feat(build): feature-dep 与已声明依赖同键时按加法合并

`mergeActiveFeatureDeps` 用的是 `try_emplace`,键已存在就丢弃 feature 的
spec。这条规则对 version/path/git 是对的(条件段不该静默覆盖无条件段),对
`tools` / `reexport` 却丢掉了这个 feature 存在的全部理由。

gRPC 就是反例,而且是本 issue 的头号用例:它**无条件**依赖 compat.protobuf,
而 `codegen` feature 需要往**同一条边**加 `tools = ["protoc"], reexport = true`。
挪到无条件条目上不可行 —— 那会让每个 grpc 消费者都构建 protoc(~157 个额外
TU),而这正是 tools 默认关闭要避免的。

于是:tools / features 取并集,host-module / reexport 取或,身份字段不合并。
与逐边 feature 请求本来遵循的规则一致。

e2e 193 补第 3 段覆盖它,并已用「不合并 tools」验证会变红
(`rulepkg: no codegen tool`)。

Author: speak-agent

.agents/docs/2026-08-06-provisions-and-build-inputs.md

@@ -146,6 +146,18 @@ visible(P)    = ⋃ {P→D}                 [ own(P→D) ∪ exported(D) ]
 2. **provision 的存在性绑定 feature 激活**,与 `featureDefines`(`prepare.cppm:3812`)同一时机。feature 关闭时不应有任何工具被构建。
 3. **`reexport` 只在依赖侧有意义。** root 写它无害但无效果(root 没有消费者)。
 
+##### feature-dep 与已声明依赖同键时按加法合并
+
+`mergeActiveFeatureDeps` 原本用 `try_emplace`,即「键已存在就丢弃 feature 的
+spec」。gRPC 恰好是反例:它**无条件**依赖 `compat.protobuf`,而 `codegen`
+feature 需要往**同一条边**加 `tools = ["protoc"], reexport = true`。把这个请求
+挪到无条件条目上不可行(那会让每个消费者都构建 protoc),丢弃又会静默丢掉这个
+feature 存在的全部理由。
+
+因此:`tools` / `features` 取并集,`host-module` / `reexport` 取或;
+`version` / `path` / `git` 这类**身份字段不合并**,「条件段绝不静默覆盖无条件
+段」这条纪律保持不变。与逐边 feature 请求本来就遵循的规则一致。
+
 #### D1.3 裸名:走索引那套命名空间阶梯,而不是「谁最后写谁赢」
 
 `env_var_name`(`tool_store.cppm:154`)今天同时发长名与短名:`compat.protobuf` → `MCPP_DEP_COMPAT_PROTOBUF_BIN_PROTOC` 与 `MCPP_DEP_PROTOBUF_BIN_PROTOC`。今天只有 root 亲自声明的工具进环境,撞车在用户眼皮底下;传递传播之后,两条互不相识的库各自提供同名短名工具时,谁赢取决于 vector 的追加顺序,且无任何诊断。这从另一扇门放回了本设计要保住的「版本错配不可表达」性质。

docs/05-mcpp-toml.md

@@ -1106,6 +1106,12 @@ int main() { return grpcgen::generate_all() ? 0 : 1; }
 - **One hop per declaration.** A re-exported provision reaches the consumers of
   the package that declared it. For it to travel further, the next package must
   re-export in turn — each package decides only what *it* hands on.
+- **A feature may add a request to a dependency you already declare.** gRPC
+  depends on protobuf unconditionally and its `codegen` feature adds
+  `tools = ["protoc"], reexport = true` to that same edge. `tools` and
+  `features` union, `host-module` and `reexport` OR together; `version` /
+  `path` / `git` do not merge, so a feature still cannot silently override the
+  unconditional entry's identity.
 - **Visibility, not execution.** `dep_bin()` returns a path; whether anything
   runs is still the consumer's `build.mcpp`'s decision. Nothing changes about
   who builds the tool or how the tool store is keyed.

docs/zh/05-mcpp-toml.md

@@ -847,6 +847,10 @@ int main() { return grpcgen::generate_all() ? 0 : 1; }
   命名空间里塞东西。「把某样东西交给消费者」是一条供应链主张,必须写下来。
 - **一次声明只走一跳。** 被再导出的提供物到达声明它的那个包的消费者;要继续
   往上走,下一个包必须自己也写 `reexport`。每个包只决定**它**交出什么。
+- **feature 可以往一条已经声明过的依赖上追加请求。** gRPC 无条件依赖 protobuf,
+  而它的 `codegen` feature 往同一条边加 `tools = ["protoc"], reexport = true`。
+  `tools` 与 `features` 取并集,`host-module` 与 `reexport` 取或;`version` /
+  `path` / `git` 不合并 —— feature 仍然无法静默覆盖无条件条目的身份。
 - **传播的是可见性,不是执行。** `dep_bin()` 只返回路径,跑不跑仍由消费者的
   `build.mcpp` 决定;谁构建了这个工具、tool store 怎么做键,都不改变。
 - **裸名由阶梯决定,而不是靠运气。** 一旦两个库都能再导出,它们可能同时提供

