Set conflicts parameter for cuda-11 (#18847)

Magma is not currently compatible with CUDA-11. While this is reflected
in the package, it is done with a comment in a `depends_on` directive,
which has the effect of trying to install a version of CUDA that may be
different from the one in the current environment, without any message
to the end user. A `conflicts` is a better way to handle this.

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

@@ -36,14 +36,16 @@ class Magma(CMakePackage, CudaPackage):
     depends_on('blas')
     depends_on('lapack')
     depends_on('cuda@8:', when='@2.5.1:')  # See PR #14471
-    depends_on('cuda@:10.99999')  # incompatible with CUDA 11
-    # The previous line would ideally include "when='@:2.5.3'", but this
-    # doesn't work due to a problem with the concretizer.
 
     conflicts('~cuda', msg='Magma requires cuda')
     conflicts('cuda_arch=none',
               msg='Please indicate a CUDA arch value or values')
 
+    # currently not compatible with CUDA-11
+    # https://bitbucket.org/icl/magma/issues/22/cuda-11-changes-issue
+    # https://bitbucket.org/icl/magma/issues/25/error-cusparsesolveanalysisinfo_t-does-not
+    conflicts('^cuda@11:', when='@:2.5.3')
+
     patch('ibm-xl.patch', when='@2.2:2.5.0%xl')
     patch('ibm-xl.patch', when='@2.2:2.5.0%xl_r')
     patch('magma-2.3.0-gcc-4.8.patch', when='@2.3.0%gcc@:4.8')