Catch attempts to compose a listsam/compose-list-exception

Attempting to compose a list property would result in an unhandled
exception:

    AttributeError: 'CommentedSeq' object has no attribute 'get'

We now at least detect the situation and produce an exception that
the frontend will report neatly:

    Error loading pipeline: element.bst [line 11 column 4]: Only values of type 'dict' can be composed.

I was getting this error from an attempt to conditionally extend the
sources list:

    sources:
      (?):
        arch == "x86_64":
          - url: http://example.com/x86_64

The correct way to do this is to move the conditional into the parent
dict, e.g.:

    (?):
      arch == "x86_64":
        sources:
          - url: https://example.com/x86_64

It would be nice if the error message could hint at how the user can do
what they want, but it doesn't seem possible in this case.

3 files changed, 28 insertions, 0 deletions

diff --git a/tests/format/list-directive-type-error/element.bst b/tests/format/list-directive-type-error/element.bst
new file mode 100644
index 000000000..4ac7d95bb
--- /dev/null
+++ b/tests/format/list-directive-type-error/element.bst
@@ -0,0 +1,6 @@
+kind: autotools
+
+sources:
+  (?):
+    - arch == "x86_64":
+      - url: https://example.com/x86_64
diff --git a/tests/format/list-directive-type-error/project.conf b/tests/format/list-directive-type-error/project.conf
new file mode 100644
index 000000000..dde56d7b8
--- /dev/null
+++ b/tests/format/list-directive-type-error/project.conf
@@ -0,0 +1,7 @@
+name: test
+
+options:
+  arch:
+    type: arch
+    description: Example architecture option
+    values: [ x86_32, x86_64 ]
diff --git a/tests/format/listdirectiveerrors.py b/tests/format/listdirectiveerrors.py
index 001b8a49d..9f2cfaaca 100644
--- a/tests/format/listdirectiveerrors.py
+++ b/tests/format/listdirectiveerrors.py
@@ -39,3 +39,18 @@ def test_element_error(cli, datafiles, target):
     assert result.exception
     assert isinstance(result.exception, LoadError)
     assert result.exception.reason == LoadErrorReason.TRAILING_LIST_DIRECTIVE
+
+
+@pytest.mark.datafiles(DATA_DIR)
+def test_project_error(cli, datafiles):
+    project = os.path.join(datafiles.dirname, datafiles.basename, 'list-directive-type-error')
+    result = cli.run(project=project, silent=True, args=[
+        'show',
+        '--deps', 'none',
+        '--format', '%{vars}',
+        'element.bst'])
+
+    assert result.exit_code != 0
+    assert result.exception
+    assert isinstance(result.exception, LoadError)
+    assert result.exception.reason == LoadErrorReason.ILLEGAL_COMPOSITE