diff --git a/lib/tests/checkAndMergeCompat.nix b/lib/tests/checkAndMergeCompat.nix
new file mode 100644
index 000000000000..dc6f26fbf917
--- /dev/null
+++ b/lib/tests/checkAndMergeCompat.nix
@@ -0,0 +1,379 @@
+{
+  pkgs ? import ../.. { },
+  currLibPath ? ../.,
+  prevLibPath ? "${
+    pkgs.fetchFromGitHub {
+      owner = "nixos";
+      repo = "nixpkgs";
+      # Parent commit of [#391544](https://github.com/NixOS/nixpkgs/pull/391544)
+      # Which was before the type.merge.v2 introduction
+      rev = "bcf94dd3f07189b7475d823c8d67d08b58289905";
+      hash = "sha256-MuMiIY3MX5pFSOCvutmmRhV6RD0R3CG0Hmazkg8cMFI=";
+    }
+  }/lib",
+}:
+let
+  lib = import currLibPath;
+
+  lib_with_merge_v2 = lib;
+  lib_with_merge_v1 = import prevLibPath;
+
+  getMatrix =
+    {
+      getType ? null,
+      # If getType is set this is only used as test prefix
+      # And the type from getType is used
+      outerTypeName,
+      innerTypeName,
+      value,
+      testAttrs,
+    }:
+    let
+      evalModules.call_v1 = lib_with_merge_v1.evalModules;
+      evalModules.call_v2 = lib_with_merge_v2.evalModules;
+      outerTypes.outer_v1 = lib_with_merge_v1.types;
+      outerTypes.outer_v2 = lib_with_merge_v2.types;
+      innerTypes.inner_v1 = lib_with_merge_v1.types;
+      innerTypes.inner_v2 = lib_with_merge_v2.types;
+    in
+    lib.mapAttrs (
+      _: evalModules:
+      lib.mapAttrs (
+        _: outerTypes:
+        lib.mapAttrs (_: innerTypes: {
+          "test_${outerTypeName}_${innerTypeName}" = testAttrs // {
+            expr =
+              (evalModules {
+                modules = [
+                  (m: {
+                    options.foo = m.lib.mkOption {
+                      type =
+                        if getType != null then
+                          getType outerTypes innerTypes
+                        else
+                          outerTypes.${outerTypeName} innerTypes.${innerTypeName};
+                      default = value;
+                    };
+                  })
+                ];
+              }).config.foo;
+          };
+        }) innerTypes
+      ) outerTypes
+    ) evalModules;
+in
+{
+  # AttrsOf string
+  attrsOf_str_ok = getMatrix {
+    outerTypeName = "attrsOf";
+    innerTypeName = "str";
+    value = {
+      bar = "test";
+    };
+    testAttrs = {
+      expected = {
+        bar = "test";
+      };
+    };
+  };
+  attrsOf_str_err_inner = getMatrix {
+    outerTypeName = "attrsOf";
+    innerTypeName = "str";
+    value = {
+      bar = 1; # not a string
+    };
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = "A definition for option `foo.bar' is not of type `string'.*";
+      };
+    };
+  };
+  attrsOf_str_err_outer = getMatrix {
+    outerTypeName = "attrsOf";
+    innerTypeName = "str";
+    value = [ "foo" ]; # not an attrset
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = "A definition for option `foo' is not of type `attribute set of string'.*";
+      };
+    };
+  };
+
+  # listOf string
+  listOf_str_ok = getMatrix {
+    outerTypeName = "listOf";
+    innerTypeName = "str";
+    value = [
+      "foo"
+      "bar"
+    ];
+    testAttrs = {
+      expected = [
+        "foo"
+        "bar"
+      ];
+    };
+  };
+  listOf_str_err_inner = getMatrix {
+    outerTypeName = "listOf";
+    innerTypeName = "str";
+    value = [
+      "foo"
+      1
+    ]; # not a string
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = ''A definition for option `foo."\[definition 1-entry 2\]"' is not of type `string'.'';
+      };
+    };
+  };
+  listOf_str_err_outer = getMatrix {
+    outerTypeName = "listOf";
+    innerTypeName = "str";
