diff --git a/lib/modules.nix b/lib/modules.nix
index cc3148b0eea7..3330579952de 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -1121,6 +1121,7 @@ let
       files = map (def: def.file) res.defsFinal;
       definitionsWithLocations = res.defsFinal;
       inherit (res) isDefined;
+      inherit (res.checkedAndMerged) valueMeta;
       # This allows options to be correctly displayed using `${options.path.to.it}`
       __toString = _: showOption loc;
     };
@@ -1164,7 +1165,14 @@ let
     # Type-check the remaining definitions, and merge them. Or throw if no definitions.
     mergedValue =
       if isDefined then
-        if all (def: type.check def.value) defsFinal then
+        if type.merge ? v2 then
+          # check and merge share the same closure
+          # .headError is either not-present, null, or a string describing the error
+          if checkedAndMerged.headError or null != null then
+            throw "A definition for option `${showOption loc}' is not of type `${type.description}'. TypeError: ${checkedAndMerged.headError.message}"
+          else
+            checkedAndMerged.value
+        else if all (def: type.check def.value) defsFinal then
           type.merge loc defsFinal
         else
           let
@@ -1177,6 +1185,43 @@ let
         throw
           "The option `${showOption loc}' was accessed but has no value defined. Try setting the option.";
 
+    checkedAndMerged =
+      (
+        # This function (which is immediately applied) checks that type.merge
+        # returns the proper attrset.
+        # Once use of the merge.v2 feature has propagated, consider removing this
+        # for an estimated one thousandth performance improvement (NixOS by nr.thunks).
+        {
+          headError,
+          value,
+          valueMeta,
+        }@args:
+        args
+      )
+        (
+          if type.merge ? v2 then
+            let
+              r = type.merge.v2 {
+                inherit loc;
+                defs = defsFinal;
+              };
+            in
+            r
+            // {
+              valueMeta = r.valueMeta // {
+                _internal = {
+                  inherit type;
+                };
+              };
+            }
+          else
+            {
+              headError = null;
+              value = mergedValue;
+              valueMeta = { };
+            }
+        );
+
     isDefined = defsFinal != [ ];
 
     optionalValue = if isDefined then { value = mergedValue; } else { };
@@ -1586,13 +1631,11 @@ let
         New option path as list of strings.
       */
       to,
-
       /**
         Release number of the first release that contains the rename, ignoring backports.
         Set it to the upcoming release, matching the nixpkgs/.version file.
       */
       sinceRelease,
-
     }:
     doRename {
       inherit from to;
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/modules.sh b/lib/tests/modules.sh
index 301808ae6651..fbbccd8172f6 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -345,14 +345,14 @@ checkConfigOutput '^true$' "$@" ./define-module-check.nix
 set --
 checkConfigOutput '^"42"$' config.value ./declare-coerced-value.nix
 checkConfigOutput '^"24"$' config.value ./declare-coerced-value.nix ./define-value-string.nix
-checkConfigError 'A definition for option .* is not.*string or signed integer convertible to it.*. Definition values:\n\s*- In .*: \[ \]' config.value ./declare-coerced-value.nix ./define-value-list.nix
+checkConfigError 'A definition for option .*. is not of type .*.\n\s*- In .*: \[ \]' config.value ./declare-coerced-value.nix ./define-value-list.nix
 
 # Check coerced option merging.
 checkConfigError 'The option .value. in .*/declare-coerced-value.nix. is already declared in .*/declare-coerced-value-no-default.nix.' config.value ./declare-coerced-value.nix ./declare-coerced-value-no-default.nix
 
 # Check coerced value with unsound coercion
 checkConfigOutput '^12$' config.value ./declare-coerced-value-unsound.nix
-checkConfigError 'A definition for option .* is not of type .*. Definition values:\n\s*- In .*: "1000"' config.value ./declare-coerced-value-unsound.nix ./define-value-string-bigint.nix
+checkConfigError 'A definition for option .* is not of type .*.\n\s*- In .*: "1000"' config.value ./declare-coerced-value-unsound.nix ./define-value-string-bigint.nix
 checkConfigError 'toInt: Could not convert .* to int' config.value ./declare-coerced-value-unsound.nix ./define-value-string-arbitrary.nix
 
