fix: make a combine error message clearer

Author: nedbat

CHANGES.rst

@@ -23,7 +23,12 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
-Nothing yet.
+- If you attempt to combine statement coverage data with branch coverage data,
+  coverage.py used to fail with the message "Can't combine arc data with line
+  data" or its reverse, "Can't combine line data with arc data."  These
+  messages used internal terminology, making it hard for people to understand
+  the problem.  They are now changed to mention "branch coverage data" and
+  "statement coverage data."
 
 
 .. scriv-start-here

coverage/sqldata.py

@@ -663,9 +663,9 @@ def update(
                 getattr(other_data, "_filename", "???"),
             ))
         if self._has_lines and other_data._has_arcs:
-            raise DataError("Can't combine arc data with line data")
+            raise DataError("Can't combine branch coverage data with statement data")
         if self._has_arcs and other_data._has_lines:
-            raise DataError("Can't combine line data with arc data")
+            raise DataError("Can't combine statement coverage data with branch data")
 
         map_path = map_path or (lambda p: p)
 

tests/test_data.py

@@ -405,10 +405,12 @@ def test_update_cant_mix_lines_and_arcs(self) -> None:
         covdata2 = DebugCoverageData(suffix='2')
         covdata2.add_arcs(ARCS_3)
 
-        with pytest.raises(DataError, match="Can't combine arc data with line data"):
+        msg = "Can't combine branch coverage data with statement data"
+        with pytest.raises(DataError, match=msg):
             covdata1.update(covdata2)
 
-        with pytest.raises(DataError, match="Can't combine line data with arc data"):
+        msg = "Can't combine statement coverage data with branch data"
+        with pytest.raises(DataError, match=msg):
             covdata2.update(covdata1)
 
     def test_update_file_tracers(self) -> None: