fix: properly measure strange use of wildcard alternatives in match/case. #1421

Author: nedbat

CHANGES.rst

@@ -20,7 +20,10 @@ development at the same time, such as 4.5.x and 5.0.
 Unreleased
 ----------
 
-Nothing yet.
+- Fixed a mis-measurement of a strange use of wildcard alternatives in
+  match/case statements, closing `issue 1421`_.
+
+.. _issue 1421: https://github.com/nedbat/coveragepy/issues/1421
 
 
 .. _changes_6-6-0b1:

coverage/parser.py

@@ -1041,7 +1041,10 @@ def _handle__Match(self, node):
         had_wildcard = False
         for case in node.cases:
             case_start = self.line_for_node(case.pattern)
-            if isinstance(case.pattern, ast.MatchAs):
+            pattern = case.pattern
+            while isinstance(pattern, ast.MatchOr):
+                pattern = pattern.patterns[-1]
+            if isinstance(pattern, ast.MatchAs):
                 had_wildcard = True
             self.add_arc(last_start, case_start, "the pattern on line {lineno} always matched")
             from_start = ArcStart(case_start, cause="the pattern on line {lineno} never matched")

tests/test_arcs.py

@@ -1362,6 +1362,19 @@ def test_match_case_without_wildcard(self):
         )
         assert self.stdout() == "None\nno go\ngo: n\n"
 
+    def test_absurd_wildcard(self):
+        # https://github.com/nedbat/coveragepy/issues/1421
+        self.check_coverage("""\
+            def absurd(x):
+                match x:
+                    case (3 | 99 | (999 | _)):
+                        print("default")
+            absurd(5)
+            """,
+            arcz=".1 15 5.  .2 23 34 4.",
+        )
+        assert self.stdout() == "default\n"
+
 
 class OptimizedIfTest(CoverageTest):
     """Tests of if statements being optimized away."""