Generated source action is skipped after clearing a workspace member target: empty placeholder is compiled #778

A generated C++ source is replaced with an empty placeholder after clearing a workspace member's target directory. If the workspace-level target (and its Ninja log) is retained, the generator is skipped and the empty source is compiled.

Environment

• mcpp 2026.10.5.3
• Windows x64, LLVM 22.1.8, Ninja 1.12.1
• MSVC 14.51.36231, Windows SDK 10.0.26100.0
• Python 3 available as python on PATH

Reproduction

Create the six files below in a new directory, then run these PowerShell commands from its root:

mcpp build -p app --release
(Get-Item app/target/.build-mcpp/out/generated.cpp).Length

# Clear the member's build directory, preserving the workspace-level target.
# Moving it aside avoids any destructive cleanup commands.
Move-Item -LiteralPath app/target -Destination saved-app-target

mcpp build -p app --release
(Get-Item app/target/.build-mcpp/out/generated.cpp).Length

The first build succeeds and generated.cpp contains the definition of resource().
The second build fails with:

lld-link: error: undefined symbol: char const * __cdecl resource(void)
>>> referenced by obj/repro_app/main.o:(main)

After the second build, generated.cpp exists but has length 0. The generator did not run: the workspace .ninja_log retains the original generation entry and adds new scan/compile entries for the empty source, without a new generation entry.

Expected behavior

Clearing a member's generated outputs should cause its source-generation action to run again. A scanner placeholder must not count as a valid generator output for incremental-build purposes.

Files

mcpp.toml

[workspace]
members = ["app"]
[toolchain]
windows = "llvm@22.1.8"

app/mcpp.toml

[package]
namespace="repro"
name="app"
version="0.0.1"
standard="c++23"
[build]
sources=[]
[targets.resource_test]
kind="bin"
main="main.cpp"

app/main.cpp

extern const char* resource();
int main() { return resource()[0] == 'R' ? 0 : 1; }

app/build.mcpp

import std;
import mcpp;
int main() {
    const auto root = std::filesystem::path(mcpp::manifest_dir());
    const auto out = (std::filesystem::path(mcpp::out_dir()) / "generated.cpp").generic_string();
    const auto generator = (root / "generate.py").generic_string();
    const auto input = (root / "data.txt").generic_string();
    mcpp::action a;
    a.id = "generate:resource";
    a.role = mcpp::roles::source;
    a.arg("python").arg(generator.c_str()).arg(input.c_str()).arg(out.c_str())
        .input(generator.c_str()).input(input.c_str()).output(out.c_str()).submit();
    return 0;
}

app/generate.py

import sys
from pathlib import Path
out=Path(sys.argv[2])
out.parent.mkdir(parents=True,exist_ok=True)
out.write_text('const char* resource() { return "RESOURCE"; }\n')
print("GENERATOR_EXECUTED")

app/data.txt

RESOURCE