fix(cmake): respect explicit PYBIND11_USE_CROSSCOMPILING under CMP0190

[henryiii]

Implemented with Claude Code Opus 4.8 (see linux kernel style trailer in commit messages)

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Description

Fixes #6043.

Cross-compiling to Emscripten/Pyodide (and any target that defines CMAKE_CROSSCOMPILING_EMULATOR) broke with CMake ≥ 4.1 and pybind11 ≥ 3.0.2, failing with a spurious Cannot run the interpreter / pointer-size mismatch.

PR #5829 added AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR to the condition that sets the internal _PYBIND11_CROSSCOMPILING flag. That check was meant only to gate whether pybind11 auto-defaults PYBIND11_USE_CROSSCOMPILING to ON, but it ended up overriding a value the project had set explicitly. Emscripten defines CMAKE_CROSSCOMPILING_EMULATOR (node) while still wanting cross-compiling mode, so the flag was forced OFF.

This moves the emulator check up so it only guards the auto-default (which also no longer fires if the project already set the variable), and restores the second condition to CMAKE_CROSSCOMPILING AND PYBIND11_USE_CROSSCOMPILING. This is the fix proposed by the original PR author (@mhsmith) in the issue thread.

Behavior preserved across all cases:

• Emscripten/Pyodide — sets PYBIND11_USE_CROSSCOMPILING=ON explicitly → respected → ON
• Android (CMake ≥ 4.1, CMP0190 NEW, no emulator, variable unset) → still auto-defaults to ON
• Explicit OFF → respected

Suggested changelog entry:

• Fixed cross-compilation to Emscripten/Pyodide with CMake ≥ 4.1 by no longer overriding an explicitly set PYBIND11_USE_CROSSCOMPILING when CMAKE_CROSSCOMPILING_EMULATOR is defined.

[henryiii]

GPT 5.5 review in Codex:

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The change fixes respecting explicit cross-compiling settings with an emulator, but it regresses the intended CMP0190 default in normal top-level/add_subdirectory builds because the option default makes the variable defined before this file is included.

Review comment:

• [P2] Restore CMP0190 auto-default for subdirectory builds — /Users/henryfs/git/pybind/pybind11/tools/pybind11Common.cmake:49-49
  When pybind11 is used as the top-level project or via add_subdirectory, CMakeLists.txt has already run option(PYBIND11_USE_CROSSCOMPILING ... OFF), so this NOT DEFINED check is false even when the user did not explicitly choose a value. With CMake 4.1+ / CMP0190 NEW cross-compiling without CMAKE_CROSSCOMPILING_EMULATOR, _PYBIND11_CROSSCOMPILING now stays OFF and find_package(Python ... Interpreter ... Development...) fails with the CMP0190 error instead of omitting the interpreter component.

[henryiii]

Claude Opus 4.8:

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Good catch — fixed in 692b031.

The NOT DEFINED PYBIND11_USE_CROSSCOMPILING guard worked for find_package() consumers but, as you noted, broke top-level / add_subdirectory builds: CMakeLists.txt runs option(PYBIND11_USE_CROSSCOMPILING ... OFF) before including pybind11Common.cmake, so the variable was already defined (OFF) and the CMP0190 auto-default never applied.

The fix records, right before the option() call, whether the project had already set the value:

if(NOT DEFINED PYBIND11_USE_CROSSCOMPILING)
  set(_PYBIND11_USE_CROSSCOMPILING_DEFAULTED ON)
endif()
option(PYBIND11_USE_CROSSCOMPILING "Respect CMAKE_CROSSCOMPILING" OFF)

and pybind11Common.cmake now applies the CMP0190 default when either the variable is undefined (the find_package case) or it was only defaulted by option():

if(CMAKE_VERSION VERSION_GREATER_EQUAL "4.1"
   AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR
   AND (NOT DEFINED PYBIND11_USE_CROSSCOMPILING OR _PYBIND11_USE_CROSSCOMPILING_DEFAULTED))

This preserves all cases: explicit user value (any flow) respected; CMP0190 auto-default restored for add_subdirectory/top-level and find_package; and the original emulator override fix (Emscripten/Pyodide sets the var explicitly) intact.