+    value = {
+      foo = 42;
+    }; # not a list
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = "A definition for option `foo' is not of type `list of string'.*";
+      };
+    };
+  };
+
+  attrsOf_submodule_ok = getMatrix {
+    getType =
+      a: b:
+      a.attrsOf (
+        b.submodule (m: {
+          options.nested = m.lib.mkOption {
+            type = m.lib.types.str;
+          };
+        })
+      );
+    outerTypeName = "attrsOf";
+    innerTypeName = "submodule";
+    value = {
+      foo = {
+        nested = "test1";
+      };
+      bar = {
+        nested = "test2";
+      };
+    };
+    testAttrs = {
+      expected = {
+        foo = {
+          nested = "test1";
+        };
+        bar = {
+          nested = "test2";
+        };
+      };
+    };
+  };
+  attrsOf_submodule_err_inner = getMatrix {
+    outerTypeName = "attrsOf";
+    innerTypeName = "submodule";
+    getType =
+      a: b:
+      a.attrsOf (
+        b.submodule (m: {
+          options.nested = m.lib.mkOption {
+            type = m.lib.types.str;
+          };
+        })
+      );
+    value = {
+      foo = [ 1 ]; # not a submodule
+      bar = {
+        nested = "test2";
+      };
+    };
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = "A definition for option `foo.foo' is not of type `submodule'.*";
+      };
+    };
+  };
+  attrsOf_submodule_err_outer = getMatrix {
+    outerTypeName = "attrsOf";
+    innerTypeName = "submodule";
+    getType =
+      a: b:
+      a.attrsOf (
+        b.submodule (m: {
+          options.nested = m.lib.mkOption {
+            type = m.lib.types.str;
+          };
+        })
+      );
+    value = [ 123 ]; # not an attrsOf
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = ''A definition for option `foo' is not of type `attribute set of \(submodule\).*'';
+      };
+    };
+  };
+
+  # either
+  either_str_attrsOf_ok = getMatrix {
+    outerTypeName = "either";
+    innerTypeName = "str_or_attrsOf_str";
+
+    getType = a: b: a.either b.str (b.attrsOf a.str);
+    value = "string value";
+    testAttrs = {
+      expected = "string value";
+    };
+  };
+  either_str_attrsOf_err_1 = getMatrix {
+    outerTypeName = "either";
+    innerTypeName = "str_or_attrsOf_str";
+
+    getType = a: b: a.either b.str (b.attrsOf a.str);
+    value = 1;
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = "A definition for option `foo' is not of type `string or attribute set of string'.*";
+      };
+    };
+  };
+  either_str_attrsOf_err_2 = getMatrix {
+    outerTypeName = "either";
+    innerTypeName = "str_or_attrsOf_str";
+
+    getType = a: b: a.either b.str (b.attrsOf a.str);
+    value = {
+      bar = 1; # not a string
+    };
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = "A definition for option `foo.bar' is not of type `string'.*";
+      };
+    };
+  };
+
+  # Coereced to
+  coerce_attrsOf_str_to_listOf_str_run = getMatrix {
+    outerTypeName = "coercedTo";
+    innerTypeName = "attrsOf_str->listOf_str";
+    getType = a: b: a.coercedTo (b.attrsOf b.str) builtins.attrValues (b.listOf b.str);
+    value = {
+      bar = "test1"; # coerced to listOf string
+      foo = "test2"; # coerced to listOf string
+    };
+    testAttrs = {
+      expected = [
+        "test1"
+        "test2"
+      ];
+    };
+  };
+  coerce_attrsOf_str_to_listOf_str_final = getMatrix {
+    outerTypeName = "coercedTo";
+    innerTypeName = "attrsOf_str->listOf_str";
+    getType = a: b: a.coercedTo (b.attrsOf b.str) (abort "This shouldnt run") (b.listOf b.str);
+    value = [
+      "test1"
+      "test2"
+    ]; # already a listOf string
