From 1b638d6cbee4045d76899d83983660d2d86941b5 Mon Sep 17 00:00:00 2001
From: FBumann <117816358+FBumann@users.noreply.github.com>
Date: Fri, 28 Aug 2026 12:44:02 +0200
Subject: [PATCH] test: a program node nothing produces is caught, rather than
 waiting for a consumer to meet it

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 tests/fixtures/every_program_node.yaml | 42 +++++++++++++++++
 tests/test_program_nodes.py            | 62 ++++++++++++++++++++++++++
 2 files changed, 104 insertions(+)
 create mode 100644 tests/fixtures/every_program_node.yaml
 create mode 100644 tests/test_program_nodes.py

diff --git a/tests/fixtures/every_program_node.yaml b/tests/fixtures/every_program_node.yaml
new file mode 100644
index 00000000..63b133f5
--- /dev/null
+++ b/tests/fixtures/every_program_node.yaml
@@ -0,0 +1,42 @@
+# SPDX-FileCopyrightText: math-spec Contributors
+#
+# SPDX-License-Identifier: MIT
+
+description: every node a program can carry, in one file that lowers
+dimensions:
+  t: { dtype: int }
+  g: { dtype: str }
+  zone: { dtype: str }
+lookups:
+  zone_of: { over: g, into: zone }
+parameters:
+  cost: { dims: [g] }
+  load: { dims: [zone] }
+  rate: { dims: [] }
+  lead: { dims: [g], dtype: int }
+  width: { dims: [g], dtype: int }
+variables:
+  p: { foreach: [t, g], bounds: { lower: 0, upper: 100 } }
+  q: { foreach: [zone], bounds: { lower: 0, upper: 100 } }
+constraints:
+  arithmetic:
+    foreach: [t, g]
+    expression: "-p + p * cost - p / cost + rate ** 2 <= 1"
+  reduced:
+    foreach: []
+    expression: "sum(p) >= 0"
+  grouped:
+    foreach: [t, zone]
+    expression: "sum(p, by=zone_of) - q <= load"
+  pulled_back:
+    foreach: [t, g]
+    expression: "p - at(q, by=zone_of) <= 0"
+  translated:
+    foreach: [t, g]
+    expression: "p - shift(p, over=t, offset=lead, edge='wrap') <= 0"
+  windowed:
+    foreach: [t, g]
+    expression: "sum_back(p, over=t, within=width) <= 10"
+objective:
+  sense: minimize
+  expression: "sum(p * cost)"
diff --git a/tests/test_program_nodes.py b/tests/test_program_nodes.py
new file mode 100644
index 00000000..da38e93e
--- /dev/null
+++ b/tests/test_program_nodes.py
@@ -0,0 +1,62 @@
+# SPDX-FileCopyrightText: math-spec Contributors
+#
+# SPDX-License-Identifier: MIT
+
+"""Every node a `Program` can carry is one some file actually lowers to.
+
+The sibling of `test_the_golden_model_carries_every_node_kind_the_walk_renders`,
+one state along. That one holds the *renderer* to the AST; this holds the
+*lowering* to the program, and the two cannot share a fixture: rendering
+accepts more than lowering does, so the golden model carries a shift over a
+variable-free expression that lowering refuses outright.
+
+Without this, a node can join `ExpressionNode` with nothing producing it and
+the suite stays green — `assert_never` fires only where some test happens to
+lower a file that uses the construct. That is how `cases:` reached a release
+candidate unlowerable.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import get_args
+
+import math_spec as ms
+from math_spec.program import ExpressionNode, Program, walk
+
+FIXTURE = Path(__file__).resolve().parent / 'fixtures' / 'every_program_node.yaml'
+
+
+def _expressions(program: Program) -> list[ExpressionNode]:
+    """Every tree a program hangs on to, wherever it hangs it.
+
+    Bounds and named expressions among them: a node reachable only from a
+    bound is still a node a consumer has to build.
+    """
+    trees = [side for c in program.constraints for side in (c.lhs, c.rhs)]
+    trees += [bound for v in program.variables for bound in (v.lower, v.upper)]
+    trees += list(program.expressions.values())
+    if program.objective is not None:
+        trees.append(program.objective.expression)
+    return trees
+
+
+def test_every_program_node_is_one_some_file_lowers_to():
+    """A node nothing produces is a node no consumer has been asked to build."""
+    program = ms.to_program(FIXTURE)
+    reached = {type(node).__name__ for node in walk(*_expressions(program))}
+    declared = {node.__name__ for node in get_args(ExpressionNode)}
+
+    assert declared <= reached, (
+        f'{FIXTURE.name} lowers to none of {sorted(declared - reached)}. A node no file reaches is '
+        f'one whose lowering nobody has run — add a declaration using the construct it stands for.'
+    )
+
+
+def test_the_fixture_carries_nothing_the_program_has_no_node_for():
+    """The other direction, so the fixture cannot drift into asserting nothing."""
+    program = ms.to_program(FIXTURE)
+    reached = {type(node).__name__ for node in walk(*_expressions(program))}
+    declared = {node.__name__ for node in get_args(ExpressionNode)}
+
+    assert reached <= declared, f'{FIXTURE.name} lowers to {sorted(reached - declared)}, which is not a program node'