fix: a glob's **/ stands for whole directories, not part of a name (#878)

relative_path_matches_glob consumed `**/` and then retried the rest of
the pattern at every character of the path, so what followed `**/`
could begin in the middle of a file or directory name. `**/main.cpp`
selected `src/domain.cpp`, `tests/**/test_*.cpp` selected
`tests/mytest_a.cpp`, and `**/modules/dnn/**/*.avx2.cpp` selected
`a/submodules/dnn/l.avx2.cpp`.

The matcher serves every glob with the `sources` grammar: `sources`,
`[build] flags`, `[scan_overrides]`, test `discover`, `[pack]
include`/`exclude`, include-dir globs and `rerun_if_changed_glob`. A
`[build] flags` entry for `**/main.cpp` therefore also compiled
`src/app/domain.cpp` with its defines.

After `**/` the rest of the pattern now resumes only where it stands
(zero directories) or just after a `/`. A `**` not followed by `/`
(`src/**`, `src/**.cpp`) keeps crossing segments and names as before.
The literal-prefix and tail pre-filters of expand_glob are supersets of
the new match and are unchanged.

Tests: Glob.ADoubleStarSlashStandsForWholeDirectories checks the matcher
directly, and Scanner.ADoubleStarSlashTailMatchesAtTheRoot gains a
`sub/domain.cpp` that the `**/main.cpp` walk must not return. docs/04
states the rule in both languages.

Closes #877

Author: NopeCharon

docs/04-mcpp-toml.md

@@ -557,6 +557,11 @@ the package/feature boundary, not on an individual target.
 > mcpp 2026.8.18.1 the two were byte-identical, so there was no spelling for
 > "nothing" and any file left under `src/` was swept in.
 
+> **`**/` stands for whole directories.** In `sources` and every glob that
+> shares its grammar, `**/` matches zero or more directories and what follows
+> it starts a path segment: `**/main.cpp` names every `main.cpp` and not
+> `src/domain.cpp`. A `*` matches within one segment.
+
 > **A `sources` entry may carry the accelerator it is for** (2026.9.5.2+):
 > `{ glob = "src/kernels/**/*.cu", accel = "cuda12.9+{sm_89}" }`. The glob
 > joins the list like any other; the constraint decides whether it applies to a

docs/zh/04-mcpp-toml.md

@@ -530,6 +530,11 @@ mcpp 刻意不在一次构建里用两种方式编译同一份共享源码：一
 > 分发包需要表达的意思。在 mcpp 2026.8.18.1 之前两者逐字节相同，所以
 > 没有拼法能表达「什么都没有」，`src/` 下留下的任何文件都会被卷进来。
 
+> **`**/` 代表整段目录。** 在 `sources` 以及沿用其文法的每一处 glob 中，
+> `**/` 匹配零个或多个目录，其后的部分从一个路径段的开头算起：
+> `**/main.cpp` 选中每个 `main.cpp`，而不选中 `src/domain.cpp`。`*` 只在
+> 一个路径段内匹配。
+
 > **一条 `sources` 条目可以携带它所面向的加速器**（2026.9.5.2+）：
 > `{ glob = "src/kernels/**/*.cu", accel = "cuda12.9+{sm_89}" }`。glob
 > 像其它条目一样加入这份列表；约束决定它是否适用于某次给定的构建。它

modules/manifest/src/glob.cppm

@@ -210,13 +210,19 @@ bool relative_path_matches_glob(std::string_view relative, std::string_view glob
             [&](std::size_t si, std::size_t pi) -> bool {
             while (pi < p.size()) {
                 if (p[pi] == '*' && pi + 1 < p.size() && p[pi + 1] == '*') {
-                    // ** : skip zero or more chars/segments
+                    // `**/` : zero or more WHOLE directories, so the rest of
+                    // the pattern resumes where it stands or just after a `/`.
+                    // Resuming inside a name made `**/main.cpp` match
+                    // `src/domain.cpp`. A `**` not followed by `/` skips any
+                    // characters, segments included.
                     pi += 2;
-                    if (pi < p.size() && p[pi] == '/') ++pi;
+                    const bool wholeDirs = pi < p.size() && p[pi] == '/';
+                    if (wholeDirs) ++pi;
                     if (pi >= p.size()) return true;
-                    while (si <= s.size()) {
+                    const std::size_t from = si;
+                    for (; si <= s.size(); ++si) {
+                        if (wholeDirs && si != from && s[si - 1] != '/') continue;
                         if (rec(si, pi)) return true;
-                        ++si;
                     }
                     return false;
                 } else if (p[pi] == '*') {

tests/unit/test_modgraph.cpp

@@ -1444,16 +1444,46 @@ TEST(ModuleResolution, OneFileReachedTwiceIsNamedAsSuchWhateverItsSpelling) {
 
