FetchContent: Ignore EXACT for redirected find_package() calls

When FetchContent_MakeAvailable() populates a dependency for which
find_package() integration is enabled, all future calls to find_package()
MUST succeed using the contents of the redirection directory. The
generated config version file was not handling calls where the EXACT
keyword was given, resulting in such calls rejecting the redirection
directory's contents and continuing its search. It is not allowed to do
that. Fix the generated file to also set PACKAGE_VERSION_EXACT to
true so that calls with EXACT now accept it, as was originally intended.

Fixes: #23950

Modules/FetchContent.cmake

@@ -303,13 +303,16 @@ Commands
       ``<lowercaseName>-extra.cmake`` or ``<name>Extra.cmake`` file with the
       ``OPTIONAL`` flag (so the files can be missing and won't generate a
       warning).  Similarly, if no config version file exists, a very simple
-      one will be written which sets ``PACKAGE_VERSION_COMPATIBLE`` to true.
+      one will be written which sets ``PACKAGE_VERSION_COMPATIBLE`` and
+      ``PACKAGE_VERSION_EXACT`` to true.  This ensures all future calls to
+      :command:`find_package()` for the dependency will use the redirected
+      config file, regardless of any version requirements.
       CMake cannot automatically determine an arbitrary dependency's version,
-      so it cannot set ``PACKAGE_VERSION`` or ``PACKAGE_VERSION_EXACT``.
+      so it cannot set ``PACKAGE_VERSION``.
       When a dependency is pulled in via :command:`add_subdirectory` in the
       next step, it may choose to overwrite the generated config version file
       in :variable:`CMAKE_FIND_PACKAGE_REDIRECTS_DIR` with one that also sets
-      ``PACKAGE_VERSION``, and if appropriate, ``PACKAGE_VERSION_EXACT``.
+      ``PACKAGE_VERSION``.
       The dependency may also write a ``<lowercaseName>-extra.cmake`` or
       ``<name>Extra.cmake`` file to perform custom processing or define any
       variables that their normal (installed) package config file would

Modules/FetchContent/package-config-version.cmake.in

@@ -1,5 +1,8 @@
 # Automatically generated by CMake's FetchContent module.
 # Do not edit this file, it will be regenerated every time CMake runs.
 
-# Version not available, assuming it is compatible
+# Version not available, assuming it is compatible. We must also say it is an
+# exact match to ensure find_package() calls with the EXACT keyword still get
+# redirected.
 set(PACKAGE_VERSION_COMPATIBLE TRUE)
+set(PACKAGE_VERSION_EXACT TRUE)

Tests/RunCMake/FetchContent_find_package/PreferFetchContent.cmake

@@ -18,3 +18,7 @@ message(STATUS "Lowercase extra file was read")
 
 # This is expected to be re-routed to a FetchContent_MakeAvailable() call
 find_package(AddedProject REQUIRED)
+
+# Verify that find_package() version constraints are fully ignored by the
+# default-generated config version file
+find_package(AddedProject 1.2.3 EXACT REQUIRED)