fix: handle unsized array symbolics in linear_expansion

Author: AayushSabharwal

src/linear_algebra.jl

@@ -328,6 +328,7 @@ function _linear_expansion(t, x)
     elseif op === getindex
         arrt, idxst... = arguments(t)
         isequal(arrt, arrx) && return (0, t, true)
+        shape(arrt) == Unknown() && return (0, t, true)
 
         indexed_t = OffsetArrays.Origin(map(first, axes(arrt)))(Symbolics.scalarize(arrt))[idxst...]
         # when indexing a registered function/callable symbolic

test/linear_solver.jl

@@ -77,4 +77,8 @@ a, b, islinear = Symbolics.linear_expansion(D(x) - x, x)
     @variables x[0:2]
     a, b, islin = Symbolics.linear_expansion(x[0] - z(x[1]), z(x[1]))
     @test islin && isequal(a, -1) && isequal(b, x[0])
+
+    @variables x::Vector{Real}
+    a, b, islin = Symbolics.linear_expansion(x[0] - z(x[1]), z(x[1]))
+    @test islin && isequal(a, -1) && isequal(b, x[0])
 end