fix: don't assume 'no branches' means 'not executed' #1896

CHANGES.rst

@@ -23,7 +23,12 @@ upgrading your version of coverage.py.
 Unreleased
 ----------
 
-Nothing yet.
+- fix: the LCOV report code assumed that a branch line that took no branches
+  meant that the entire line was unexecuted.  This isn't true in a few cases:
+  the line might always raise an exception, or might have been optimized away.
+  Fixes `issue 1896`_.
+
+.. _issue 1896: https://github.com/nedbat/coveragepy/issues/1896
 
 
 .. start-releases

coverage/lcovreport.py

@@ -118,10 +118,8 @@ def lcov_arcs(
 
         if taken == 0:
             # When _none_ of the out arcs from 'line' were executed,
-            # this probably means 'line' was never executed at all.
-            # Cross-check with the line stats.
+            # it can mean the line always raised an exception.
             assert len(executed_arcs[line]) == 0
-            assert line in analysis.missing
             destinations = [
                 (dst, "-") for dst in missing_arcs[line]
             ]

tests/test_lcov.py

@@ -526,3 +526,35 @@ def foo(a):
             """)
         actual_result = self.get_lcov_report_content()
         assert expected_result == actual_result
+
+    def test_always_raise(self) -> None:
+        self.make_file("always_raise.py", """\
+            try:
+                if not_defined:
+                    print("Yes")
+                else:
+                    print("No")
+            except Exception:
+                pass
+        """)
+        cov = coverage.Coverage(source=".", branch=True)
+        self.start_import_stop(cov, "always_raise")
+        cov.lcov_report()
+        expected_result = textwrap.dedent("""\
+            SF:always_raise.py
+            DA:1,1
+            DA:2,1
+            DA:3,0
+            DA:5,0
+            DA:6,1
+            DA:7,1
+            LF:6
+            LH:4
+            BRDA:2,0,jump to line 3,-
+            BRDA:2,0,jump to line 5,-
+            BRF:2
+            BRH:0
+            end_of_record
+            """)
+        actual_result = self.get_lcov_report_content()
+        assert expected_result == actual_result