fix(manifest): the emptiness predicate is a free function, as its sibling is

Windows CI reported `test_modgraph.cpp` failing to compile with

  optional:262: error: no matching constructor for initialization of
  '_SMF_control<_Optional_construct_base<basic_string<char,...>>, ...>'

against `types.cppm`'s `Profile`, a struct this change never touched. The chain
is `Manifest` -> `std::map<std::string, Profile>` -> `Profile`'s
`std::optional<std::string>`, and that member's own comment already records the
same breakage under clang with the MSVC standard library on an earlier occasion.

The cause was making the new emptiness predicate an inline MEMBER of two structs
this module exports: that changes what importers materialise from its BMI, and
what it materialised here was a copy constructor that does not compile. `append`,
the operation this one is the sibling of, has been a free function since it was
written. `is_empty` now matches it, and BuildInputs and ConditionalConfig have
exactly the member sets they had before.

Also records where the four new e2e tests run. All declare `# requires: gcc`,
which `run_all.sh` grants only on Linux, so the end-to-end leg is Linux-only and
the cross-dialect coverage -- including the MSVC `/ifcOutput` spelling that no
Linux runner can reach -- is in the unit tests, which run everywhere.

Author: speak-agent

.agents/docs/2026-09-09-two-answers-and-two-silences.md

@@ -430,7 +430,7 @@ Landed in mcpp 2026.9.9.1, one change per mechanism.
 | A.2 BOM | the same rule for every TOML document mcpp reads | `libs/toml.cppm` |
 | B | `module_lang` and `module_output` become PER-EDGE, bound from `providesModule` | `ninja_backend.cppm` |
 | B | `moduleImplLangFlag` — the spelling for a module-extension file that is not an interface | `toolchain-model/model.cppm` |
-| C | `BuildInputs::empty()` and `ConditionalConfig::empty()`, replacing two hand-written gates | `manifest/types.cppm`, `toml.cppm`, `xpkg.cppm` |
+| C | `is_empty(BuildInputs)` and `is_empty(ConditionalConfig)`, replacing two hand-written gates | `manifest/types.cppm`, `toml.cppm`, `xpkg.cppm` |
 | D | unknown-key sweeps for `[runtime]` and `[target.<pred>.runtime]` | `manifest/toml.cppm` |
 
 Reading B's row against §3: the recommendation there was A, and the emitter now
@@ -451,12 +451,25 @@ READ. This is not a new restriction: GCC 16.1 refuses the same declaration with
 `unrecognized 'MODULE-EXPORT ...'`, so the two versions of mcpp differ in which
 sentence the author gets, not in whether the file builds.
 
-`ConditionalConfig::empty()` composes rather than enumerating: it calls
-`BuildInputs::empty()` and `XlingsConfig::empty()`, the latter of which already
+`is_empty(ConditionalConfig)` composes rather than enumerating: it calls
+`is_empty(BuildInputs)` and `XlingsConfig::empty()`, the latter of which already
 existed. That is #258's medicine applied one level further out, and it closed a
 fourth instance found while writing it — `xpkg.cppm`'s gate omitted
 `privateIncludeDirs`, which its own loop fills.
 
+**They are free functions, and that was forced by a measurement rather than
+chosen.** The first version made them members, which is the obvious shape and
+the wrong one: adding an inline member to a struct this module exports changes
+what importers materialise from its BMI, and `Profile` carries a
+`std::optional<std::string>` that is already known to break under clang with the
+MSVC standard library — the note on that member records the earlier occasion.
+Windows CI failed on `test_modgraph.cpp` with
+`no matching constructor for _SMF_control<_Optional_construct_base<...>>`,
+reported against a struct this change never touched, reached through
+`Manifest` -> `std::map<std::string, Profile>` -> `Profile`. `append`, the
+sibling operation, has been a free function since it was written; these now
+match it, and the structs' member sets are exactly what they were.
+
 ### Measurements after
 
 Each row was measured before the change and after, on the same machine, with
@@ -482,7 +495,7 @@ fail on the reverted tree and all four new e2e tests fail against the released
 `CorrectlySpelledRuntimeKeysAreSilent` — which exist to fail if the new refusals
 are too broad, and are supposed to pass in both.
 
