Revert "Remove symbol type assertion"

This reverts commit c22ac5c.

Author: MilesCranmer

ext/DynamicExpressionsOptimExt.jl

@@ -18,7 +18,7 @@ struct ExpressionOptimizationResults{R<:OptimizationResults,N<:AbstractExpressio
     tree::N # The final expression tree
 end
 #! format: on
-function Base.getproperty(r::ExpressionOptimizationResults, s)
+function Base.getproperty(r::ExpressionOptimizationResults, s::Symbol)
     if s == :tree || s == :minimizer
         return getfield(r, :tree)
     else

src/EquationUtils.jl

@@ -111,12 +111,12 @@ struct NodeConstantRef{T,N<:AbstractExpressionNode{T}}
         return new{_T,_N}(Ref(node))
     end
 end
-function Base.getproperty(cr::NodeConstantRef{T}, s) where {T}
+function Base.getproperty(cr::NodeConstantRef{T}, s::Symbol) where {T}
     s != :x && error("Only :x is a valid property for NodeConstantRef")
 
     return getfield(cr, :_node).x.val::T
 end
-function Base.setproperty!(cr::NodeConstantRef{T}, s, v) where {T}
+function Base.setproperty!(cr::NodeConstantRef{T}, s::Symbol, v) where {T}
     s != :x && error("Only :x is a valid property for NodeConstantRef")
 
     return getfield(cr, :_node).x.val::T = v::T