Add test for package dev'ed in its own test/Manifest.toml

Covers the reproducer from #120: a package whose test/Manifest.toml
dev's the package itself (what `julia --project=test -e 'using Pkg;
Pkg.develop(path=".")'` produces) could not be activated on Julia
1.4-1.10 before the merge-project guard was ported.

The new `test/sources/SelfDevTestManifest` fixture is copied to a
temporary directory and both manifests are generated at test time, so
the manifest format always matches the running Julia version.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: davidanthoff

test/activate_set.jl

@@ -90,6 +90,43 @@
         end
     end
 
+    if VERSION >= v"1.4-"
+        # https://github.com/JuliaTesting/TestEnv.jl/pull/120
+        # A package whose `test/Manifest.toml` dev's the package itself, i.e. what
+        # `julia --project=test -e 'using Pkg; Pkg.develop(path=".")'` produces.
+        @testset "activate package dev'ed in its own test/Manifest" begin
+            mktempdir() do p
+                pkg_path = joinpath(p, "SelfDevTestManifest")
+                cp(joinpath(@__DIR__, "sources", "SelfDevTestManifest"), pkg_path)
+
+                orig_project_toml_path = Base.active_project()
+                orig_load_path = copy(LOAD_PATH)
+                push!(LOAD_PATH, mktempdir())  # put something weird in LOAD_PATH for testing
+                try
+                    # Generate the test/Manifest.toml that dev's the package itself
+                    Pkg.activate(joinpath(pkg_path, "test"))
+                    Pkg.develop(PackageSpec(path=pkg_path))
+
+                    # ... and the package's own manifest
+                    Pkg.activate(pkg_path)
+                    Pkg.resolve()
+
+                    TestEnv.activate()
+                    new_project_toml_path = Base.active_project()
+                    @test new_project_toml_path != orig_project_toml_path
+
+                    @eval using SelfDevTestManifest
+                    @test Base.invokelatest(isdefined, @__MODULE__, :SelfDevTestManifest)
+                    @test (@eval SelfDevTestManifest.foo()) == 42
+                finally
+                    Pkg.activate(orig_project_toml_path)
+                    pop!(LOAD_PATH)
+                    @test orig_load_path == LOAD_PATH
+                end
+            end
+        end
+    end
+
     if VERSION >= v"1.11"
         @testset "activate with [sources]" begin
             orig_project_toml_path = Base.active_project()

test/sources/SelfDevTestManifest/Project.toml

@@ -0,0 +1,3 @@
+name = "SelfDevTestManifest"
+uuid = "1cda0850-20f2-4a85-a119-95c26fe5cbc8"
+version = "0.0.0"

test/sources/SelfDevTestManifest/src/SelfDevTestManifest.jl

@@ -0,0 +1,5 @@
+module SelfDevTestManifest
+
+foo() = 42
+
+end

test/sources/SelfDevTestManifest/test/Project.toml

@@ -0,0 +1,3 @@
+[deps]
+SelfDevTestManifest = "1cda0850-20f2-4a85-a119-95c26fe5cbc8"
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"

test/sources/SelfDevTestManifest/test/runtests.jl

@@ -0,0 +1,4 @@
+using SelfDevTestManifest
+using Test
+
+@test SelfDevTestManifest.foo() == 42