conflicts('^amdfftw~openmp', when='+openmp')
     conflicts('^openblas threads=none', when='+openmp')

var/spack/repos/builtin/packages/gcc/package.py

@@ -11,8 +11,7 @@
 import llnl.util.tty as tty
 import spack.architecture
 import spack.util.executable
-
+from spack.operating_systems.mac_os import macos_sdk_path, macos_version
-from spack.operating_systems.mac_os import macos_version, macos_sdk_path
 
 
 class Gcc(AutotoolsPackage, GNUMirrorPackage):
@@ -25,7 +24,7 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     list_url = 'http://ftp.gnu.org/gnu/gcc/'
     list_depth = 1
 
-    maintainers = ['michaelkuhn']
+    maintainers = ['michaelkuhn', 'alalazo']
 
     version('master', branch='master')
 
@@ -120,11 +119,13 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     #   GCC 5.4 https://github.com/spack/spack/issues/6902#issuecomment-433072097
     #   GCC 7.3 https://github.com/spack/spack/issues/6902#issuecomment-433030376
     #   GCC 9+  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86724
-    depends_on('isl@0.14', when='@5.0:5.2 +graphite')
+    with when('+graphite'):
-    depends_on('isl@0.15', when='@5.3:5.9 +graphite')
+        depends_on('isl@0.14', when='@5.0:5.2')
-    depends_on('isl@0.15:0.18', when='@6:8.9 +graphite')
+        depends_on('isl@0.15', when='@5.3:5.9')
-    depends_on('isl@0.15:0.20', when='@9:9.9 +graphite')
+        depends_on('isl@0.15:0.18', when='@6:8.9')
-    depends_on('isl@0.15:', when='@10: +graphite')
+        depends_on('isl@0.15:0.20', when='@9:9.9')
+        depends_on('isl@0.15:', when='@10:')
+
     depends_on('zlib', when='@6:')
     # GCC only tries to link with -lzstd but it requires
     # -pthread too when linking against libzstd.a, so
@@ -135,26 +136,10 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     depends_on('gnat', when='languages=ada')
     depends_on('binutils+gas+ld+plugins~libiberty', when='+binutils', type=('build', 'link', 'run'))
     depends_on('zip', type='build', when='languages=java')
-    depends_on('cuda', when='+nvptx')
 
     # The server is sometimes a bit slow to respond
     timeout = {'timeout': 60}
 
-    resource(name='newlib',
-             url='ftp://sourceware.org/pub/newlib/newlib-3.0.0.20180831.tar.gz',
-             sha256='3ad3664f227357df15ff34e954bfd9f501009a647667cd307bf0658aefd6eb5b',
-             destination='newlibsource',
-             when='+nvptx',
-             fetch_options=timeout)
-
-    # nvptx-tools does not seem to work as a dependency,
-    # but does fine when the source is inside the gcc build directory
-    # nvptx-tools doesn't have any releases, so grabbing the last commit
-    resource(name='nvptx-tools',
-             git='https://github.com/MentorEmbedded/nvptx-tools',
-             commit='d0524fbdc86dfca068db5a21cc78ac255b335be5',
-             when='+nvptx')
-
     # TODO: integrate these libraries.
     # depends_on('ppl')
     # depends_on('cloog')
@@ -167,14 +152,20 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     depends_on('guile@1.4.1:', type='test')
 
     # See https://golang.org/doc/install/gccgo#Releases
-    provides('golang',        when='languages=go @4.6:')
+    with when('languages=go'):
-    provides('golang@:1',     when='languages=go @4.7.1:')
+        provides('golang',        when='@4.6:')
-    provides('golang@:1.1',   when='languages=go @4.8:')
+        provides('golang@:1',     when='@4.7.1:')
-    provides('golang@:1.1.2', when='languages=go @4.8.2:')
+        provides('golang@:1.1',   when='@4.8:')
-    provides('golang@:1.2',   when='languages=go @4.9:')
+        provides('golang@:1.1.2', when='@4.8.2:')
-    provides('golang@:1.4',   when='languages=go @5:')
+        provides('golang@:1.2',   when='@4.9:')
-    provides('golang@:1.6.1', when='languages=go @6:')
+        provides('golang@:1.4',   when='@5:')
-    provides('golang@:1.8',   when='languages=go @7:')
+        provides('golang@:1.6.1', when='@6:')
+        provides('golang@:1.8',   when='@7:')
+        # GCC 4.6 added support for the Go programming language.
+        # See https://gcc.gnu.org/gcc-4.6/changes.html
+        conflicts('@:4.5', msg='support for Go has been added in GCC 4.6')
+        # Go is not supported on macOS
+        conflicts('platform=darwin', msg='Go not supported on MacOS')
 
     # For a list of valid languages for a specific release,
     # run the following command in the GCC source directory:
@@ -194,13 +185,6 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     # but this is the first version that accepts 'c' as a valid language.
     conflicts('languages=c', when='@:4.7')
 
-    # GCC 4.6 added support for the Go programming language.
-    # See https://gcc.gnu.org/gcc-4.6/changes.html
-    conflicts('languages=go', when='@:4.5')
-
-    # Go is not supported on macOS
-    conflicts('languages=go', when='platform=darwin')
-
     # The GCC Java frontend and associated libjava runtime library
     # have been removed from GCC as of GCC 7.
     # See https://gcc.gnu.org/gcc-7/changes.html
@@ -210,17 +194,34 @@ class Gcc(AutotoolsPackage, GNUMirrorPackage):
     # See https://gcc.gnu.org/gcc-5/changes.html
     conflicts('languages=jit', when='@:4')
 
-    # NVPTX offloading supported in 7 and later by limited languages
+    with when('+nvptx'):
-    conflicts('+nvptx', when='@:6', msg='NVPTX only supported in gcc 7 and above')
+        depends_on('cuda')
-    conflicts('languages=ada', when='+nvptx')
+        resource(
-    conflicts('languages=brig', when='+nvptx')
+            name='newlib',
-    conflicts('languages=go', when='+nvptx')
+            url='ftp://sourceware.org/pub/newlib/newlib-3.0.0.20180831.tar.gz',
-    conflicts('languages=java', when='+nvptx')
+            sha256='3ad3664f227357df15ff34e954bfd9f501009a647667cd307bf0658aefd6eb5b',
-    conflicts('languages=jit', when='+nvptx')
+            destination='newlibsource',
-    conflicts('languages=objc', when='+nvptx')
+            fetch_options=timeout
-    conflicts('languages=obj-c++', when='+nvptx')
+        )
-    # NVPTX build disables bootstrap
+        # nvptx-tools does not seem to work as a dependency,
-    conflicts('+bootstrap', when='+nvptx')
+        # but does fine when the source is inside the gcc build directory
+        # nvptx-tools doesn't have any releases, so grabbing the last commit
+        resource(
+            name='nvptx-tools',
+            git='https://github.com/MentorEmbedded/nvptx-tools',
+            commit='d0524fbdc86dfca068db5a21cc78ac255b335be5',
+        )
+        # NVPTX offloading supported in 7 and later by limited languages
+        conflicts('@:6', msg='NVPTX only supported in gcc 7 and above')
+        conflicts('languages=ada')
+        conflicts('languages=brig')
+        conflicts('languages=go')
+        conflicts('languages=java')
+        conflicts('languages=jit')
+        conflicts('languages=objc')
+        conflicts('languages=obj-c++')
+        # NVPTX build disables bootstrap
+        conflicts('+bootstrap')
 
     # Binutils can't build ld on macOS
     conflicts('+binutils', when='platform=darwin')