+    testAttrs = {
+      expected = [
+        "test1"
+        "test2"
+      ]; # Order should be kept
+    };
+  };
+  coerce_attrsOf_str_to_listOf_err_coercer_input = getMatrix {
+    outerTypeName = "coercedTo";
+    innerTypeName = "attrsOf_str->listOf_str";
+    getType = a: b: a.coercedTo (b.attrsOf b.str) builtins.attrValues (b.listOf b.str);
+    value = [
+      { }
+      { }
+    ]; # not coercible to listOf string, with the given coercer
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = ''A definition for option `foo."\[definition 1-entry 1\]"' is not of type `string'.*'';
+      };
+    };
+  };
+  coerce_attrsOf_str_to_listOf_err_coercer_ouput = getMatrix {
+    outerTypeName = "coercedTo";
+    innerTypeName = "attrsOf_str->listOf_str";
+    getType = a: b: a.coercedTo (b.attrsOf b.str) builtins.attrValues (b.listOf b.str);
+    value = {
+      foo = {
+        bar = 1;
+      }; # coercer produces wrong type -> [ { bar = 1; } ]
+    };
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = ''A definition for option `foo."\[definition 1-entry 1\]"' is not of type `string'.*'';
+      };
+    };
+  };
+  coerce_str_to_int_coercer_ouput = getMatrix {
+    outerTypeName = "coercedTo";
+    innerTypeName = "int->str";
+    getType = a: b: a.coercedTo b.int builtins.toString a.str;
+    value = [ ];
+    testAttrs = {
+      expectedError = {
+        type = "ThrownError";
+        msg = ''A definition for option `foo' is not of type `string or signed integer convertible to it.*'';
+      };
+    };
+  };
+
+  # Submodule
+  submodule_with_ok = getMatrix {
+    outerTypeName = "submoduleWith";
+    innerTypeName = "mixed_types";
+    getType =
+      a: b:
+      a.submodule (m: {
+        options.attrs = m.lib.mkOption {
+          type = b.attrsOf b.str;
+        };
+        options.list = m.lib.mkOption {
+          type = b.listOf b.str;
+        };
+        options.either = m.lib.mkOption {
+          type = b.either a.str a.int;
+        };
+      });
+    value = {
+      attrs = {
+        foo = "bar";
+      };
+      list = [
+        "foo"
+        "bar"
+      ];
+      either = 123; # int
+    };
+    testAttrs = {
+      expected = {
+        attrs = {
+          foo = "bar";
+        };
+        list = [
+          "foo"
+          "bar"
+        ];
+        either = 123;
+      };
+    };
+  };
+}
diff --git a/lib/tests/nix-unit.nix b/lib/tests/nix-unit.nix
new file mode 100644
index 000000000000..ff62aff4d96b
--- /dev/null
+++ b/lib/tests/nix-unit.nix
@@ -0,0 +1,25 @@
+{
+  pkgs ? import ../.. { },
+}:
+let
+  prevNixpkgs = pkgs.fetchFromGitHub {
+    owner = "nixos";
+    repo = "nixpkgs";
+    # Parent commit of [#391544](https://github.com/NixOS/nixpkgs/pull/391544)
+    # Which was before the type.merge.v2 introduction
+    rev = "bcf94dd3f07189b7475d823c8d67d08b58289905";
+    hash = "sha256-MuMiIY3MX5pFSOCvutmmRhV6RD0R3CG0Hmazkg8cMFI=";
+  };
+in
+(pkgs.runCommand "lib-cross-eval-merge-v2"
+  {
+    nativeBuildInputs = [ pkgs.nix-unit ];
+  }
+  ''
+    export HOME=$TMPDIR
+    nix-unit --eval-store "$HOME" ${./checkAndMergeCompat.nix} \
+      --arg currLibPath "${../.}" \
+      --arg prevLibPath "${prevNixpkgs}/lib"
+    mkdir $out
+  ''
+)
diff --git a/lib/tests/release.nix b/lib/tests/release.nix
index 3eb62912ffc4..5a515a55dcdc 100644
--- a/lib/tests/release.nix
+++ b/lib/tests/release.nix
@@ -29,6 +29,9 @@ in
 pkgsBB.symlinkJoin {
   name = "nixpkgs-lib-tests";
   paths = map testWithNix nixVersions ++ [
+    (import ./nix-unit.nix {
+      inherit pkgs;
+    })
     (import ./maintainers.nix {
       inherit pkgs;
       lib = import ../.;