ALIASED_TARGET: Make property read-only

The ALIAS_GLOBAL property is read-only, so ALIASED_TARGET should be as
well. Even when set, there is no way to read the property value, so this
change gives an explicit error for users.

Help/prop_tgt/ALIASED_TARGET.rst

@@ -1,7 +1,10 @@
 ALIASED_TARGET
 --------------
 
-Name of target aliased by this target.
+Read-only property containing the name of target aliased by this target.
+
+.. versionchanged:: 4.5
+  CMake 4.4 and earlier did not enforce the read-only nature of this property.
 
 If this is an :ref:`Alias Target <Alias Targets>`, this property contains
 the name of the target aliased.

Help/release/dev/alias-transparency.rst

@@ -4,3 +4,5 @@ alias-transparency
 * The :command:`set_property` and :command:`set_target_properties` commands
   now accept :ref:`Alias Targets`. Properties are set on the target which
   the alias references.
+
+* The :prop_tgt:`ALIASED_TARGET` target property is now read-only.

Source/cmTarget.cxx

@@ -2116,6 +2116,7 @@ bool IsSettableProperty(cmMakefile* context, cmTarget* target,
     { "SYMBOLIC", { ROC::All } },
     { "TYPE", { ROC::All } },
     { "ALIAS_GLOBAL", { ROC::All, cmPolicies::CMP0160 } },
+    { "ALIASED_TARGET", { ROC::All } },
     { "BINARY_DIR", { ROC::All, cmPolicies::CMP0160 } },
     { "CXX_MODULE_SETS", { ROC::All, cmPolicies::CMP0160 } },
     { "IMPORTED", { ROC::All, cmPolicies::CMP0160 } },