fix(dist): 图供给 C++ 运行时时,契约是 SelfContained,由对象兑现而不是由点名一个库

    ld64.lld: error: library not found for -lc++

⚠️ 我的第一版修法把整块跳过了,那说错了话,而且丢东西:那一块还负责产物格式、
MinGW 判定、macOS 下限这些下游要用的事实,跳过它们一起没了。

⭐⭐ 而这一块里本来就有正确形状的先例 —— mi.freestanding:

    // 裸机短路整张表:下面找到的归档是**宿主的**
    if (in.freestanding) { m.effective = SelfContained; m.unitFlags = " -nostdlib++"; }

图供给运行时是同一件事的 hosted 形态。表里三个答案全都在「点名一个要链的运行时」
(系统的 / 工具链的 / 其中之一的静态形式),三个在「运行时是产物必须被接上的东西」
时都对,在「它已经在里面」时都错;而它会找到的归档是宿主的。

⇒ 加 mi.graphCxxRuntime,与 freestanding 走同一条短路。

⭐ 并且它比「跳过」更强:-nostdlib++ 是**主动**告诉驱动别加它自己那套,而不是
沉默。答案不是「在这里改挑 openkal 的那个」—— openkal 的那个**就是那些对象**,
没有库可点名,诚实的 flag 是阻止驱动自作主张的那一个。

实测:unit_ldflags = -nostdlib++;四个目标全部产出,三个能在本机跑的输出逐字相同。

Author: speak-agent

src/build/distribution.cppm

@@ -256,6 +256,27 @@ struct MechanismInput {
     // (measured 2026-08-19). A target-side C++ runtime, if one is wanted, is
     // an ordinary package — the same way the libc is.
     bool             freestanding = false;
+    // ⭐⭐ THE HOSTED FORM OF THE LINE ABOVE: a package in the graph supplies
+    // the C++ runtime, built for this target, and its objects are already on
+    // the link line.
+    //
+    // The table below has three answers and all of them name a runtime to LINK
+    // — the system's, the toolchain's, or a static form of one. Each is right
+    // when the runtime is something the artifact has to be JOINED to, and each
+    // is wrong here, where it is already inside. The archives it would find are
+    // the host's, which is the same defect the `freestanding` flag above
+    // exists for; the difference is only that this target has an OS.
+    //
+    // ⚠️ Measured 2026-08-23, cross-building for `aarch64-macos` over openkal
+    // right after the format decision was corrected to key on the target — the
+    // wrong format had been masking this:
+    //
+    //     ld64.lld: error: library not found for -lc++
+    //
+    // ⇒ Not "pick openkal's here". openkal's IS the objects; there is no
+    // library to name, and the honest flag is the one that stops the driver
+    // from adding its own.
+    bool             graphCxxRuntime = false;
 };
 
 struct Mechanism {
@@ -351,7 +372,7 @@ Mechanism resolve(const MechanismInput& in) {
     // ELF here, and every ELF cell below reaches for the toolchain's HOST
     // archives. One of them silently produced a link line with
     // x86-64 libc++.a on a riscv64 link.
-    if (in.freestanding) {
+    if (in.freestanding || in.graphCxxRuntime) {
         m.effective = Contract::SelfContained;
         m.unitFlags = " -nostdlib++";
         return m;

src/build/flags.cppm

@@ -891,24 +891,6 @@ CompileFlags compute_flags(const BuildPlan& plan) {
     // nothing below re-decides it. The flags land in the PER-UNIT channel
     // (`unit_ldflags`) rather than the global one because two roles in the
     // same build may hold different contracts.
-    // ⭐⭐ AND NOT AT ALL WHEN THE C++ RUNTIME IS ALREADY IN THE OBJECTS.
-    //
-    // Every answer this block can give names a runtime to LINK — the system's
-    // (`-lc++`), the toolchain's (`-load_hidden …/libc++.a`), or a static form
-    // of one of them. All three are right when the runtime is something the
-    // artifact has to be joined to, and all three are wrong when a package in
-    // the graph already compiled one for this target and its objects are on
-    // the link line.
-    //
-    // ⚠️ Measured 2026-08-23, immediately after the format decision above was
-    // corrected to key on the target. `aarch64-macos` had been falling through
-    // to `Elf`, where this block contributes nothing — so a wrong answer to one
-    // question was masking a second wrong answer to another:
-    //
-    //     ld64.lld: error: library not found for -lc++
-    //
-    // ⇒ There is nothing to find, and nothing to look for.
-    if (!plan.toolchain.targetCxxRuntime)
     {
         namespace dist = mcpp::build::dist;
         auto const& bc = plan.manifest.buildConfig;
@@ -1065,6 +1047,11 @@ CompileFlags compute_flags(const BuildPlan& plan) {
         // "incompatible with elf64lriscv". See MechanismInput::freestanding.
         if (auto ft = mcpp::toolchain::triple::parse(plan.toolchain.targetTriple))
             mi.freestanding = ft->is_freestanding();
+        // ⭐⭐ AND THE HOSTED FORM OF THE SAME FACT. A package in the graph has
+        // compiled a C++ runtime FOR THIS TARGET and its objects are on the
+        // link line — so, exactly as on bare metal, every archive the table
+        // below would reach for is the HOST's.
+        mi.graphCxxRuntime = plan.toolchain.targetCxxRuntime;
 
         const bool wantsArchives =
             (base == dist::Contract::SelfContained