Similar to the range-or-specific-version ambiguity of `@1.2` in the past,
which was solved with `@1.2` vs `@=1.2` we still have the ambiguity of
`name=a,b,c` in multi-valued variants. Do they mean "at least a,b,c" or
"exactly a,b,c"?
This issue comes up in for example `gcc languages=c,cxx`; there's no
way to exclude `fortran`.
The ambiguity is resolved with syntax `:=` to distinguish concrete from
abstract.
The following strings parse as **concrete** variants:
* `name:=a,b,c` => values exactly {a, b, c}
* `name:=a` => values exactly {a}
* `+name` => values exactly {True}
* `~name` => values exactly {False}
The following strings parse as **abstract** variants:
* `name=a,b,c` values at least {a, b, c}
* `name=*` special case for testing existence of a variant; values are at
least the empty set {}
As a reminder
* `satisfies(lhs, rhs)` means `concretizations(lhs)` ⊆ `concretizations(rhs)`
* `intersects(lhs, rhs)` means `concretizations(lhs)` ∩ `concretizations(rhs)` ≠ ∅
where `concretizations(...)` is the set of sets of variant values in this case.
The satisfies semantics are:
* rhs abstract: rhs values is a subset of lhs values (whether lhs is abstract or concrete)
* lhs concrete, rhs concrete: set equality
* lhs abstract, rhs concrete: false
and intersects should mean
* lhs and rhs abstract: true (the union is a valid concretization under both)
* lhs or rhs abstract: true iff the abstract variant's values are a subset of the concrete one
* lhs concrete, rhs concrete: set equality
Concrete specs with single-valued variants are printed `+foo`, `~foo` and `foo=bar`;
only multi-valued variants are printed with `foo:=bar,baz` to reduce the visual noise.
6bfe83106d