A selected workspace member declares as the root; the clang MSVC row states its compiler version

The workspace plan's virtual root declares only its members, and the rules
that grant the root's own declarations a privilege read its edges alone, so a
rooted workspace's own package and a member selected with -p lost the
position each held when planned as its own root (2026.9.28.3). The release
canary on mcpp-language-server, whose root package overrides openkal-linux by
path against a transitive version request, was refused with "Pick one".
PrepareState::declaredByRoot now answers for the root and each selected
member, and the kind and reference clashes, the git lock, the identity
write-back, edge linkage, direct target-side candidates and the index refresh
ask it. Two selected members that disagree about one dependency's checkout or
link form are refused, naming both (e2e 850).

mcpp#746: the version of the toolset's cl.exe, read from its
VS_FIXEDFILEINFO as clang reads it, is passed as -fms-compatibility-version
with the toolset words, so it is on every command and in every key (e2e 760,
unit tests).

e2e 851 states that a member two configurations use is compiled once in each.

Author: speak-agent

.agents/docs/2026-09-30-build-wall-time-progress-count-and-hang-plan.md

@@ -1018,6 +1018,42 @@ resolved as stated.
   held only `/usr/bin:/bin`. The test's `PATH` now holds System32, as every
   Windows `PATH` does, and as the other Windows tests with a restricted `PATH`
   do.
+- **The release canary on mcpp-language-server.** Its
+  commit of 07:44 overrides `openkal-linux` by `path` in its root package,
+  against the `version` request of `openkal-musl`, which the rule "the root's
+  declaration wins" settles. 2026.9.28.3 builds it; 2026.9.29.1 to 2026.9.30.2
+  refuse it ("Pick one"): the virtual root of the workspace plan declares
+  only its members, and the rule read the virtual root's edges alone. The
+  same reading was inventoried across the planner; a selected member now
+  declares as the root for the kind and reference clashes, the git lock, the
+  identity write-back, `linkage` on its dependency edges, the target-side
+  candidates it names directly, and the index refresh. The refusal to mangle
+  the root's sources stays with the root: a member reaches it only when two
+  members pin two versions of one package, which one plan builds by mangling
+  and which each member built on its own before. Two selected members that
+  disagree about one dependency's checkout (its kind or its reference) or its
+  link form are refused, naming both. An independent review of the change
+  found three defects before it was pushed, each fixed: the arguments of the
+  link-form refusal were out of order, two selected members pointing one
+  dependency at two directories were only warned about, and link-form
+  conflicts were compared by the short key, which two packages can share.
+  e2e 850 states each; every criterion fails on 2026.9.30.2 as released
+  before the fix.
+- **mcpp#746, e2e 760 on Windows.** A job on an older runner image restored
+  the sandbox cache that a job on a newer image had saved; clang refused the
+  std module compiled for `msvc19.51.36260` against a translation unit
+  compiled for `msvc19.51.36257`. The key of the clang MSVC row held the
+  toolset directory, and a directory name does not determine `cl.exe`'s
+  build. The version of the toolset's `cl.exe`, read from its
+  `VS_FIXEDFILEINFO` as clang reads it, is now passed as
+  `-fms-compatibility-version` with the toolset words, so it is on every
+  command and in every key.
+- **A shared member across two configurations (e2e 851).** A member that two
+  members of different configurations use is compiled once in each build
+  directory; in one configuration it is compiled once with the union of the
+  features its consumers ask for, and a consumer's own flags stay out of it.
+  The behaviour was correct; the criterion was absent (e2e 833 builds a
+  member of another standard that uses no shared member).
 
