fix: correct the missing branch message for the last case of a match/case

Author: nedbat

CHANGES.rst

@@ -23,6 +23,10 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
+- Fix: the last case of a match/case statement had an incorrect message if the
+  branch was missed.  It said the pattern never matched, when actually the
+  branch is missed if the last case always matched.
+
 - Fix: clicking a line number in the HTML report now positions more accurately.
 
 

coverage/parser.py

@@ -1112,7 +1112,9 @@ def _handle__Match(self, node: ast.Match) -> set[ArcStart]:
                 exits |= self.add_body_arcs(case.body, from_start=from_start)
                 last_start = case_start
             if not had_wildcard:
-                exits.add(from_start)
+                exits.add(
+                    ArcStart(case_start, cause="the pattern on line {lineno} always matched"),
+                )
             return exits
 
     def _handle__NodeList(self, node: NodeList) -> set[ArcStart]:

tests/test_parser.py

@@ -902,21 +902,23 @@ def test_missing_arc_descriptions_bug460(self) -> None:
         assert parser.missing_arc_description(2, -3) == "line 3 didn't finish the lambda on line 3"
 
     @pytest.mark.skipif(not env.PYBEHAVIOR.match_case, reason="Match-case is new in 3.10")
-    def test_match_case_with_default(self) -> None:
-        parser = self.parse_text("""\
-            for command in ["huh", "go home", "go n"]:
-                match command.split():
-                    case ["go", direction] if direction in "nesw":
-                        match = f"go: {direction}"
-                    case ["go", _]:
-                        match = "no go"
-                print(match)
+    def test_match_case(self) -> None:
+        parser = self.parse_text("""\
+            match command.split():
+                case ["go", direction] if direction in "nesw":      # 2
+                    match = f"go: {direction}"
+                case ["go", _]:                                     # 4
+                    match = "no go"
+            print(match)                                            # 6
             """)
-        assert parser.missing_arc_description(3, 4) == (
-            "line 3 didn't jump to line 4, because the pattern on line 3 never matched"
+        assert parser.missing_arc_description(2, 3) == (
+            "line 2 didn't jump to line 3, because the pattern on line 2 never matched"
         )
-        assert parser.missing_arc_description(3, 5) == (
-            "line 3 didn't jump to line 5, because the pattern on line 3 always matched"
+        assert parser.missing_arc_description(2, 4) == (
+            "line 2 didn't jump to line 4, because the pattern on line 2 always matched"
+        )
+        assert parser.missing_arc_description(4, 6) == (
+            "line 4 didn't jump to line 6, because the pattern on line 4 always matched"
         )