 # Check `graph` attribute
@@ -761,6 +761,26 @@ checkConfigOutput '"bar"' config.sub.conditionalImportAsNixos.foo ./specialArgs-
 checkConfigError 'attribute .*bar.* not found' config.sub.conditionalImportAsNixos.bar ./specialArgs-class.nix
 checkConfigError 'attribute .*foo.* not found' config.sub.conditionalImportAsDarwin.foo ./specialArgs-class.nix
 checkConfigOutput '"foo"' config.sub.conditionalImportAsDarwin.bar ./specialArgs-class.nix
+# Check that some types expose the 'valueMeta'
+checkConfigOutput '\{\}' options.str.valueMeta ./types-valueMeta.nix
+checkConfigOutput '["foo", "bar"]' config.attrsOfResult ./types-valueMeta.nix
+checkConfigOutput '2' config.listOfResult ./types-valueMeta.nix
+
+# Check that composed types expose the 'valueMeta'
+# attrsOf submodule (also on merged options,types)
+checkConfigOutput '42' options.attrsOfModule.valueMeta.attrs.foo.configuration.options.bar.value ./composed-types-valueMeta.nix
+checkConfigOutput '42' options.mergedAttrsOfModule.valueMeta.attrs.foo.configuration.options.bar.value ./composed-types-valueMeta.nix
+
+# listOf submodule (also on merged options,types)
+checkConfigOutput '42' config.listResult ./composed-types-valueMeta.nix
+checkConfigOutput '42' config.mergedListResult ./composed-types-valueMeta.nix
+
+# Add check
+checkConfigOutput '^0$' config.v1CheckedPass ./add-check.nix
+checkConfigError 'A definition for option .* is not of type .signed integer.*' config.v1CheckedFail ./add-check.nix
+checkConfigOutput '^true$' config.v2checkedPass ./add-check.nix
+checkConfigError 'A definition for option .* is not of type .attribute set of signed integer.*' config.v2checkedFail ./add-check.nix
+
 