 ## 8. Appendix: readings
 

.agents/docs/2026-09-30-build-wall-time-verify.sh

@@ -158,6 +158,24 @@ if git clone -q --depth 1 https://github.com/openxlings/xlings.git xlings > s8-c
     else fail "8 xlings from its main branch" ../s8.log; fi
 else skip "8 xlings could not be cloned"; fi
 
+# ── 9. CHANGE: a rooted workspace's own path override wins ────────────────
+# The shape of the release canary on mcpp-language-server: the workspace's
+# own package declares `framework` by `path`, a library it uses asks for it by
+# `git`. 2026.9.30.1 refuses it ("Pick one").
+rm -rf "$W/s9"; mkdir -p "$W/s9/fw/src" "$W/s9/libg/src" "$W/s9/ws/src"; cd "$W/s9"
+git init -q fw && git -C fw config user.email t@l && git -C fw config user.name t
+printf '[package]\nname = "framework"\nversion = "0.1.0"\n\n[build]\nsources = ["src/*.c"]\n\n[targets.framework]\nkind = "lib"\n' > fw/mcpp.toml
+printf 'int framework_marker(void) { return 101; }\n' > fw/src/framework.c
+git -C fw add -A && git -C fw commit -qm A && rev=$(git -C fw rev-parse HEAD)
+printf 'int framework_marker(void) { return 199; }\n' > fw/src/framework.c
+printf '[package]\nname = "libg"\nversion = "0.1.0"\n\n[build]\nsources = ["src/*.c"]\n\n[targets.libg]\nkind = "lib"\n\n[dependencies.framework]\ngit = "%s"\nrev = "%s"\n' "$W/s9/fw" "$rev" > libg/mcpp.toml
+printf 'extern int framework_marker(void);\nint libg_marker(void) { return framework_marker(); }\n' > libg/src/libg.c
+printf '[package]\nname = "app"\nversion = "0.1.0"\n\n[dependencies]\nframework = { path = "%s" }\nlibg = { path = "%s" }\n\n[targets.app]\nkind = "bin"\nmain = "src/main.cpp"\n\n[workspace]\nmembers = ["."]\n' "$W/s9/fw" "$W/s9/libg" > ws/mcpp.toml
+printf '#include <cstdio>\nextern "C" int libg_marker(void);\nint main() { std::printf("%%d\\n", libg_marker()); }\n' > ws/src/main.cpp
+if (cd ws && "$MCPP" build > ../s9.log 2>&1) && [ "$("$(bin_of ws/target app)")" = 199 ]; then
+    pass "9 CHANGE: a rooted workspace's own path override wins over a library's git declaration"
+else fail "9 CHANGE: the rooted workspace's override" s9.log; fi
+
 echo
 echo "RESULT: $passes passed, $fails failed, $skips skipped (mcpp $VER)"
 [ "$fails" -eq 0 ]

CHANGELOG.md

@@ -54,6 +54,27 @@ of 10.5 s; a build with nothing to do is unchanged at 0.05 s.
   BMI after that compile, so a fresh build could compile the consumer first and
   fail with `failed to read compiled module`. The stage edge of such a BMI now
   waits for those BMIs (e2e 849).
+- **A selected workspace member declares as the root.** Since 2026.9.29.1 a
+  workspace is planned from a virtual root that declares only its members, and
+  the rules that grant the root's own declarations a privilege read that
+  root's edges alone. A rooted workspace's own package, and a member selected
+  with `-p`, again hold the position each held when planned as its own root:
+  its `path` or `git` override of a dependency that another package requests
+  by another kind wins (the release canary on mcpp-language-server, whose root
+  package overrides `openkal-linux` by `path`, was refused with "Pick one");
+  `linkage` on its dependency edges is honoured; the identity its declarations
+  adopt is written back for the lock names; and its registry dependencies are
+  considered for the index refresh. Two selected members that disagree about
+  one dependency's checkout (its kind or its reference) or its link form are
+  refused, naming both (e2e 850).
+- **The clang MSVC row states the compiler version (mcpp#746).** The version
+  of the toolset's `cl.exe`, read from its file version as clang reads it, is
+  passed as `-fms-compatibility-version` on every command and enters every
+  key. Without it the driver chose the version itself and wrote it into each
+  BMI, and a std module compiled under one runner image's `cl.exe` was served
+  to another image whose `cl.exe` differed under the same toolset directory
+  name (`std.pcm was compiled for ... msvc19.51.36260 ... msvc19.51.36257`).
+  A clang build for `*-windows-msvc` rebuilds once after the upgrade.
 
