Add error hint on implicit Float conversion from Num

src/Symbolics.jl

@@ -143,6 +143,8 @@ export lambertw
 include("parsing.jl")
 export parse_expr_to_symbolic
 
+include("error_hints.jl")
+
 # Hacks to make wrappers "nicer"
 const NumberTypes = Union{AbstractFloat,Integer,Complex{<:AbstractFloat},Complex{<:Integer}}
 (::Type{T})(x::SymbolicUtils.Symbolic) where {T<:NumberTypes} = throw(ArgumentError("Cannot convert Sym to $T since Sym is symbolic and $T is concrete. Use `substitute` to replace the symbolic unwraps."))

src/error_hints.jl

@@ -0,0 +1,52 @@
+const FLOAT_NUM_CONVERSION_ERROR = """
+
+An implict conversion of symbolic Num into a Float64 was encountered. Common causes for this error include:
+
+1. A substitution resulted in numerical values, but is not automatically converted to numerical values. For example:
+
+```julia
+@variables x y
+result = [1.0]
+A = [x x^2
+     y+x 2x + y]
+sub = substitute(A, Dict([x=>2.0, y=>2]))
+result[1] = sub[1]
+```
+
+The reason is because the result of `sub` is still symbolic:
+
+```julia
+julia> sub = substitute(A, Dict([x=>2.0, y=>2]))
+2×2 Matrix{Num}:
+ 2.0  4.0
+ 4.0  6.0
+```
+
+Notice that the return is a `Matrix{Num}`, not a `Matrix{Float64}`. To fix this, ensure that the type is converted, i.e.
+`sub = Symbolics.unwrap.(substitute(A, Dict([x=>2.0, y=>2])))`, or `result[1] = Symbolics.unwrap(sub[1])`.
+
+2. Captured symbolic values inside of registered functions. An example of this version of the error is the following:
+
+```julia
+@variables x y
+ff(x) = x + y
+@register_symbolic ff(x)
+result = [0.0]
+result[1] = eval(build_function(ff(x),[x,y]))(1.0) # Error
+```
+
+Notice that the is the fact that the generated function call `eval(build_function(ff(x),x))(1.0)` returns a symbolic
+value (`1.0 + y`), not a numerical value. This is because the value `y` is a global symbol enclosed into `ff`. To fix
+this, ensure that any symbolic value that is used in registered functions is passed in, i.e. :
+
+```julia
+ff(x) = x + y
+@register_symbolic ff(x)
+```
+"""
+
+Base.Experimental.register_error_hint(MethodError) do io, e, args, kwargs
+  if e isa MethodError && Num in args && e.f <: Number
+      println(io, FLOAT_NUM_CONVERSION_ERROR)
+  end
+end