fix(ci): keep an expression matrix directive out of the comparison

`include:` or `exclude:` written as `${{ ... }}` read back as a string, and
the sweep treated that as the directive being absent, so it expanded every
combination GitHub would have dropped. A job whose `name:` holds no matrix
value then looked like it repeated one name across combinations that never
run. An absent directive still means no rows; anything that is not a list
of rows now joins the names left out of the comparison

tests/code_coverage_tests/check_workflow_job_name_collisions.py

@@ -162,8 +162,10 @@ def listed_values(matrix: Mapping[str, object]) -> tuple[tuple[str, tuple[str, .
 def directive_rows(matrix: Mapping[str, object], directive: str) -> tuple[Mapping[str, str], ...] | Opaque:
     """One `include` or `exclude` row, or why the combinations they shape cannot be worked out."""
     rows: Final = matrix.get(directive)
-    if not isinstance(rows, Sequence) or isinstance(rows, str):
+    if rows is None:
         return ()
+    if not isinstance(rows, Sequence) or isinstance(rows, str):
+        return Opaque(f"a matrix `{directive}` is itself an expression rather than a list of rows")
     mappings: Final = tuple(row for row in rows if isinstance(row, Mapping))
     if len(mappings) != len(rows):
         return Opaque(f"a matrix `{directive}` row is not a mapping of values")

tests/code_coverage_tests/test_workflow_job_name_collisions.py

@@ -846,3 +846,35 @@ def test_a_file_holding_two_yaml_documents_is_reported_rather_than_raising() ->
 
     assert len(unreadable(sources)) == 1
     assert exit_code(sources) == 1
+
+
+def test_an_exclude_that_is_itself_an_expression_is_reported_rather_than_ignored() -> None:
+    sources: Final = {
+        "a.yml": (
+            "on: pull_request\njobs:\n  one:\n    name: build\n    runs-on: ubuntu-latest\n"
+            "    strategy:\n      matrix:\n        python: ['3.11', '3.12']\n"
+            "        exclude: ${{ fromJson(vars.SKIP) }}\n"
+        ),
+    }
+
+    found: Final = blind_spots(sources)
+
+    assert collisions(sources) == ()
+    assert len(found) == 1
+    assert "a matrix `exclude` is itself an expression" in found[0]
+
+
+def test_an_include_that_is_itself_an_expression_is_reported_rather_than_ignored() -> None:
+    sources: Final = {
+        "a.yml": (
+            "on: pull_request\njobs:\n  one:\n    name: build-${{ matrix.python }}\n    runs-on: ubuntu-latest\n"
+            "    strategy:\n      matrix:\n        python: ['3.11']\n"
+            "        include: ${{ fromJson(vars.EXTRA) }}\n"
+        ),
+    }
+
+    found: Final = blind_spots(sources)
+
+    assert collisions(sources) == ()
+    assert len(found) == 1
+    assert "a matrix `include` is itself an expression" in found[0]