fix Conan recipe: add system_context option and correct package_type/package_id

- Add system_context option (default False) to reflect that stdexec is
  header-only by default; opt in to build the compiled system_context library
- Set package_type dynamically in configure(): header-library or static-library
- package_id() clears settings only when system_context=False (header-only)
- Pass STDEXEC_BUILD_SYSTEM_CONTEXT to CMake based on the option
- Declare system_context as a Conan component so CMakeDeps generates the
  STDEXEC::system_context target for consumers
- test_package: conditionally link and test system_context when available

Author: solarispika

conanfile.py

@@ -14,6 +14,12 @@ class StdexecPackage(ConanFile):
   license = "Apache 2.0"
 
   settings = "os", "arch", "compiler", "build_type"
+  options = {
+    "system_context": [True, False],
+  }
+  default_options = {
+    "system_context": False,
+  }
   exports_sources = (
     "include/*",
     "src/*",
@@ -24,6 +30,12 @@ class StdexecPackage(ConanFile):
   )
   generators = "CMakeToolchain"
 
+  def configure(self):
+    if self.options.system_context:
+      self.package_type = "static-library"
+    else:
+      self.package_type = "header-library"
+
   def validate(self):
     check_min_cppstd(self, "20")
 
@@ -37,18 +49,21 @@ def layout(self):
 
   def build(self):
     tests = "OFF" if self.conf.get("tools.build:skip_test", default=False) else "ON"
+    system_context = "ON" if self.options.system_context else "OFF"
 
     cmake = CMake(self)
     cmake.configure(variables={
       "STDEXEC_BUILD_TESTS": tests,
       "STDEXEC_BUILD_EXAMPLES": tests,
+      "STDEXEC_BUILD_SYSTEM_CONTEXT": system_context,
     })
     cmake.build()
     cmake.test()
 
   def package_id(self):
-    # Clear settings because this package is header-only.
-    self.info.clear()
+    if not self.info.options.system_context:
+      # Clear settings because this package is header-only.
+      self.info.clear()
 
   def package(self):
     cmake = CMake(self)
@@ -58,4 +73,8 @@ def package_info(self):
     self.cpp_info.set_property("cmake_file_name", "P2300")
     self.cpp_info.set_property("cmake_target_name", "P2300::P2300")
     self.cpp_info.set_property("cmake_target_aliases", ["STDEXEC::stdexec"])
-    self.cpp_info.libs = ["system_context"]
+    if self.options.system_context:
+      self.cpp_info.components["system_context"].libs = ["system_context"]
+      self.cpp_info.components["system_context"].set_property(
+        "cmake_target_name", "STDEXEC::system_context"
+      )

test_package/CMakeLists.txt

@@ -5,5 +5,8 @@ enable_testing()
 find_package(P2300 REQUIRED)
 
 add_executable(test_stdexec test.cpp)
-target_link_libraries(test_stdexec STDEXEC::stdexec)
+target_link_libraries(test_stdexec STDEXEC::stdexec $<TARGET_NAME_IF_EXISTS:STDEXEC::system_context>)
+if(TARGET STDEXEC::system_context)
+  target_compile_definitions(test_stdexec PRIVATE STDEXEC_TEST_SYSTEM_CONTEXT)
+endif()
 add_test(test_stdexec test_stdexec)

test_package/test.cpp

@@ -1,11 +1,20 @@
+#ifdef STDEXEC_TEST_SYSTEM_CONTEXT
 #include <exec/system_context.hpp>
+#else
+#include <exec/static_thread_pool.hpp>
+#endif
 #include <stdexec/execution.hpp>
 
 #include <cstdlib>
 
 int main()
 {
+#ifdef STDEXEC_TEST_SYSTEM_CONTEXT
   auto x   = stdexec::starts_on(exec::get_parallel_scheduler(), stdexec::just(42));
+#else
+  exec::static_thread_pool pool{1};
+  auto x   = stdexec::starts_on(pool.get_scheduler(), stdexec::just(42));
+#endif
   auto [a] = stdexec::sync_wait(std::move(x)).value();
   return a == 42 ? EXIT_SUCCESS : EXIT_FAILURE;
 }