feat(features): propagate active-feature defines to consumers (interface defines); v0.0.72#184
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…ace defines); v0.0.72 A dependency's active-feature `defines` are interface requirements: a header-only library's feature switch only takes effect in the translation unit that includes its headers. The canonical case is Eigen's `use_blas` feature, whose `EIGEN_USE_BLAS` macro must be defined when the *consumer* compiles `a * b` — not only when Eigen's own anchor TU compiles. Previously feature defines landed only on the owning package's private build flags, so the switch was inert for header-only providers and the consumer silently used the library's built-in path. Feature defines now flow into every consumer's compile flags along Public/Interface dependency edges, mirroring `include_dirs`: they are recorded on `PackageRoot::publicUsage` (cflags/cxxflags) during feature activation, and the `computeUsageRequirements()` fixpoint propagates them into each consumer's `privateBuild`. The fixpoint is re-run after feature activation (the first pass precedes it). The automatic `MCPP_FEATURE_<NAME>` macro stays private to the owning package — it is a build signal, not a public contract. This completes the eigen[backend-openblas] closed loop: the produced binary now links and calls OpenBLAS `dgemm_` instead of Eigen's built-in GEMM. Tests: tests/e2e/83_feature_defines_propagate.sh (consumer observes a dependency's feature define). Regression: 80/81/82, transitive-dep 09/31/55/56, full unit suite.
… interface-define rationale The 83_feature_defines_propagate.sh runtime check assumed a POSIX binary name (`app`); on Windows the artifact is `app.exe`, so `find -name app` returned empty and `"$BIN"` ran as an empty command (exit 127). Match both names and assert the binary was found. The build-time assertion (the #error guard) already passed on Windows, confirming interface-define propagation is cross-platform; only the test's runtime lookup was non-portable. Doc: record that a feature's `defines` are interface requirements (propagated to consumers along Public/Interface edges) — the realization of the "package-owned namespaced define" rule for header-only providers, with the ODR-safety, visibility-bounding, and MCPP_FEATURE_* non-propagation rationale.
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Summary
A dependency's active-feature
definesare now interface requirements: they propagate to every consumer's own compile flags along Public/Interface dependency edges, mirroringinclude_dirs.Motivation
For a header-only library, a feature's preprocessor switch only takes effect in the translation unit that includes the headers. The canonical case is Eigen's
use_blasfeature →EIGEN_USE_BLAS, which must be defined when the consumer compilesa * b, not only when Eigen's own anchor TU compiles. Previously feature defines landed solely on the owning package's private build flags, so the switch was inert for header-only providers — the consumer silently fell back to the library's built-in code path.Change
definesare recorded onPackageRoot::publicUsage(cflags/cxxflags) during feature activation (prepare.cppm).computeUsageRequirements()is extended to propagatecflags/cxxflags(not onlyincludeDirs) into each consumer'sprivateBuildalong Public/Interface edges, and is re-run after feature activation (the first pass precedes it).MCPP_FEATURE_<NAME>macro stays private to the owning package — it is a build signal, not a public contract.Impact
Completes the
eigen[backend-openblas]ecosystem closed loop: the produced binary now links and calls OpenBLASdgemm_instead of Eigen's built-in GEMM.Tests
tests/e2e/83_feature_defines_propagate.sh— a consumer observes a dependency's feature define (compile-time#errorguard + runtime check).80/81/82, transitive-dep09/31/55/56, full unit/integration suite (all green locally).Version
0.0.71→0.0.72.