feat(macos): built-in 14.0 deployment floor + static libc++ by default

A fresh user's `mcpp new hello && mcpp run` on macOS 14 died at dyld
(`__ZNSt3__119__is_posix_terminalEP7__sFILE` missing from the system
libc++): with no declared floor the binary compiled against libc++-20
headers but dynamically linked the host system libc++. Caught by the
macos-14 fresh-install CI lane.

The resolver (platform::macos::deployment_target) now falls back to a
built-in 14.0 floor (rustc-style baseline; the official LLVM static
archive floor), so the declared-floor-implies-static semantic becomes
portable-by-default: every macOS build gets minos=14.0 + static LLVM
libc++ out of the box. Opt-out: [build] static_stdlib = false.

Both blockers that deferred this default are resolved: split-brain
SIGSEGV fixed by -Wl,-load_hidden (#117 forensics, shipped in 0.0.50),
fingerprint/stdmod drift fixed by the single resolver (#119) — which
is why the default lives in the resolver, not at any single consumer.

Author: Sunrisepeak

docs/05-mcpp-toml.md

@@ -98,15 +98,17 @@ macos_deployment_target = "14.0"   # macOS 产物的最低支持系统版本(仅
 (`LC_BUILD_VERSION minos`),即二进制能运行的最老 macOS。优先级与各生态
 惯例一致:环境变量 `MACOSX_DEPLOYMENT_TARGET`(单次调用的显式覆盖,
 cargo/rustc、cc 等同样尊重该变量)> 本字段(项目默认,类似 SwiftPM 的
-`platforms:`)> 工具链/SDK 默认。该值会进入 BMI 指纹——切换 target 会
-自动重建模块缓存。
-
-**声明 floor 即静态运行时**:显式设置了 deployment target(env 或本
-字段)且 `static_stdlib = true`(默认)时,macOS 链接会静态链入 LLVM
-自带的 libc++/libc++abi —— 系统 libc++ 会把实际可运行版本钉死在构建机
-的 OS(老系统缺新符号,如 `std::print` 的支撑符号),静态化才能真正
-兑现声明的 floor。注意 LLVM 官方静态库自身的下限是 **14.0**。未声明
-floor 时保持动态系统 libc++(产物只保证在构建机同版本及以上运行)。
+`platforms:`)> **内建默认 `14.0`**(rustc 风格——每个 target 都有基线,
+14.0 即 LLVM 官方静态库自身的下限)。该值会进入 BMI 指纹——切换 target
+会自动重建模块缓存。
+
+**默认即静态运行时(portable by default)**:`static_stdlib = true`
+(默认)时,macOS 链接会静态链入 LLVM 自带的 libc++/libc++abi ——
+系统 libc++ 会把实际可运行版本钉死在构建机的 OS(老系统缺新符号,
+如 `std::print` 的支撑符号),静态化才能真正兑现 floor。因此默认构建的
+产物在任何 macOS ≥ 14 上开箱即用。设 `static_stdlib = false` 退回动态
+系统 libc++(产物只保证在构建机同版本及以上运行)。更低 floor(11–13)
+需自建 libc++ 归档(已验证可行,数据级切换,按需提供)。
 
 C++ 标准不要通过 `build.cxxflags = ["-std=..."]` 配置。请使用:
 

src/build/flags.cppm

@@ -369,23 +369,22 @@ CompileFlags compute_flags(const BuildPlan& plan) {
         //    lld ships with the exact toolchain doing the compile.
         f.ldStdlibDefault = " -lc++";
         f.ldStdlibTest    = " -lc++";
-        // Static libc++ is tied to an EXPLICIT deployment floor: when the
-        // user (or the release pipeline) declares a minimum macOS via the
-        // env var or [build] macos_deployment_target, the static LLVM
-        // libc++ is what makes that floor real (the system libc++ caps it
-        // at the build host's OS). With no declared floor, keep the
-        // 0.0.49 behavior — dynamic system libc++, host-coupled.
-        //
-        // TODO(macos-static-default): flip static to the unconditional
-        // default (rust-style "portable by default") once two tracked
-        // issues are fixed — (1) mixed C/C++ static binaries SIGSEGV at
-        // runtime (e2e 36_llvm_toolchain: answer.c + std::cout main.cpp,
-        // exit 139; root cause not yet isolated), (2) the std-module
-        // staging/fingerprint boundary (see canonical_compile_flags).
+        // Static libc++ + the deployment floor are the DEFAULT (rust-style
+        // "portable by default"): the resolver always yields a floor on
+        // macOS (built-in 14.0 unless env/manifest override), and the
+        // static LLVM libc++ is what makes that floor real — the system
+        // libc++ caps binaries at the build host's OS (a fresh user's
+        // std::println hello on macOS 14 died at dyld against the system
+        // libc++ before this). Opt-out: [build] static_stdlib = false
+        // (host-coupled dynamic libc++, the pre-0.0.52 no-declaration
+        // behavior). The two blockers that deferred this default are
+        // resolved: (1) mixed C/C++ split-brain SIGSEGV — fixed by
+        // -load_hidden (PR #117 forensics), (2) std-module staging /
+        // fingerprint drift — fixed by the single resolver (PR #119).
         // TODO(macos-floor-11): the official LLVM archives are built for
         // macOS 14; supporting 11-13 needs a custom libc++ build shipped
         // via xlings-res (data-only change — swap the archive source).
-        // Both tracked in xlings
+        // Tracked in xlings
         // .agents/docs/2026-06-05-macos-min-version-support.md §5.
         if (f.staticStdlib && !macosDeploymentTarget.empty()
             && !llvmRootForStdlib.empty()) {

