Fix handling unregistered deps when using force_latest_compatible_version

src/Operations.jl

@@ -3313,6 +3313,15 @@ function apply_force_latest_compatible_version!(
         uuid,
         old_compat_spec,
     )
+    if latest_compatible_version === nothing
+        if name != target_name
+            @warn(
+                "Skipping force_latest_compatible_version for unregistered dependency",
+                name, uuid, target_name,
+            )
+        end
+        return nothing
+    end
     earliest_backwards_compatible_version = get_earliest_backwards_compatible_version(latest_compatible_version)
     if allow_earlier_backwards_compatible_versions
         version_for_intersect = only_major_minor_patch(earliest_backwards_compatible_version)
@@ -3342,6 +3351,9 @@ function get_latest_compatible_version(
     )
     all_registered_versions = get_all_registered_versions(ctx, uuid)
     compatible_versions = filter(in(compat_spec), all_registered_versions)
+    if isempty(compatible_versions)
+        return nothing  # No registered versions found
+    end
     latest_compatible_version = maximum(compatible_versions)
     return latest_compatible_version
 end

test/force_latest_compatible_version.jl

@@ -327,6 +327,23 @@ const test_package_parent_dir = joinpath(
             end
         end
     end
+
+    @testset "UnregisteredDep: package with unregistered dependencies" begin
+        mktempdir() do tmp_dir
+            test_package = joinpath(tmp_dir, "UnregisteredDep")
+            cp(joinpath(test_package_parent_dir, "UnregisteredDep"), test_package; force = true)
+            Utils.isolate(loaded_depot = true) do
+                Pkg.activate(test_package)
+                # Note: we can't actually instantiate this package since it has unregistered deps
+                # But we can test that force_latest_compatible_version handles it gracefully
+
+                # Test that it throws and shows warning for unregistered dependency
+                @test_logs (:warn, r"Skipping force_latest_compatible_version for unregistered dependency") begin
+                    @test_throws Pkg.Types.PkgError "expected package `SomeUnregisteredPackage [deadbeef]` to be registered" Pkg.test(; force_latest_compatible_version = true)
+                end
+            end
+        end
+    end
 end
 
 end # module

test/test_packages/force_latest_compatible_version/UnregisteredDep/Project.toml

@@ -0,0 +1,16 @@
+name = "UnregisteredDep"
+uuid = "abc047b9-6ad4-4533-a374-d50ae908e3e9"
+authors = ["Test <[email protected]>"]
+version = "0.1.0"
+
+[deps]
+SomeUnregisteredPackage = "deadbeef-dead-beef-dead-beefdeadbeef"
+
+[compat]
+SomeUnregisteredPackage = "^1.0"
+
+[extras]
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+
+[targets]
+test = ["Test"]

test/test_packages/force_latest_compatible_version/UnregisteredDep/src/UnregisteredDep.jl

@@ -0,0 +1,5 @@
+module UnregisteredDep
+
+# This is a dummy module for testing
+
+end

test/test_packages/force_latest_compatible_version/UnregisteredDep/test/runtests.jl

@@ -0,0 +1,5 @@
+using Test
+
+@testset "UnregisteredDep tests" begin
+    @test true  # Just a dummy test
+end