[Dependencies] Default build_version to the minimum compatible version (#150)

In a `Dependency` we have to specify two versions numbers: the actual build
version and the compat version, which 99.99% of the time are the same.  This is
annoying and error-prone.  With this change the build version defaults to the
minimum compatible version specified by `compat`, if any.

Author: giordano

Project.toml

@@ -1,7 +1,7 @@
 name = "BinaryBuilderBase"
 uuid = "7f725544-6523-48cd-82d1-3fa08ff4056e"
 authors = ["Elliot Saba <staticfloat@gmail.com>"]
-version = "0.6.6"
+version = "0.6.7"
 
 [deps]
 CodecZlib = "944b1d66-785c-5afd-91f1-9de20f533193"

src/Dependencies.jl

@@ -6,6 +6,8 @@ export Dependency, BuildDependency, HostBuildDependency,
 
 # Pkg.PackageSpec return different types in different Julia versions so...
 const PkgSpec = typeof(Pkg.PackageSpec(name="dummy"))
+const PKG_VERSIONS = Base.VERSION >= v"1.7-" ? Pkg.Versions : Pkg.Types
+
 
 """
 An `AbstractDependency` is a binary dependency of the JLL package.  Dependencies
@@ -52,7 +54,7 @@ Return whether `dep` is a runtime dependency or not.
 is_runtime_dependency
 
 """
-    Dependency(dep::Union{PackageSpec,String})
+    Dependency(dep::Union{PackageSpec,String}, build_version; compat)
 
 Define a binary dependency that is necessary to build the package and load the
 generated JLL package.  The argument can be either a string with the name of the
@@ -61,20 +63,28 @@ JLL package or a `Pkg.PackageSpec`.
 The optional keyword argument `build_version` can be used to specify the version
 of the dependency to be installed when building it.
 
-The optional keyword argument `compat` can be used to specify a string for
-use in the `Project.toml` of the generated Julia package.
+The optional keyword argument `compat` can be used to specify a string for use
+in the `Project.toml` of the generated Julia package.  If `compat` is non-empty
+and `build_version` is not passed, the latter defaults to the minimum version
+compatible with the `compat` specifier.
 """
 struct Dependency <: AbstractDependency
     pkg::PkgSpec
     build_version::Union{VersionNumber,Nothing}
     compat::String  # semver string for use in Project.toml of the JLL
     function Dependency(pkg::PkgSpec, build_version = nothing; compat::String = "")
         if length(compat) > 0
-            spec = Pkg.Types.semver_spec(compat) # verify compat is valid
-            if build_version !== nothing && !(build_version in spec)
+            spec = PKG_VERSIONS.semver_spec(compat) # verify compat is valid
+            if build_version === nothing
+                # Since we usually want to build against the oldest compatible
+                # version, if `build_version` isn't set but `compat` is, make it
+                # default to the minimum compatible version.
+                build_version = minimum(VersionNumber(rng.lower.t) for rng in spec.ranges)
+            end
+            if build_version ∉ spec
                 throw(ArgumentError("build_version and compat for $(pkg) are incompatible"))
             end
-            if pkg.version != Pkg.Types.VersionSpec("*") && !(pkg.version in spec)
+            if pkg.version != PKG_VERSIONS.VersionSpec("*") && !(pkg.version in spec)
                 throw(ArgumentError("PackageSpec version and compat for $(pkg) are incompatible"))
             end
         end

test/dependencies.jl

@@ -42,6 +42,13 @@ end
     @test getpkg(dep_buildver) == PackageSpec(; name = name, version = build_version)
     @test getcompat(dep_buildver) == "~1.2"
 
+    # the same but only with compat specifier
+    dep_compat = Dependency(PackageSpec(; name); compat = "2, ~$(build_version)")
+    @test Dependency(name, build_version) == dep_compat
+    @test getname(dep_compat) == name
+    @test getpkg(dep_compat) == PackageSpec(; name, version = build_version)
+    @test getcompat(dep_compat) == "2, ~$(build_version)"
+
     # if build_version and compat don't match, an error should be thrown
     @test_throws ArgumentError Dependency(PackageSpec(; name = name), build_version, compat = "2.0")