Make Test a weak dependency

Project.toml

@@ -1,24 +1,27 @@
 name = "InverseFunctions"
 uuid = "3587e190-3f89-42d0-90ee-14403ec27112"
-version = "0.1.15"
+version = "0.1.16"
 
 [deps]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [weakdeps]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [extensions]
-DatesExt = "Dates"
+InverseFunctionsDatesExt = "Dates"
+InverseFunctionsTestExt = "Test"
 
 [compat]
 julia = "1"
 
 [extras]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 Unitful = "1986cc42-f94f-5a68-af5c-568840ba703d"
 
 [targets]
-test = ["Dates", "Documenter", "Unitful"]
+test = ["Dates", "Documenter", "Test", "Unitful"]

README.md

@@ -9,8 +9,7 @@
 
 InverseFunctions.jl defines an interface to invert functions.
 
-`InverseFunctions` is a very lightweight package and has no dependencies
-beyond `Base` and `Test`.
+`InverseFunctions` is a very lightweight package and has no dependencies.
 
 ## Documentation
 

ext/DatesExt.jl → ext/InverseFunctionsDatesExt.jl

@@ -1,4 +1,4 @@
-module DatesExt
+module InverseFunctionsDatesExt
 
 using Dates
 import InverseFunctions: inverse

ext/InverseFunctionsTestExt.jl

@@ -0,0 +1,17 @@
+module InverseFunctionsTestExt
+
+using Test: @test, @testset
+using InverseFunctions: InverseFunctions, inverse
+
+function InverseFunctions.test_inverse(f, x; compare=isapprox, kwargs...)
+    @testset "test_inverse: $f with input $x" begin
+        y = f(x)
+        inverse_f = inverse(f)
+        @test compare(inverse_f(y), x; kwargs...)
+        inverse_inverse_f = inverse(inverse_f)
+        @test compare(inverse_inverse_f(x), y; kwargs...)
+    end
+    return nothing
+end
+
+end

src/InverseFunctions.jl

@@ -6,15 +6,43 @@ Lightweight package that defines an interface to invert functions.
 """
 module InverseFunctions
 
-using Test
-
 include("functions.jl")
 include("inverse.jl")
 include("setinverse.jl")
-include("test.jl")
+
+"""
+    InverseFunctions.test_inverse(f, x; compare=isapprox, kwargs...)
+
+Test if [`inverse(f)`](@ref) is implemented correctly.
+
+The function tests (as a `Test.@testset`) if
+
+* `compare(inverse(f)(f(x)), x) == true` and
+* `compare(inverse(inverse(f))(x), f(x)) == true`.
+
+`kwargs...` are forwarded to `compare`.
+
+!!! Note
+    On Julia >= 1.9, you have to load the `Test` standard library to be able to use
+    this function.
+"""
+function test_inverse end
 
 @static if !isdefined(Base, :get_extension)
-    include("../ext/DatesExt.jl")
+    include("../ext/InverseFunctionsDatesExt.jl")
+    include("../ext/InverseFunctionsTestExt.jl") 
+end
+
+# Better error message if users forget to load Test
+if isdefined(Base, :get_extension) && isdefined(Base.Experimental, :register_error_hint)
+    function __init__()
+        Base.Experimental.register_error_hint(MethodError) do io, exc, _, _
+            if exc.f === test_inverse &&
+                (Base.get_extension(InverseFunctions, :InverseFunctionsTest) === nothing)
+                print(io, "\nDid you forget to load Test?")
+            end
+        end
+    end
 end
 
 end # module