feat: implicitly combine relative paths

3 files changed, 25 insertions, 11 deletions

diff --git a/coverage/control.py b/coverage/control.py
index 5e1e54bf..91e604e0 100644
--- a/coverage/control.py
+++ b/coverage/control.py
@@ -741,16 +741,14 @@ class Coverage:
         self._post_init()
         self.get_data()
 
-        aliases = None
-        if self.config.paths:
-            aliases = PathAliases(
-                debugfn=(self._debug.write if self._debug.should("pathmap") else None),
-                relative=self.config.relative_files,
-            )
-            for paths in self.config.paths.values():
-                result = paths[0]
-                for pattern in paths[1:]:
-                    aliases.add(pattern, result)
+        aliases = PathAliases(
+            debugfn=(self._debug.write if self._debug.should("pathmap") else None),
+            relative=self.config.relative_files,
+        )
+        for paths in self.config.paths.values():
+            result = paths[0]
+            for pattern in paths[1:]:
+                aliases.add(pattern, result)
 
         combine_parallel_data(
             self._data,
diff --git a/coverage/files.py b/coverage/files.py
index 87a18bc2..2c520b8a 100644
--- a/coverage/files.py
+++ b/coverage/files.py
@@ -409,6 +409,20 @@ class PathAliases:
                     f"producing {new!r}"
                 )
                 return new
+
+        # If we get here, no pattern matched.
+
+        if self.relative and not isabs_anywhere(path):
+            parts = re.split(r"[/\\]", path)
+            if len(parts) > 1:
+                dir1 = parts[0]
+                pattern = f"*/{dir1}"
+                regex = rf"^(.*[\\/])?{re.escape(dir1)}[\\/]"
+                result = f"{dir1}{os.sep}"
+                self.debugfn(f"Generating rule: {pattern!r} -> {result!r} using regex {regex!r}")
+                self.aliases.append((pattern, re.compile(regex), result))
+                return self.map(path)
+
         self.debugfn(f"No rules match, path {path!r} is unchanged")
         return path
 
diff --git a/coverage/sqldata.py b/coverage/sqldata.py
index 5d62b15b..5d7fbecf 100644
--- a/coverage/sqldata.py
+++ b/coverage/sqldata.py
@@ -596,7 +596,9 @@ class CoverageData(SimpleReprMixin):
         """Update this data with data from several other :class:`CoverageData` instances.
 
         If `aliases` is provided, it's a `PathAliases` object that is used to
-        re-map paths to match the local machine's.
+        re-map paths to match the local machine's.  Note: `aliases` is None
+        only when called directly from the test suite.
+
         """
         if self._debug.should("dataop"):
             self._debug.write("Updating with data from {!r}".format(