Fix the dead Test-extension guard and let the extension implement a stub

Project.toml

@@ -1,6 +1,6 @@
 name = "InverseFunctions"
 uuid = "3587e190-3f89-42d0-90ee-14403ec27112"
-version = "0.1.17"
+version = "0.1.18"
 
 [deps]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"

ext/InverseFunctionsTestExt.jl

@@ -3,7 +3,7 @@ module InverseFunctionsTestExt
 using Test: @test, @testset
 using InverseFunctions: InverseFunctions, inverse
 
-function InverseFunctions.test_inverse(f, x; compare=isapprox, kwargs...)
+function InverseFunctions._test_inverse(f, x; compare=isapprox, kwargs...)
     @testset "test_inverse: $f with input $x" begin
         y = f(x)
         inverse_f = inverse(f)

src/InverseFunctions.jl

@@ -26,23 +26,19 @@ The function tests (as a `Test.@testset`) if
     On Julia >= 1.9, you have to load the `Test` standard library to be able to use
     this function.
 """
-function test_inverse end
+function test_inverse(f, x; kwargs...)
+    hasmethod(_test_inverse, Tuple{Any,Any}) || throw(ArgumentError(
+        "InverseFunctions.test_inverse requires the Test standard library: `using Test`."
+    ))
+    return _test_inverse(f, x; kwargs...)
+end
+
+# implemented by InverseFunctionsTestExt
+function _test_inverse end
 
 @static if !isdefined(Base, :get_extension)
     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

test/test_inverse.jl

@@ -43,6 +43,27 @@ end
     InverseFunctions.test_inverse(inverse, log, compare = ===)
 end
 
+@testset "test_inverse" begin
+    @test !isdefined(InverseFunctions, :__init__)
+
+    err = try
+        InverseFunctions.test_inverse(identity)
+    catch e
+        e
+    end
+    @test err isa MethodError
+    @test !occursin("requires the Test standard library", sprint(showerror, err))
+
+    # Test can't be unloaded, so the missing-Test error is checked in a subprocess
+    if isdefined(Base, :get_extension)
+        script = "using InverseFunctions; InverseFunctions.test_inverse(identity, 1)"
+        cmd = `$(Base.julia_cmd()) --startup-file=no --project=$(Base.active_project()) -e $script`
+        output = IOBuffer()
+        run(pipeline(ignorestatus(cmd); stdout=output, stderr=output))
+        @test occursin("ArgumentError: InverseFunctions.test_inverse requires the Test standard library", String(take!(output)))
+    end
+end
+
 @testset "maths" begin
     InverseFunctions.test_inverse(!, false)