Fix argument-formatter. (#288)

* Check that we never get to _afmt on CI

* Add failing test

* Fix string formatter

* Fix formatting

test/test_check_arities.py

@@ -68,3 +68,19 @@ def test_complex_arities():
 
     with pytest.raises(ArityMismatch):
         compute_form_data(inner(conj(v), u) * dx, complex_mode=True)
+
+
+def test_product_arity():
+    cell = tetrahedron
+    D = Mesh(FiniteElement("Lagrange", cell, 1, (3,), identity_pullback, H1))
+    V = FunctionSpace(D, FiniteElement("Lagrange", cell, 2, (3,), identity_pullback, H1))
+    v = TestFunction(V)
+    u = TrialFunction(V)
+
+    with pytest.raises(ArityMismatch):
+        F = inner(u, u) * dx
+        compute_form_data(F, complex_mode=True)
+
+    with pytest.raises(ArityMismatch):
+        L = inner(v, v) * dx
+        compute_form_data(L, complex_mode=False)

ufl/algorithms/check_arities.py

@@ -1,6 +1,7 @@
 """Check arities."""
 
 from itertools import chain
+from typing import Tuple
 
 from ufl.classes import Argument, Zero
 from ufl.corealg.map_dag import map_expr_dag
@@ -14,9 +15,10 @@ class ArityMismatch(BaseException):
     pass
 
 
-def _afmt(atuple):
+def _afmt(atuple: Tuple[Argument, bool]) -> str:
     """Return a string representation of an arity tuple."""
-    return tuple(f"conj({arg})" if conj else str(arg) for arg, conj in atuple)
+    arg, conj = atuple
+    return f"conj({arg})" if conj else str(arg)
 
 
 class ArityChecker(MultiFunction):