Set BUILD_SHARED_LIBS and CMAKE_POSITION_INDEPENDENT_CODE as Cache va…

…riables in CMakeToolchain

conan/tools/cmake/toolchain/blocks.py

@@ -153,7 +153,7 @@ class FPicBlock(Block):
     template = textwrap.dedent("""
         {% if fpic %}
         message(STATUS "Conan toolchain: Setting CMAKE_POSITION_INDEPENDENT_CODE={{ fpic }} (options.fPIC)")
-        set(CMAKE_POSITION_INDEPENDENT_CODE {{ fpic }})
+        set(CMAKE_POSITION_INDEPENDENT_CODE {{ fpic }} CACHE BOOL "Position independent code") 
         {% endif %}
         """)
 
@@ -239,7 +239,7 @@ def context(self):
 class SharedLibBock(Block):
     template = textwrap.dedent("""
         message(STATUS "Conan toolchain: Setting BUILD_SHARED_LIBS = {{ shared_libs }}")
-        set(BUILD_SHARED_LIBS {{ shared_libs }})
+        set(BUILD_SHARED_LIBS {{ shared_libs }} CACHE BOOL "Build shared libraries")
         """)
 
     def context(self):

conans/test/functional/toolchains/cmake/test_cmake_toolchain.py

@@ -1061,3 +1061,66 @@ def package(self):
     client.save({"conanfile.py": conanfile, "CMakeLists.txt": cmake, "my_license": "MIT"})
     client.run("create .", assert_error=True)
     assert "Cannot install stuff" in client.out
+
+
+def test_cmake_tooclahin_vars_when_option_declared():
+    t = TestClient()
+
+    cmakelists = textwrap.dedent("""
+    cmake_minimum_required(VERSION 2.8) # <---- set this to an old version for old policies
+    project(mylib CXX)
+
+    # Set the options AFTER the call to project, that is, after toolchain is loaded
+    option(BUILD_SHARED_LIBDS "" ON)
+    option(CMAKE_POSITION_INDEPENDENT_CODE "" OFF)
+
+    add_library(mylib src/mylib.cpp)
+    target_include_directories(mylib PUBLIC include)
+
+    get_target_property(MYLIB_TARGET_TYPE mylib TYPE)
+    get_target_property(MYLIB_PIC mylib POSITION_INDEPENDENT_CODE)
+    message("mylib target type: ${MYLIB_TARGET_TYPE}")
+    message("MYLIB_PIC value: ${MYLIB_PIC}")
+    if(MYLIB_PIC)
+        #Note: the value is "True"/"False" if set by cmake,
+        #       and "ON"/"OFF" if set by Conan
+        message("mylib position independent code: ON")
+    else()
+        message("mylib position independent code: OFF")
+    endif()
+
+    set_target_properties(mylib PROPERTIES PUBLIC_HEADER "include/mylib.h")
+    install(TARGETS mylib)
+    """)
+
+    t.run("new mylib/1.0 --template cmake_lib")
+    t.save({"CMakeLists.txt": cmakelists})
+
+    # The generated toolchain should set this `BUILD_SHARED_LIBS` to `OFF`
+    # and othe call to `option()` should respect the pre-existing value
+    t.run("create . -o mylib:shared=False -o mylib:fPIC=True --test-folder=None")
+    assert "mylib target type: STATIC_LIBRARY" in t.out
+    assert "mylib position independent code: ON" in t.out
+
+    t.run("create . -o mylib:shared=True -o mylib:fPIC=False --test-folder=None")
+    assert "mylib target type: SHARED_LIBRARY" in t.out
+    # Note: cmake will set the target property as to `ON` because being a shared library
+    # takes precedence (won't produce a shared library with fPIC=false)
+    assert "mylib position independent code: ON" in t.out
+
+    # When building manually, ensure the value passed by the toolchain overrides the ones in 
+    # the CMakeLists
+    t.run("install . -o mylib:shared=False -o mylib:fPIC=False")
+    t.run_command("cmake -S . -B build/ -DCMAKE_TOOLCHAIN_FILE=build/generators/conan_toolchain.cmake")
+    assert "mylib target type: STATIC_LIBRARY" in t.out
+    assert "mylib position independent code: OFF" in t.out
+
+    # When explicitly overriding `BUILD_SHARED_LIBS` via command line, ensure
+    # this takes precedence to the value defined by the toolchain
+    t.run_command("cmake -S . -B build/ -DCMAKE_TOOLCHAIN_FILE=build/generators/conan_toolchain.cmake -DCMAKE_POSITION_INDEPENDENT_CODE=ON")
+    assert "mylib target type: STATIC_LIBRARY" in t.out
+    assert "mylib position independent code: ON" in t.out
+
+    t.run_command("cmake -S . -B build/ -DCMAKE_TOOLCHAIN_FILE=build/generators/conan_toolchain.cmake -DBUILD_SHARED_LIBS=ON")
+    assert "mylib target type: SHARED_LIBRARY" in t.out
+    assert "mylib position independent code: ON" in t.out