 // A kept walk matched by the literal tail of a glob: `**/` also matches no
 // directory, so `**/main.cpp` matches a `main.cpp` at the root as well as one
-// below it, as the per-pattern walk did.
+// below it, as the per-pattern walk did. `domain.cpp` ends with the same tail
+// and is still not a `main.cpp`.
 TEST(Scanner, ADoubleStarSlashTailMatchesAtTheRoot) {
     auto dir = make_tempdir("mcpp-glob-tail");
     write(dir / "main.cpp", "int main() {}\n");
     write(dir / "sub" / "main.cpp", "int main() {}\n");
     write(dir / "sub" / "other.cpp", "int x;\n");
+    write(dir / "sub" / "domain.cpp", "int y;\n");
     auto files = expand_glob(dir, "**/main.cpp");
     ASSERT_EQ(files.size(), 2u);
     EXPECT_EQ(files[0], dir / "main.cpp");
     EXPECT_EQ(files[1], dir / "sub" / "main.cpp");
     std::filesystem::remove_all(dir);
 }
 
+// `**/` stands for zero or more whole directories: what follows it starts a
+// path segment. It used to resume at any character, so `**/main.cpp` named
+// `src/domain.cpp` and `tests/**/test_*.cpp` named `tests/mytest_a.cpp`.
+// A `**` not followed by `/` still crosses segments and names.
+TEST(Glob, ADoubleStarSlashStandsForWholeDirectories) {
+    EXPECT_TRUE(relative_path_matches_glob("main.cpp", "**/main.cpp"));
+    EXPECT_TRUE(relative_path_matches_glob("src/main.cpp", "**/main.cpp"));
+    EXPECT_TRUE(relative_path_matches_glob("src/a/b/main.cpp", "**/main.cpp"));
+    EXPECT_FALSE(relative_path_matches_glob("domain.cpp", "**/main.cpp"));
+    EXPECT_FALSE(relative_path_matches_glob("src/domain.cpp", "**/main.cpp"));
+    EXPECT_FALSE(relative_path_matches_glob("src/a/domain.cpp", "**/main.cpp"));
+
+    EXPECT_TRUE(relative_path_matches_glob("tests/test_a.cpp", "tests/**/test_*.cpp"));
+    EXPECT_TRUE(relative_path_matches_glob("tests/x/test_a.cpp", "tests/**/test_*.cpp"));
+    EXPECT_FALSE(relative_path_matches_glob("tests/mytest_a.cpp", "tests/**/test_*.cpp"));
+    EXPECT_FALSE(relative_path_matches_glob("tests/x/mytest_a.cpp", "tests/**/test_*.cpp"));
+
+    const std::string_view dnn = "**/modules/dnn/**/*.avx2.cpp";
+    EXPECT_TRUE(relative_path_matches_glob("modules/dnn/l.avx2.cpp", dnn));
+    EXPECT_TRUE(relative_path_matches_glob("a/modules/dnn/b/l.avx2.cpp", dnn));
+    EXPECT_FALSE(relative_path_matches_glob("a/submodules/dnn/l.avx2.cpp", dnn));
+    EXPECT_FALSE(relative_path_matches_glob("modules/xdnn/l.avx2.cpp", dnn));
+
+    EXPECT_TRUE(relative_path_matches_glob("src/a/b.cpp", "src/**"));
+    EXPECT_TRUE(relative_path_matches_glob("src/a/b.cpp", "src/**.cpp"));
+    EXPECT_TRUE(relative_path_matches_glob("src/a/b.cpp", "**"));
+    EXPECT_FALSE(relative_path_matches_glob("srcx/a.cpp", "src/**/*.cpp"));
+}