 ### Changed
 

docs/05-dependencies.md

@@ -174,6 +174,18 @@ disagrees with another dependency, with neither being the root, is never
 settled by guessing which one was declared first — that is exactly the
 "accident of queue order" this section replaces.
 
+**In a workspace (mcpp 2026.9.30.2+).** The root is the package a command
+builds: a rooted workspace's own package, and each member the command selects
+(every member under `--workspace`, `X` under `-p X`). Each holds the root's
+position for its own declarations, as it did when every member was planned as
+its own root, so a member's `path` override of a dependency that another
+package requests by `version` wins, with the warning above. Two selected
+members that declare one identity by two kinds, or by two references of one
+kind, are refused, naming both: one configuration builds one checkout of a
+package. The same holds for `linkage`
+on a selected member's dependency edges, which is honoured, and refused when
+two selected members ask for two forms of one package.
+
 ### The identity of a `path` or `git` dependency (mcpp 2026.9.14.2+)
 
 A `path` or `git` dependency is the package its manifest declares, whatever key

docs/07-workspace.md

@@ -435,7 +435,18 @@ member that several members use is compiled once.
   that member's commands. Editing them recompiles that member and what
   imports it; the build directory stays the same.
 - **Features.** `--features f` activates `f` in each selected member that
-  declares it, and is refused when no selected member declares it.
+  declares it, and is refused when no selected member declares it. A package
+  that several members of one configuration use is compiled once, with the
+  union of the features they ask for; a package that members of two
+  configurations use is compiled once in each.
+- **The root's declarations.** Each selected member declares as the root did
+  when it was planned alone: its `path` or `git` override of a dependency wins
+  over another package's declaration, `linkage` on its dependency edges is
+  honoured, and its registry dependencies are considered for the index
+  refresh (2026.9.30.2+). Two selected members that disagree about one
+  dependency's checkout (its kind or its reference) or its link form are
+  refused, naming both. See
+  [05 — When two declarations of one dependency disagree](05-dependencies.md#when-two-declarations-of-one-dependency-disagree).
 - **Hooks.** The `[hooks]` of every selected member run around the build, in
   member order.
 - **Resources.** A member's `[resources]` and `windows_code_page` are

docs/zh/05-dependencies.md

@@ -160,6 +160,14 @@ sdk = { version = "1.0", visibility = "private" }
 冲突、且都不是根的情形，绝不会靠猜哪个先被声明来解决 —— 那正是本节要替换掉
 的"队列顺序的意外"。
 
+**工作区中（mcpp 2026.9.30.2+）。** 根是命令所构建的包：带根包工作区自己的包，
+以及命令选中的每个成员（`--workspace` 下是全部成员，`-p X` 下是 `X`）。每个都
+以根的位置对待自己的声明，与每个成员各自作为根规划时一致；因此成员用 `path`
+覆盖另一个包以 `version` 请求的依赖时，成员的声明胜出，并给出上文的警告。两个
+被选中的成员以不同种类、或以同一种类的不同引用声明同一身份时被拒绝，并点名双方：
+一个配置只构建一个包的一份 checkout。被选中成员依赖边上的 `linkage` 同理：它会被采纳；两个被选中成员
+为同一个包要求两种形态时被拒绝。
+
 ### `path` 或 `git` 依赖的身份（mcpp 2026.9.14.2+）
 