-`ConditionalConfig::empty()` replaced a gate in the descriptor reader as well
+`is_empty(ConditionalConfig)` replaced a gate in the descriptor reader as well
 as in the manifest reader, and a descriptor is read by every consumer of the
 index rather than by one project. All 228 descriptors in `mcpplibs/mcpp-index`
 were parsed with both binaries and compared: 228 identical, 0 differing, 0
@@ -499,6 +512,17 @@ today. It is a correctness fix for a channel nothing currently exercises. The
 but it is not evidence about the gate, and recording it as though it were is
 the failure this record is otherwise about.
 
+WHERE THE NEW e2e TESTS RUN, STATED RATHER THAN ASSUMED. All four declare
+`# requires: gcc`, and `run_all.sh` grants that capability only in its `Linux`
+branch — so they run on the Linux shards and are skipped on macOS and Windows.
+That is the right boundary for what they assert (three of the four defects are
+invisible on gcc, so their criteria are assertions on the emitted graph rather
+than on a build outcome, and the graph is the same everywhere), but it means the
+end-to-end leg exists on one platform only. The cross-dialect coverage is in the
+unit tests, which construct Clang, GCC and MSVC plans explicitly and run in
+every platform's unit job — including the MSVC spelling, whose empty-value
+symptom (`/ifcOutput` consuming the next token) no Linux runner could reach.
+
 One denominator is worth recording because it is empty where a reader would
 expect it not to be: mcpp's own build emits **zero** `cxx_module` edges, since
 every module unit in it takes the two-phase split path (`cxx_precompile` /

CHANGELOG.md

@@ -62,8 +62,12 @@ GCC 的接口拼法本就是纯语言,因此同一个工程在 GCC 上能构建
 结构体时没有同时加入它。于是该块被解析、被填充,然后丢弃;在同一谓词下随便再写一条
 无关的 `defines` 就能让它生效。
 
-闸门改为 `ConditionalConfig::empty()`,定义在字段旁边,并由 mcpp.toml 与 xpkg 两个读者
-共用;它进一步组合 `BuildInputs::empty()` 与既有的 `XlingsConfig::empty()`。同一形状的
+闸门改为 `is_empty(ConditionalConfig)`,与既有的 `append` 并列为自由函数,并由 mcpp.toml
+与 xpkg 两个读者共用;它进一步组合 `is_empty(BuildInputs)` 与既有的 `XlingsConfig::empty()`。
+自由函数而非成员是被实测逼出来的:给这个模块导出的结构体加内联成员会改变导入方从它的 BMI
+里实例化的东西,而 `Profile` 的 `std::optional<std::string>` 成员在 clang + MSVC 标准库下
+本就会崩——初版写成成员,Windows CI 上 `test_modgraph.cpp` 因此编译失败,报错指向一个本次
+改动从未碰过的结构体。同一形状的
 第四处随之关闭:xpkg 的闸门漏掉了它自己会填的 `privateIncludeDirs`。索引里 228 份
 descriptor 用新旧两个二进制解析,结果逐字节相同。
 

modules/manifest/src/toml.cppm

@@ -2918,11 +2918,11 @@ std::expected<Manifest, ManifestError> parse_string(std::string_view content,
                                               std::vector<std::string>{});
                 }
             }
-            // `ConditionalConfig::empty()` and not a disjunction written here:
+            // `is_empty(ConditionalConfig)` and not a disjunction written here:
             // this list omitted `libraries` and `linkLibraryDirs`, so a
             // predicate carrying only a `[target.<pred>.runtime]` table was
             // parsed and then dropped. See the note on that member.
-            if (!cc.empty())
+            if (!mcpp::manifest::is_empty(cc))
                 m.conditionalConfigs.push_back(std::move(cc));
         }
     }

