fix(mysql-connector-cpp): state the C++ standard library as a layer requirement

.github/workflows/validate.yml

@@ -1206,13 +1206,18 @@ jobs:
           #     ld.lld: error: undefined symbol:
           #         std::__cxx11::basic_string<...>::_M_create(...)
           #
-          # Its install hook cannot adapt, because nothing tells it what the
-          # consumer chose: MCPP_CXX_STDLIB is exported only when mcpp runs a
-          # BUILD PROGRAM (`src/build/build_program.cppm`), `make_xlings_env`
-          # carries `{binary, home, projectDir}` and no toolchain, and a probe
-          # in the hook measured `MCPP_CXX_STDLIB=nil` on this very leg (#392).
-          # mcpp-community/mcpp#613; when the hook can see the consumer's stdlib,
-          # this entry comes out and the descriptor does the work instead.
+          # Its install hook cannot adapt: mcpp resolves the toolchain after the
+          # dependency graph, so the consumer's standard library does not exist
+          # yet when the hook runs, and MCPP_CXX_STDLIB is nil (older mcpp) or
+          # empty (2026.9.12.2 and later) there (#392; mcpp-community/mcpp#613,
+          # measured by its tests/e2e/648). The descriptor states the
+          # requirement instead, `requires = { "mcpp:c++-abi=libstdc++" }` in its
+          # `linux` table, and a libc++ consumer is refused at resolution naming
+          # both implementations. (On macOS the libraries are built with Apple
+          # clang, and the `macosx` table requires libc++, which the default
+          # toolchain there resolves.)
+          # That refusal follows the hook's source build, so the member stays off
+          # this leg: the leg would spend the build to reach a known refusal.
           #
           # SKIPPED, NOT MARKED GREEN: the incompatibility is real for users on
           # a libc++ toolchain. This stops CI from re-measuring a known answer

pkgs/c/compat.mysql-connector-cpp.lua

@@ -51,13 +51,22 @@ package = {
             ["compat.openssl"]        = "3.5.1",
         },
 
+        -- THE STANDARD LIBRARY THE STATIC LIBRARIES WERE COMPILED AGAINST, per
+        -- platform, because install() uses each platform's system compiler:
+        -- g++ with libstdc++ on Linux, Apple clang with libc++ on macOS. A
+        -- consumer whose toolchain resolves the other one is refused at
+        -- resolution, naming both, instead of failing at link. An mcpp older
+        -- than the layer grammar reports the key as unknown and proceeds as
+        -- before.
         linux = {
+            requires = { "mcpp:c++-abi=libstdc++" },
             ldflags = {
                 "-Llib", "-l:libmysqlcppconnx-static.a",
                 "-l:libmysqlcppconn-static.a", "-lresolv",
             },
         },
         macosx = {
+            requires = { "mcpp:c++-abi=libc++" },
             ldflags = {
                 "-Llib", "-lmysqlcppconnx-static",
                 "-lmysqlcppconn-static", "-lresolv",
@@ -267,8 +276,8 @@ function install()
 
     -- THE STANDARD LIBRARY THIS IS BUILT AGAINST HAS TO MATCH THE CONSUMER'S.
     --
-    -- CMake picks the system compiler below, so the static libs come out
-    -- against libstdc++ whatever the consumer uses. On the llvm leg, which
+    -- CMake picks the system compiler below, so on Linux the static libs come
+    -- out against libstdc++ whatever the consumer uses. On the llvm leg, which
     -- links libc++, the member then fails at link with the libstdc++ half of
     -- its own dependency undefined:
     --
@@ -282,13 +291,12 @@ function install()
     --   1. MEASURED. The probe below logged `MCPP_CXX_STDLIB=nil` on the llvm
     --      leg (mcpplibs/mcpp-index#392, `workspace (linux llvm 0/4)`), and
     --      the link failed exactly as before.
-    --   2. THE ONLY SETTER is `src/build/build_program.cppm` (`e.emplace_back
-    --      ("MCPP_CXX_STDLIB", env.cxxStdlib)`) -- mcpp exports it when it runs
-    --      a BUILD PROGRAM. An xlings install hook is not that.
-    --   3. THE CALL CARRIES NOTHING ELSE either: `make_xlings_env` builds an
-    --      `xlings::Env` of `{binary, home, projectDir}`, and
-    --      `install_packages` is invoked with `XLINGS_HOME` and PATH. No
-    --      toolchain, no compiler, no stdlib crosses that boundary.
+    --   2. BY CONSTRUCTION, NOT BY OMISSION. mcpp 2026.9.12.2 emits
+    --      MCPP_CXX_STDLIB to a dependency's install hook, and it is empty
+    --      there: mcpp resolves the toolchain after the dependency graph,
+    --      because a package in the graph may supply a target-side layer, so no
+    --      standard library exists yet when this hook runs
+    --      (mcpp-community/mcpp#613, measured by its tests/e2e/648).
     --
     -- So neither route is reachable from HERE. `llamacpp` refuses a libc++
     -- toolchain by name with `mcpp::cxx_stdlib()`, but that lives in its
@@ -300,17 +308,17 @@ function install()
     -- before the linker says it: these are STATIC libraries built by CMake
     -- with the system compiler, so `std::__cxx11::` and friends cross the
     -- boundary into the consumer. Consuming them from a libc++ toolchain does
-    -- not work and cannot be made to work from inside this hook. Tracked as
-    -- mcpp-community/mcpp#613 (install hooks need the consumer's stdlib);
-    -- until then
-    -- `validate.yml` keeps this member off the llvm leg, with the same reason
-    -- written there.
+    -- not work and cannot be made to work from inside this hook. The
+    -- descriptor states it instead (`requires` in the per-platform tables of
+    -- the mcpp segment above), and
+    -- `validate.yml` keeps this member off the llvm leg, because the refusal
+    -- follows this hook's source build and the leg would spend the build to
+    -- reach an answer it already knows.
     --
-    -- The probe stays. It costs one log line, it is the evidence for point 1,
-    -- and the day mcpp does pass the variable through, this line reports it
-    -- and the fix becomes a three-line change directly below.
+    -- The probe stays. It costs one log line and is the evidence for points 1
+    -- and 2: nil from an mcpp older than 2026.9.12.2, empty from a newer one.
     hook_log("MCPP_CXX_STDLIB=" .. tostring(os.getenv("MCPP_CXX_STDLIB"))
-             .. " (expected nil; see the comment above)")
+             .. " (expected nil or empty; see the comment above)")
     if os.host() == "macosx" then
         -- Connector 在 project() 前启动 bootstrap CMake；必须通过环境变量
         -- 将最低系统版本同步给 bootstrap 及其后续的内置依赖构建。