 `path` 或 `git` 依赖就是它的 manifest 所声明的那个包，与指向它的键无关。来源

docs/zh/07-workspace.md

@@ -403,7 +403,12 @@ mcpp test --workspace --workspace-timeout 1800   # whole fan-out (default 0 = no
 - **编译参数。** 成员的 `cflags`、`cxxflags`、`ldflags` 与 defines 作用于该成员自己的命令。
   修改它们会重新编译该成员以及导入它的单元，构建目录保持不变。
 - **Feature。** `--features f` 在每个声明了 `f` 的被选成员中启用它；没有被选成员声明它时，
-  命令被拒绝。
+  命令被拒绝。同一配置中被多个成员使用的包只编译一次，feature 取它们请求的并集；被两个
+  配置的成员使用的包在每个配置中各编译一次。
+- **根的声明。** 每个被选成员都像单独规划时的根那样声明依赖：它对某个依赖的 `path` 或
+  `git` 覆盖胜过其他包的声明，它依赖边上的 `linkage` 被采纳，它的注册表依赖参与索引刷新
+  的判断（2026.9.30.2+）。两个被选成员对同一依赖的 checkout（种类或引用）或链接形态意见不一
+  时被拒绝，并点名双方。见 [05 —— 同一依赖两条声明冲突的处理](05-dependencies.md#同一依赖两条声明冲突的处理)。
 - **Hooks。** 每个被选成员的 `[hooks]` 按成员顺序在构建前后运行。
 - **资源。** 成员的 `[resources]` 与 `windows_code_page` 按该成员的目录与 include 目录编译，
   只嵌入该成员自己的程序与共享库（2026.9.29.2+）。

modules/toolchain-model/src/linkmodel.cppm

@@ -80,6 +80,7 @@ struct ToolchainLinkModel {
     std::filesystem::path msvcToolsDir;
     std::filesystem::path winSdkRoot;
     std::string           winSdkVersion;
+    std::string           msvcCompilerVersion;
 
     bool clangDriver   = false;  // clang: -isystem headers; gcc: -idirafter
                                  // gcc:   -idirafter (…#include_next), -B/-L only
@@ -117,6 +118,15 @@ struct ToolchainLinkModel {
     // and the SDK always travel together: given only the toolset, the driver
     // stops reading %INCLUDE% and takes the registry's newest SDK instead.
     // Read by the compile line, the link line and the cache key alike.
+    //
+    // THE COMPILER VERSION TRAVELS WITH THEM (mcpp#746). Without
+    // `-fms-compatibility-version` the driver reads it from a cl.exe it
+    // locates itself and writes it into every BMI's target
+    // (`x86_64-pc-windows-msvc19.51.36260`), and a BMI whose version differs
+    // from the importer's is refused. Two runner images held different cl.exe
+    // builds under one toolset directory name, so the directory, which was
+    // the key's only view of the toolset, served one image's std module to
+    // the other. Said here, the version is on every command and in every key.
     std::vector<std::string> msvc_driver_tokens(const PathEscape& esc) const {
         std::vector<std::string> out;
         if (msvcToolsDir.empty()) return out;
@@ -128,6 +138,8 @@ struct ToolchainLinkModel {
             out.push_back("-Xmicrosoft-windows-sdk-version");
             out.push_back(winSdkVersion);
         }
+        if (!msvcCompilerVersion.empty())
+            out.push_back("-fms-compatibility-version=" + msvcCompilerVersion);
         return out;
     }
 
@@ -405,6 +417,7 @@ ToolchainLinkModel resolve_link_model(const Toolchain& tc) {
             lm.msvcToolsDir  = tc.msvcToolsDir;
             lm.winSdkRoot    = tc.windowsSdkRoot;
             lm.winSdkVersion = tc.windowsSdkVersion;
+            lm.msvcCompilerVersion = tc.msvcCompilerVersion;
         }
         return lm;
     }

modules/toolchain-model/src/model.cppm

@@ -279,6 +279,10 @@ struct Toolchain {
     // the cl.exe row carries the same answer in its own path and envOverrides.
     std::filesystem::path               msvcToolsDir;       // <vs>/VC/Tools/MSVC/<v>
     std::string                         msvcToolsVersion;   // "14.44.35207"
+    // The version of that toolset's cl.exe ("19.44.35211"), from its file
+    // version: what clang would otherwise read from the same file itself, and
+    // what a toolset directory name does not determine (mcpp#746).
+    std::string                         msvcCompilerVersion;
     std::string                         msvcOrigin;         // "system" | "managed"
     std::string                         msvcProduct;        // for the one printed line
     std::filesystem::path               windowsSdkRoot;     // <kits>/10 or the payload