modules/manifest/src/types.cppm

@@ -285,17 +285,6 @@ struct BuildInputs {
     // glibc and wrong for picolibc — and while this lived beside the package's
     // identity there was no spelling for that difference.
     std::vector<std::string>           stdModuleFlags;
-
-    // Does this carry anything? The sibling of `append` below: that function
-    // is the one place a contribution is combined, and this is the one place
-    // it is weighed. Both enumerate every field, and both are here so a field
-    // added above is a field the compiler puts in front of a reader twice.
-    bool empty() const {
-        return sources.empty() && cflags.empty() && cxxflags.empty()
-            && ldflags.empty() && defines.empty() && globFlags.empty()
-            && includeDirs.empty() && includeDirsAfter.empty()
-            && privateIncludeDirs.empty() && stdModuleFlags.empty();
-    }
 };
 
 // The single additive merge. Every conditional axis folds through this, so
@@ -322,6 +311,27 @@ inline void append(BuildInputs& dst, const BuildInputs& src) {
                               src.stdModuleFlags.end());
 }
 
+// Does this carry anything? The sibling of `append` above: that function is the
+// one place a contribution is combined, and this is the one place it is weighed.
+// Both enumerate every field, and they sit together so that a field added to
+// BuildInputs is a field the reader meets twice.
+//
+// A FREE FUNCTION, NOT A MEMBER, AND THAT IS NOT STYLE. Adding an inline member
+// to a struct exported from this module changes what importers materialise from
+// its BMI, and `Profile`'s `std::optional<std::string>` member is already known
+// to break under clang with the MSVC standard library — see the note on that
+// member for the measurement. The first version of this was a member, and it
+// failed exactly there: `test_modgraph.cpp` stopped compiling with
+// `no matching constructor for _SMF_control<_Optional_construct_base<...>>`,
+// reported against a struct the change never touched. `append` has been a free
+// function since it was written; this one matches it.
+inline bool is_empty(const BuildInputs& b) {
+    return b.sources.empty() && b.cflags.empty() && b.cxxflags.empty()
+        && b.ldflags.empty() && b.defines.empty() && b.globFlags.empty()
+        && b.includeDirs.empty() && b.includeDirsAfter.empty()
+        && b.privateIncludeDirs.empty() && b.stdModuleFlags.empty();
+}
+
 // A build-graph node declared by a build program (`mcpp:action=`).
 //
 // The architectural point (see
@@ -1156,14 +1166,17 @@ struct ConditionalConfig {
     // This is #258's medicine: a field added below and forgotten here is a
     // question asked at the point where the field is written, instead of in two
     // files that do not mention each other.
-    bool empty() const {
-        return inputs.empty() && linkLibraryDirs.empty() && libraries.empty()
-            && dependencies.empty() && devDependencies.empty()
-            && buildDependencies.empty() && featureDeps.empty()
-            && xlings.empty();
-    }
 };
 
+// The same question for a whole conditional block, and a free function for the
+// same reason `is_empty(BuildInputs)` is one.
+inline bool is_empty(const ConditionalConfig& c) {
+    return is_empty(c.inputs) && c.linkLibraryDirs.empty() && c.libraries.empty()
+        && c.dependencies.empty() && c.devDependencies.empty()
+        && c.buildDependencies.empty() && c.featureDeps.empty()
+        && c.xlings.empty();
+}
+
 // `[lib]` — library "root" interface convention.
 //
 // Convention-over-configuration: a library package's primary module

modules/manifest/src/xpkg.cppm

@@ -1530,7 +1530,7 @@ synthesize_from_xpkg_lua(std::string_view luaContent,
                 // The same question the mcpp.toml reader asks, asked the same
                 // way. This list omitted `privateIncludeDirs`, which this loop
                 // fills.
-                if (!cc.empty())
+                if (!mcpp::manifest::is_empty(cc))
                     m.conditionalConfigs.push_back(std::move(cc));
                 cur.skip_ws_and_comments();
             }