lib/types: fix toCoerced's typeMerge

lib/tests/modules.sh

@@ -294,6 +294,9 @@ 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
 
+# 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

lib/tests/modules/declare-coerced-value-no-default.nix

@@ -0,0 +1,9 @@
+{ lib, ... }:
+
+{
+  options = {
+    value = lib.mkOption {
+      type = lib.types.coercedTo lib.types.int builtins.toString lib.types.str;
+    };
+  };
+}

lib/types.nix

@@ -1035,7 +1035,7 @@ rec {
         getSubOptions = finalType.getSubOptions;
         getSubModules = finalType.getSubModules;
         substSubModules = m: coercedTo coercedType coerceFunc (finalType.substSubModules m);
-        typeMerge = t1: t2: null;
+        typeMerge = t: null;
         functor = (defaultFunctor name) // { wrapped = finalType; };
         nestedTypes.coercedType = coercedType;
         nestedTypes.finalType = finalType;