 cat <<EOF
 ====== module tests ======
diff --git a/lib/tests/modules/add-check.nix b/lib/tests/modules/add-check.nix
new file mode 100644
index 000000000000..3bd993694994
--- /dev/null
+++ b/lib/tests/modules/add-check.nix
@@ -0,0 +1,36 @@
+(
+  { lib, ... }:
+  let
+    inherit (lib) types mkOption;
+    inherit (types) addCheck int attrsOf;
+
+    # type with a v1 merge
+    v1Type = addCheck int (v: v == 0);
+
+    # type with a v2 merge
+    v2Type = addCheck (attrsOf int) (v: v ? foo);
+  in
+  {
+    options.v1CheckedPass = mkOption {
+      type = v1Type;
+      default = 0;
+    };
+    options.v1CheckedFail = mkOption {
+      type = v1Type;
+      default = 1;
+    };
+    options.v2checkedPass = mkOption {
+      type = v2Type;
+      default = {
+        foo = 1;
+      };
+      # plug the value to make test script regex simple
+      apply = v: v.foo == 1;
+    };
+    options.v2checkedFail = mkOption {
+      type = v2Type;
+      default = { };
+      apply = v: lib.deepSeq v v;
+    };
+  }
+)
diff --git a/lib/tests/modules/composed-types-valueMeta.nix b/lib/tests/modules/composed-types-valueMeta.nix
new file mode 100644
index 000000000000..734917f8b67d
--- /dev/null
+++ b/lib/tests/modules/composed-types-valueMeta.nix
@@ -0,0 +1,75 @@
+{ lib, ... }:
+let
+  inherit (lib) types mkOption;
+
+  attrsOfModule = mkOption {
+    type = types.attrsOf (
+      types.submodule {
+        options.bar = mkOption {
+          type = types.int;
+        };
+      }
+    );
+  };
+
+  listOfModule = mkOption {
+    type = types.listOf (
+      types.submodule {
+        options.bar = mkOption {
+          type = types.int;
+        };
+      }
+    );
+  };
+
+in
+{
+  imports = [
+    #  Module A
+    ({
+      options.attrsOfModule = attrsOfModule;
+      options.mergedAttrsOfModule = attrsOfModule;
+      options.listOfModule = listOfModule;
+      options.mergedListOfModule = listOfModule;
+    })
+    # Module B
+    ({
+      options.mergedAttrsOfModule = attrsOfModule;
+      options.mergedListOfModule = listOfModule;
+    })
+    # Values
+    # It is important that the value is defined in a separate module
+    # Without valueMeta the actual value and sub-options wouldn't be accessible via:
+    # options.attrsOfModule.type.getSubOptions
+    ({
+      attrsOfModule = {
+        foo.bar = 42;
+      };
+      mergedAttrsOfModule = {
+        foo.bar = 42;
+      };
+    })
+    (
+      { options, ... }:
+      {
+        config.listOfModule = [
+          {
+            bar = 42;
+          }
+        ];
+        config.mergedListOfModule = [
+          {
+            bar = 42;
+          }
+        ];
+        # Result options to expose the list module to bash as plain attribute path
+        options.listResult = mkOption {
+          default = (builtins.head options.listOfModule.valueMeta.list).configuration.options.bar.value;
+        };
+        options.mergedListResult = mkOption {
+          default = (builtins.head options.mergedListOfModule.valueMeta.list).configuration.options.bar.value;
+        };
+      }
+    )
+  ];
+}
diff --git a/lib/tests/modules/types-valueMeta.nix b/lib/tests/modules/types-valueMeta.nix
new file mode 100644
index 000000000000..3004898f112a
--- /dev/null
+++ b/lib/tests/modules/types-valueMeta.nix
@@ -0,0 +1,60 @@
+{ lib, ... }:
+let
+  inherit (lib) types mkOption;
+
+  inherit (types)
+    # attrsOf uses attrsWith internally
+    attrsOf
+    listOf
+    submoduleOf
+    str
+    ;
+in
+{
+  imports = [
+    (
+      { options, ... }:
+      {
+        # Should have an empty valueMeta
+        options.str = mkOption {
+          type = str;
+        };
+
+        # Should have some valueMeta which is an attribute set of the nested valueMeta
+        options.attrsOf = mkOption {
+          type = attrsOf str;
+          default = {
+            foo = "foo";
+            bar = "bar";
+          };
+        };
+        options.attrsOfResult = mkOption {
+          default = builtins.attrNames options.attrsOf.valueMeta.attrs;
+        };
+
+        # Should have some valueMeta which is the list of the nested valueMeta of types.str
+        # [ {} {} ]
+        options.listOf = mkOption {
+          type = listOf str;
+          default = [
+            "foo"
+            "bar"
+          ];
+        };
+        options.listOfResult = mkOption {
+          default = builtins.length options.listOf.valueMeta.list;
+        };
+
+        # Should have some valueMeta which is the submodule evaluation
+        # { _module, options, config, ...}
+        options.submoduleOf = mkOption {
+          type = submoduleOf {
+            options.str = mkOption {
+              type = str;
+            };
+          };
+        };
+      }
+    )
+  ];
+}
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 $out
+  ''
+)
diff --git a/lib/tests/release.nix b/lib/tests/release.nix
index 4209fd9274b6..54d62f87b3d5 100644
--- a/lib/tests/release.nix
+++ b/lib/tests/release.nix
@@ -29,6 +29,9 @@ in
 pkgsBB.symlinkJoin {
   name = "nixpkgs-lib-tests";
   paths = map testWithNix nixVersions ++ [
+    (import ./nix-unit.nix {
+      inherit pkgs;
+    })
     (import ./maintainers.nix {
       inherit pkgs;
       lib = import ../.;
diff --git a/lib/types.nix b/lib/types.nix
index dcb536c4723b..6b51f9254a00 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -20,7 +20,6 @@ let
     toList
     ;
   inherit (lib.lists)
-    all
     concatLists
     count
     elemAt
@@ -48,6 +47,7 @@ let
     mergeOneOption
     mergeUniqueOption
     showFiles
+    showDefs
     showOption
     ;
   inherit (lib.strings)
@@ -99,6 +99,13 @@ let
       }is accessed, use `${lib.optionalString (loc != null) "type."}nestedTypes.elemType` instead.
     '' payload.elemType;
 
+  checkDefsForError =
+    check: loc: defs:
+    let
+      invalidDefs = filter (def: !check def.value) defs;
+    in
+    if invalidDefs != [ ] then { message = "Definition values: ${showDefs invalidDefs}"; } else null;
+
   outer_types = rec {
     isType = type: x: (x._type or "") == type;
 
