Catch "symbol piracy" with LinearAlgebra

src/MatrixFields/MatrixFields.jl

@@ -43,7 +43,7 @@ multiples of `LinearAlgebra.I`. This comes with the following functionality:
 """
 module MatrixFields
 
-import LinearAlgebra: I, UniformScaling, Adjoint, AdjointAbsVec, mul!, inv, norm
+import LinearAlgebra: I, UniformScaling, Adjoint, AdjointAbsVec, mul!, inv, norm, dot
 import StaticArrays: SMatrix, SVector
 import BandedMatrices: BandedMatrix, band, _BandedMatrix
 import RecursiveArrayTools: recursive_bottom_eltype

src/MatrixFields/matrix_multiplication.jl

@@ -478,3 +478,17 @@ if hasfield(Method, :recursion_relation)
         m.recursion_relation = dont_limit
     end
 end
+
+# TODO: Explain this
+function Base.Broadcast.broadcasted(
+    ::typeof(dot),              # This is LinearAlgebra.dot
+    arg1,
+    arg2,
+)
+    args_bced = (Base.Broadcast.broadcastable(arg1), Base.Broadcast.broadcastable(arg2))
+    if eltype(args_bced[1]) <: BandMatrixRow
+        error("Detected usage of LinearAlgebra.dot in MatrixFields. Did you mean to `import ClimaCore.MatrixFields: ⋅`?")
+    else
+        Base.Broadcast.broadcasted(Base.Broadcast.combine_styles(args_bced...), dot, args_bced...)
+    end
+end

src/Operators/finitedifference.jl

@@ -3432,3 +3432,4 @@ Base.@propagate_inbounds function apply_stencil!(
     end
     return field_out
 end
+

test/MatrixFields/linearalgebra_dot.jl

@@ -0,0 +1,29 @@
+# LinearAlgebra exports ⋅ and it is easy to get confused with MatrixFields.⋅.
+# This test checks that we have a mechanism in place to inform the user when
+# they are using the wrong ⋅.
+
+import ClimaCore: Domains, Geometry, Meshes, Spaces, MatrixFields
+import ClimaCore.MatrixFields: @name
+
+import LinearAlgebra: ⋅, I
+
+@testset "LinearAlgebra dot" begin
+    n_elem_z = 2
+
+    domain = Domains.IntervalDomain(Geometry.ZPoint(0.0), Geometry.ZPoint(1.0), boundary_names = (:bottom, :top))
+    mesh = Meshes.IntervalMesh(domain, nelems = 2)
+    space = Spaces.FaceFiniteDifferenceSpace(mesh)
+
+    diverg = Operators.DivergenceC2F(; bottom = Operators.SetDivergence(0.0), top = Operators.SetDivergence(0.0))
+    grad = Operators.GradientF2C()
+
+    diverg_matrix = MatrixFields.operator_matrix(diverg)
+    grad_matrix = MatrixFields.operator_matrix(grad)
+
+    name = @name(u)
+    jacobian = MatrixFields.FieldMatrix(
+        (@name(u), @name(u)) => similar(zeros(space), ClimaCore.MatrixFields.TridiagonalMatrixRow{Float64}),
+    )
+
+    @test_throws ErrorException @. jacobian[name, name] = diverg_matrix() ⋅ grad_matrix() - (I,)
+end