Currently, we have the following in the llvm package:

$ spack spec llvm~clang+z3
==> Error: Cannot set variant 'z3' for package 'llvm' because the variant condition cannot be satisfied for the given spec

Good. Now, what about llvm~clang~z3? As a user, I would expect it to be possible but it is not:

$ spack spec llvm~clang~z3
==> Error: Cannot set variant 'z3' for package 'llvm' because the variant condition cannot be satisfied for the given spec

With the modifications from this PR, it is possible to declare the z3 variant differently:

variant(
    "z3",
    values=(conditional(True, when="+clang @8:"), False),
    default=False,
    description="Use Z3 for the clang static analyzer",
)

And it looks like we can have the expected behaviour. We still get an error for llvm~clang+z3 (with a different message though):

$ spack spec llvm~clang+z3
==> Error: 'llvm' required multiple values for single-valued variant 'clang'
    Requested '~clang' and '+clang'

And we can also explicitly disable z3:

$ spack spec llvm~clang~z3
Input spec
--------------------------------
llvm~clang~z3

Concretized
--------------------------------
[email protected]%[email protected]~clang~cuda+gold~ipo~link_llvm_dylib+lld+llvm_dylib~mlir~omp_debug~omp_tsan+polly~python~split_dwarf~z3 build_type=Release patches=6379168,d85ef51,f920173 shlib_symbol_version=none targets=none version_suffix=none arch=linux-debian11-skylake
...

NOTE: The order of the values is important:

variant(..., values=(True, False), default=False,...)

seems to be the same as

variant(..., values=None, default=False,...)

but if we swap the order of values, i.e.

variant(..., values=(False, True), default=False,...)

we get:

$ spack spec llvm+z3
==> Error: invalid values for variant "z3" in package "llvm": ['True']
$ spack spec llvm~z3
==> Error: invalid values for variant "z3" in package "llvm": ['False']

The same problem is when values has a conditional as above, i.e.:

variant(..., values=(False, conditional(True, when="+clang @8:")), default=False,...)

It might be worth fixing too.

@alalazo