test(materials): add material field trait tests

test/materials/test_field_traits.jl

@@ -0,0 +1,24 @@
+# SPDX-FileCopyrightText: 2015-2026 Jukka Aho
+# SPDX-License-Identifier: MIT
+
+using Test
+using JuliaFEM
+using Tensors
+
+@testset "Material field traits" begin
+    E3 = required_material_fields(Elasticity{3}())
+    E2 = required_material_fields(Elasticity{2}())
+    @test E2 isa DataType
+    z3 = create_zero_field(E3)
+    @test z3.σ == zero(z3.σ)
+
+    T3 = required_material_fields(Thermal{3}())
+    zt = create_zero_field(T3)
+    @test zt.q == zero(zt.q)
+
+    mat = LinearElastic(E=210e9, ν=0.3)
+    @test material_field_type(mat) === required_material_fields(Elasticity{3}())
+
+    @test_throws ErrorException required_material_fields(Elasticity{1}())
+    @test_throws ErrorException required_material_fields(Thermal{1}())
+end