@@ -705,26 +712,36 @@ let
           }";
           descriptionClass = "composite";
           check = isList;
-          merge =
-            loc: defs:
-            map (x: x.value) (
-              filter (x: x ? value) (
-                concatLists (
-                  imap1 (
-                    n: def:
+          merge = {
+            __functor =
+              self: loc: defs:
+              (self.v2 { inherit loc defs; }).value;
+            v2 =
+              { loc, defs }:
+              let
+                evals = filter (x: x.optionalValue ? value) (
+                  concatLists (
                     imap1 (
-                      m: def':
-                      (mergeDefinitions (loc ++ [ "[definition ${toString n}-entry ${toString m}]" ]) elemType [
-                        {
-                          inherit (def) file;
-                          value = def';
-                        }
-                      ]).optionalValue
-                    ) def.value
-                  ) defs
-                )
-              )
-            );
+                      n: def:
+                      imap1 (
+                        m: def':
+                        (mergeDefinitions (loc ++ [ "[definition ${toString n}-entry ${toString m}]" ]) elemType [
+                          {
+                            inherit (def) file;
+                            value = def';
+                          }
+                        ])
+                      ) def.value
+                    ) defs
+                  )
+                );
+              in
+              {
+                headError = checkDefsForError check loc defs;
+                value = map (x: x.optionalValue.value or x.mergedValue) evals;
+                valueMeta.list = map (v: v.checkedAndMerged.valueMeta) evals;
+              };
+          };
           emptyValue = {
             value = [ ];
           };
@@ -801,42 +818,43 @@ let
           lazy ? false,
           placeholder ? "name",
         }:
-        mkOptionType {
+        mkOptionType rec {
           name = if lazy then "lazyAttrsOf" else "attrsOf";
           description =
             (if lazy then "lazy attribute set" else "attribute set")
             + " of ${optionDescriptionPhrase (class: class == "noun" || class == "composite") elemType}";
           descriptionClass = "composite";
           check = isAttrs;
-          merge =
-            if lazy then
-              (
-                # Lazy merge Function
-                loc: defs:
-                zipAttrsWith
-                  (
-                    name: defs:
-                    let
-                      merged = mergeDefinitions (loc ++ [ name ]) elemType defs;
-                      # mergedValue will trigger an appropriate error when accessed
-                    in
-                    merged.optionalValue.value or elemType.emptyValue.value or merged.mergedValue
-                  )
-                  # Push down position info.
-                  (pushPositions defs)
-              )
-            else
-              (
-                # Non-lazy merge Function
-                loc: defs:
-                mapAttrs (n: v: v.value) (
-                  filterAttrs (n: v: v ? value) (
-                    zipAttrsWith (name: defs: (mergeDefinitions (loc ++ [ name ]) elemType (defs)).optionalValue)
-                      # Push down position info.
-                      (pushPositions defs)
-                  )
-                )
-              );
+          merge = {
+            __functor =
+              self: loc: defs:
+              (self.v2 { inherit loc defs; }).value;
+            v2 =
+              { loc, defs }:
+              let
+                evals =
+                  if lazy then
+                    zipAttrsWith (name: defs: mergeDefinitions (loc ++ [ name ]) elemType defs) (pushPositions defs)
+                  else
+                    # Filtering makes the merge function more strict
+                    # Meaning it is less lazy
+                    filterAttrs (n: v: v.optionalValue ? value) (
+                      zipAttrsWith (name: defs: mergeDefinitions (loc ++ [ name ]) elemType defs) (pushPositions defs)
+                    );
+              in
+              {
+                headError = checkDefsForError check loc defs;
+                value = mapAttrs (
+                  n: v:
+                  if lazy then
+                    v.optionalValue.value or elemType.emptyValue.value or v.mergedValue
+                  else
+                    v.optionalValue.value
+                ) evals;
+                valueMeta.attrs = mapAttrs (n: v: v.checkedAndMerged.valueMeta) evals;
+              };
+          };
+
           emptyValue = {
             value = { };
           };
@@ -1218,6 +1236,7 @@ let
 
           name = "submodule";
 