src/cli.cppm

@@ -594,13 +594,11 @@ std::string canonical_compile_flags(const mcpp::manifest::Manifest& m) {
     // into the fingerprint so switching targets rebuilds the BMI cache
     // instead of dying with a module config mismatch.
     //
-    // TODO(macos-default-floor): a built-in default floor (rustc-style,
-    // see the 0.0.50 revert) cannot land until the std-module prebuild /
-    // staging pipeline consumes the SAME resolved value as this rule and
-    // flags.cppm — injecting a default here alone left the test build's
+    // The built-in default floor (rustc-style) lives in the single
+    // resolver (platform::macos::deployment_target), so this rule, the
+    // flags and the std-module prebuild always agree — the 0.0.50-era
+    // attempt to inject a default here alone left the test build's
     // std.pcm unstaged (import std failed wholesale on macos CI).
-    // Centralize the resolution in one helper, then re-land.
-    // See xlings .agents/docs/2026-06-05-macos-min-version-support.md §5.
     if constexpr (mcpp::platform::is_macos) {
         auto dtv = mcpp::platform::macos::deployment_target(
             m.buildConfig.macosDeploymentTarget);

src/platform/macos.cppm

@@ -32,22 +32,34 @@ bool has_xcode_clt();
 // Returns the SDK path if found, or nullopt.
 std::optional<std::filesystem::path> sdk_path();
 
+// Built-in default deployment floor (rustc-style: every target has a
+// baseline). 14.0 = the floor of the official LLVM static libc++
+// archives; with the default-static stdlib this makes `mcpp run`
+// binaries portable to any macOS ≥ 14 out of the box (no declaration
+// needed — a fresh user's std::println hello on macOS 14 used to die
+// at dyld against the system libc++). Lower floors need a custom
+// libc++ build (tracked; data-only swap via xlings-res).
+inline constexpr std::string_view default_deployment_target = "14.0";
+
 // Resolve the effective macOS deployment target: the
 // MACOSX_DEPLOYMENT_TARGET env var (explicit per-invocation override,
 // the convention cargo/rustc/cc honor) wins over `manifestValue` (the
-// [build] macos_deployment_target project default); empty means
-// toolchain/SDK default. THE single source of truth — flags.cppm, the
-// BMI fingerprint rule and the std-module prebuild must all consume
-// this same resolution, or cached std.pcm modules drift from the TUs
-// (config-mismatch / unstaged-module failures observed on macos CI).
+// [build] macos_deployment_target project default), which wins over
+// the built-in default floor — the result is never empty on macOS.
+// THE single source of truth — flags.cppm, the BMI fingerprint rule
+// and the std-module prebuild must all consume this same resolution,
+// or cached std.pcm modules drift from the TUs (config-mismatch /
+// unstaged-module failures observed on macos CI).
 std::string deployment_target(std::string_view manifestValue);
 
 // Return macOS-specific runtime library directories for LLVM toolchains.
 std::string deployment_target(std::string_view manifestValue) {
 #if defined(__APPLE__)
     if (const char* dt = std::getenv("MACOSX_DEPLOYMENT_TARGET"); dt && *dt)
         return dt;
-    return std::string(manifestValue);
+    if (!manifestValue.empty())
+        return std::string(manifestValue);
+    return std::string(default_deployment_target);
 #else
     (void)manifestValue;
     return {};