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Document that JSON_Diagnostics CMake option doesn't apply to pre-installed packages (#5270)

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Closes #3106. set(JSON_Diagnostics ON) before find_package() has no
effect on a package built and installed elsewhere (Homebrew, vcpkg, a
system package, etc.) -- the compile definition is baked into the
exported nlohmann_jsonTargets.cmake at install time and the generated
config script never re-reads that variable. Verified empirically
against the real Homebrew-installed 3.12.0 package: the exported
target carries a fixed $<$<BOOL:OFF>:JSON_DIAGNOSTICS=1>, and the
suggested set(JSON_Diagnostics ON) snippet produces no change in
exception output.

Documents the actual working fix (overriding the imported target's
INTERFACE_COMPILE_DEFINITIONS property after find_package()) and the
multi-target "JSON_DIAGNOSTICS redefined" pitfall reported earlier in
the issue thread.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Niels Lohmann
2026-07-11 23:58:18 +02:00
committed by GitHub
parent ca76c37650
commit 6a406ee141
2 changed files with 27 additions and 1 deletions
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docs/mkdocs/docs/api/macros/json_diagnostics.md
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@@ -38,7 +38,8 @@ When the macro is not defined, the library will define it to its default value.
Diagnostic messages can also be controlled with the CMake option
[`JSON_Diagnostics`](../../integration/cmake.md#json_diagnostics) (`OFF` by default)
which defines `JSON_DIAGNOSTICS` accordingly.
which defines `JSON_DIAGNOSTICS` accordingly. Note this only applies when building the
library from source — see the pre-installed-package caveat on that page.
## Examples
docs/mkdocs/docs/integration/cmake.md
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@@ -135,6 +135,31 @@ Enable CI build targets. The exact targets are used during the several CI steps
Enable [extended diagnostic messages](../home/exceptions.md#extended-diagnostic-messages) by defining macro [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md). This option is `OFF` by default.
!!! warning "Does not apply to a pre-installed package"
This option only takes effect when building nlohmann/json from source as part of your own
CMake project (e.g. via [`FetchContent`](#fetchcontent) or [`add_subdirectory`](#external)).
It has **no effect** on a package that was already built and installed elsewhere (Homebrew,
vcpkg, a system package, etc.) — the resulting compile definition is baked into the exported
`nlohmann_jsonTargets.cmake` at install time, and `set(JSON_Diagnostics ON)` before
`find_package()` does not change it (verified against the Homebrew-installed package: the
exported target still carries a fixed `$<$<BOOL:OFF>:JSON_DIAGNOSTICS=1>`, regardless of any
variable set in the consuming project).
To enable extended diagnostics for a pre-installed package, override the imported target's
property directly after `find_package()`:
```cmake
find_package(nlohmann_json REQUIRED)
set_target_properties(nlohmann_json::nlohmann_json PROPERTIES
INTERFACE_COMPILE_DEFINITIONS "JSON_DIAGNOSTICS=1")
```
This only works cleanly when your project is the sole consumer of that imported target. If
nlohmann_json is pulled in from more than one place in your dependency graph with different
`JSON_DIAGNOSTICS` values, you may see a `"JSON_DIAGNOSTICS" redefined` compiler error, since
conflicting `-D` flags can end up on the same compile command line.
### `JSON_Diagnostic_Positions`
Enable position diagnostics by defining macro [`JSON_DIAGNOSTIC_POSITIONS`](../api/macros/json_diagnostic_positions.md). This option is `OFF` by default.