+          check = x: isAttrs x || isFunction x || path.check x;
         in
         mkOptionType {
           inherit name;
@@ -1229,13 +1248,25 @@ let
                 docsEval = base.extendModules { modules = [ noCheckForDocsModule ]; };
               in
               docsEval._module.freeformType.description or name;
-          check = x: isAttrs x || isFunction x || path.check x;
-          merge =
-            loc: defs:
-            (base.extendModules {
-              modules = [ { _module.args.name = last loc; } ] ++ allModules defs;
-              prefix = loc;
-            }).config;
+          inherit check;
+          merge = {
+            __functor =
+              self: loc: defs:
+              (self.v2 { inherit loc defs; }).value;
+            v2 =
+              { loc, defs }:
+              let
+                configuration = base.extendModules {
+                  modules = [ { _module.args.name = last loc; } ] ++ allModules defs;
+                  prefix = loc;
+                };
+              in
+              {
+                headError = checkDefsForError check loc defs;
+                value = configuration.config;
+                valueMeta = { inherit configuration; };
+              };
+          };
           emptyValue = {
             value = { };
           };
@@ -1383,17 +1414,50 @@ let
               }";
           descriptionClass = "conjunction";
           check = x: t1.check x || t2.check x;
-          merge =
-            loc: defs:
-            let
-              defList = map (d: d.value) defs;
-            in
-            if all (x: t1.check x) defList then
-              t1.merge loc defs
-            else if all (x: t2.check x) defList then
-              t2.merge loc defs
-            else
-              mergeOneOption loc defs;
+          merge = {
+            __functor =
+              self: loc: defs:
+              (self.v2 { inherit loc defs; }).value;
+            v2 =
+              { loc, defs }:
+              let
+                t1CheckedAndMerged =
+                  if t1.merge ? v2 then
+                    t1.merge.v2 { inherit loc defs; }
+                  else
+                    {
+                      value = t1.merge loc defs;
+                      headError = checkDefsForError t1.check loc defs;
+                      valueMeta = { };
+                    };
+                t2CheckedAndMerged =
+                  if t2.merge ? v2 then
+                    t2.merge.v2 { inherit loc defs; }
+                  else
+                    {
+                      value = t2.merge loc defs;
+                      headError = checkDefsForError t2.check loc defs;
+                      valueMeta = { };
+                    };
+
+                checkedAndMerged =
+                  if t1CheckedAndMerged.headError == null then
+                    t1CheckedAndMerged
+                  else if t2CheckedAndMerged.headError == null then
+                    t2CheckedAndMerged
+                  else
+                    rec {
+                      valueMeta = {
+                        inherit headError;
+                      };
+                      headError = {
+                        message = "The option `${showOption loc}` is neither a value of type `${t1.description}` nor `${t2.description}`, Definition values: ${showDefs defs}";
+                      };
+                      value = abort "(t.merge.v2 defs).value must only be accessed when `.headError == null`. This is a bug in code that consumes a module system type.";
+                    };
+              in
+              checkedAndMerged;
+          };
           typeMerge =
             f':
             let
@@ -1434,12 +1498,47 @@ let
             optionDescriptionPhrase (class: class == "noun") coercedType
           } convertible to it";
           check = x: (coercedType.check x && finalType.check (coerceFunc x)) || finalType.check x;
-          merge =
-            loc: defs:
-            let
-              coerceVal = val: if coercedType.check val then coerceFunc val else val;
-            in
-            finalType.merge loc (map (def: def // { value = coerceVal def.value; }) defs);
+          merge = {
+            __functor =
+              self: loc: defs:
+              (self.v2 { inherit loc defs; }).value;
+            v2 =
+              { loc, defs }:
+              let
+                finalDefs = (
+                  map (
+                    def:
+                    def
+                    // {
+                      value =
+                        let
+                          merged = coercedType.merge.v2 {
+                            inherit loc;
+                            defs = [ def ];
+                          };
+                        in
+                        if coercedType.merge ? v2 then
+                          if merged.headError == null then coerceFunc def.value else def.value
+                        else if coercedType.check def.value then
+                          coerceFunc def.value
+                        else
+                          def.value;
+                    }
+                  ) defs
+                );
+              in
+              if finalType.merge ? v2 then
+                finalType.merge.v2 {
+                  inherit loc;
+                  defs = finalDefs;
+                }
+              else
+                {
+                  value = finalType.merge loc finalDefs;
+                  valueMeta = { };
+                  headError = checkDefsForError check loc defs;
+                };
+          };
           emptyValue = finalType.emptyValue;
           getSubOptions = finalType.getSubOptions;
           getSubModules = finalType.getSubModules;
@@ -1451,6 +1550,7 @@ let
           nestedTypes.coercedType = coercedType;
           nestedTypes.finalType = finalType;
         };
+
       /**
         Augment the given type with an additional type check function.
 
@@ -1459,8 +1559,33 @@ let
         Fixing is not trivial, we appreciate any help!
         :::
       */
-      addCheck = elemType: check: elemType // { check = x: elemType.check x && check x; };
-
+      addCheck =
+        elemType: check:
+        if elemType.merge ? v2 then
+          elemType
+          // {
+            check = x: elemType.check x && check x;
+            merge = {
+              __functor =
+                self: loc: defs:
+                (self.v2 { inherit loc defs; }).value;
+              v2 =
+                { loc, defs }:
+                let
+                  orig = elemType.merge.v2 { inherit loc defs; };
+                  headError' = if orig.headError != null then orig.headError else checkDefsForError check loc defs;
+                in
+                orig
+                // {
+                  headError = headError';
+                };
+            };
+          }
+        else
+          elemType
+          // {
+            check = x: elemType.check x && check x;
+          };
     };
 
     /**