src/build/prepare.cppm

@@ -3148,7 +3148,34 @@ prepare_build(bool print_fingerprint,
         for (auto& f : activateFeatures(pm, requested, seedDefault)) {
             auto it = pm.featureDeps.find(f);
             if (it == pm.featureDeps.end()) continue;
-            for (auto& [k, spec] : it->second) pm.dependencies.try_emplace(k, spec);
+            for (auto& [k, spec] : it->second) {
+                auto [pos, fresh] = pm.dependencies.try_emplace(k, spec);
+                if (fresh) continue;
+                // #359: the key already exists unconditionally, and dropping
+                // the feature's spec here loses REQUESTS the feature exists to
+                // make. gRPC is the shape: it depends on compat.protobuf
+                // always, and its `codegen` feature has to add
+                // `tools = ["protoc"], reexport = true` to that same edge —
+                // which is precisely what must NOT be paid for by a consumer
+                // who did not ask for codegen, so moving it to the
+                // unconditional entry is not an option either.
+                //
+                // Additive fields merge; identity fields (version/path/git) do
+                // not, keeping "a conditional section never silently
+                // overrides an unconditional one" intact. Same rule the
+                // per-edge feature request already follows.
+                auto& dst = pos->second;
+                for (auto const& t : spec.tools)
+                    if (std::find(dst.tools.begin(), dst.tools.end(), t)
+                        == dst.tools.end())
+                        dst.tools.push_back(t);
+                for (auto const& f2 : spec.features)
+                    if (std::find(dst.features.begin(), dst.features.end(), f2)
+                        == dst.features.end())
+                        dst.features.push_back(f2);
+                dst.hostModule = dst.hostModule || spec.hostModule;
+                dst.reexport   = dst.reexport   || spec.reexport;
+            }
         }
     };
 

tests/e2e/193_provision_reexport.sh

@@ -179,4 +179,58 @@ if grep -q "rulepkg" <(nm -C "target"/*/*/bin/app 2>/dev/null || true); then
     echo "FAIL: the rule package was linked into the consumer's binary"; exit 1
 fi
 
+# ── 3. a feature ADDING a request to an already-declared dependency ────────
+# The real shape: gRPC depends on protobuf always, and its `codegen` feature
+# has to add `tools = ["protoc"], reexport = true` to that SAME edge. Moving
+# the request to the unconditional entry is not an option — it would make every
+# consumer build protoc — and dropping the feature's spec (try_emplace keeps
+# the existing key) would silently lose the request.
+cd "$TMP"
+cat > lib/mcpp.toml <<'EOF'
+[package]
+name    = "mylib"
+version = "0.1.0"
+
+[build]
+sources = ["src/mylib.cpp"]
+
+# Unconditional: the library links against this package no matter what.
+[dependencies]
+toolpkg = { path = "../toolpkg" }
+rulepkg = { path = "../rulepkg", host-module = true, reexport = true }
+
+# The feature adds a REQUEST to the edge above, and nothing else.
+[feature-deps.codegen]
+toolpkg = { path = "../toolpkg", tools = ["codegen"], reexport = true }
+EOF
+cat > app/mcpp.toml <<'EOF'
+[package]
+name    = "app"
+version = "0.1.0"
+
+[dependencies]
+mylib = { path = "../lib", features = ["codegen"] }
+EOF
+cd app && rm -rf target
+"$MCPP" build > b3.log 2>&1 || {
+    cat b3.log
+    echo "FAIL: a feature could not add a tool request to an existing dependency"
+    exit 1
+}
+out="$("$MCPP" run 2>&1 | grep '^ANSWER=' | tail -1)"
+[[ "$out" == "ANSWER=42" ]] || { echo "FAIL: expected ANSWER=42, got '$out'"; exit 1; }
+
+# … and without the feature, nothing is built.
+cd "$TMP"
+sed 's/, features = \["codegen"\]//' app/mcpp.toml > app/mcpp.toml.new
+mv app/mcpp.toml.new app/mcpp.toml
+cd app && rm -rf target
+if "$MCPP" build > b4.log 2>&1; then
+    cat b4.log
+    echo "FAIL: the build succeeded without the codegen feature — the tool ran anyway"
+    exit 1
+fi
+grep -q "host tool" b4.log && {
+    cat b4.log; echo "FAIL: a tool was built for a consumer that did not enable the feature"; exit 1; }
+
 echo "PASS: 193_provision_reexport"