diff --git a/Project.toml b/Project.toml
index 360f38ea4..f4460520b 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "Mooncake"
 uuid = "da2b9cff-9c12-43a0-ae48-6db2b0edb7d6"
 authors = ["Will Tebbutt, Hong Ge, and contributors"]
-version = "0.4.65"
+version = "0.4.66"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"
diff --git a/src/interpreter/abstract_interpretation.jl b/src/interpreter/abstract_interpretation.jl
index bd6c019c0..8a1c32ba7 100644
--- a/src/interpreter/abstract_interpretation.jl
+++ b/src/interpreter/abstract_interpretation.jl
@@ -116,12 +116,6 @@ else
     get_inference_world(interp::CC.AbstractInterpreter) = CC.get_inference_world(interp)
 end
 
-_type(x::Type) = x
-_type(x::CC.Const) = _typeof(x.val)
-_type(x::CC.PartialStruct) = x.typ
-_type(x::CC.Conditional) = Union{_type(x.thentype),_type(x.elsetype)}
-_type(::CC.PartialTypeVar) = TypeVar
-
 struct NoInlineCallInfo <: CC.CallInfo
     info::CC.CallInfo # wrapped call
     tt::Any # signature
diff --git a/src/interpreter/ir_normalisation.jl b/src/interpreter/ir_normalisation.jl
index 3d70954a4..8b9cbbaea 100644
--- a/src/interpreter/ir_normalisation.jl
+++ b/src/interpreter/ir_normalisation.jl
@@ -111,7 +111,7 @@ to be the return type given by the code cache.
 function fix_up_invoke_inference!(ir::IRCode)::IRCode
     stmts = ir.stmts
     for n in 1:length(stmts)
-        if Meta.isexpr(stmt(stmts)[n], :invoke) && _type(stmts.type[n]) == Any
+        if Meta.isexpr(stmt(stmts)[n], :invoke) && CC.widenconst(stmts.type[n]) == Any
             mi = stmt(stmts)[n].args[1]::Core.MethodInstance
             R = isdefined(mi, :cache) ? mi.cache.rettype : CC.return_type(mi.specTypes)
             stmts.type[n] = R
diff --git a/src/interpreter/ir_utils.jl b/src/interpreter/ir_utils.jl
index d4a518bb5..48334fbdd 100644
--- a/src/interpreter/ir_utils.jl
+++ b/src/interpreter/ir_utils.jl
@@ -128,7 +128,7 @@ end
 # https://gist.github.com/oxinabox/cdcffc1392f91a2f6d80b2524726d802#file-example-jl-L54
 function __get_toplevel_mi_from_ir(ir, _module::Module)
     mi = ccall(:jl_new_method_instance_uninit, Ref{Core.MethodInstance}, ())
-    mi.specTypes = Tuple{map(_type, ir.argtypes)...}
+    mi.specTypes = Tuple{map(CC.widenconst, ir.argtypes)...}
     mi.def = _module
     return mi
 end
diff --git a/src/interpreter/s2s_reverse_mode_ad.jl b/src/interpreter/s2s_reverse_mode_ad.jl
index 77afbe2c4..b4b4db970 100644
--- a/src/interpreter/s2s_reverse_mode_ad.jl
+++ b/src/interpreter/s2s_reverse_mode_ad.jl
@@ -167,12 +167,13 @@ end
 # ADInfo struct for information regarding `interp` and `debug_mode`.
 function ADInfo(interp::MooncakeInterpreter, ir::BBCode, debug_mode::Bool)
     arg_types = Dict{Argument,Any}(
-        map(((n, t),) -> (Argument(n) => _type(t)), enumerate(ir.argtypes))
+        map(((n, t),) -> (Argument(n) => CC.widenconst(t)), enumerate(ir.argtypes))
     )
     stmts = collect_stmts(ir)
     ssa_insts = Dict{ID,NewInstruction}(stmts)
     is_used_dict = characterise_used_ids(stmts)
-    zero_lazy_rdata_ref = Ref{Tuple{map(lazy_zero_rdata_type ∘ _type, ir.argtypes)...}}()
+    Tlazy_rdata_ref = Tuple{map(lazy_zero_rdata_type ∘ CC.widenconst, ir.argtypes)...}
+    zero_lazy_rdata_ref = Ref{Tlazy_rdata_ref}()
     return ADInfo(
         interp, arg_types, ssa_insts, is_used_dict, debug_mode, zero_lazy_rdata_ref
     )
@@ -209,7 +210,7 @@ is_used(info::ADInfo, id::ID)::Bool = info.is_used_dict[id]
 Returns the static / inferred type associated to `x`.
 """
 get_primal_type(info::ADInfo, x::Argument) = info.arg_types[x]
-get_primal_type(info::ADInfo, x::ID) = _type(info.ssa_insts[x].type)
+get_primal_type(info::ADInfo, x::ID) = CC.widenconst(info.ssa_insts[x].type)
 get_primal_type(::ADInfo, x::QuoteNode) = _typeof(x.value)
 get_primal_type(::ADInfo, x) = _typeof(x)
 function get_primal_type(::ADInfo, x::GlobalRef)
@@ -238,10 +239,10 @@ Create the statements which initialise the reverse-data `Ref`s.
 function reverse_data_ref_stmts(info::ADInfo)
     return vcat(
         map(collect(info.arg_rdata_ref_ids)) do (k, id)
-            (id, new_inst(Expr(:call, __make_ref, _type(info.arg_types[k]))))
+            (id, new_inst(Expr(:call, __make_ref, CC.widenconst(info.arg_types[k]))))
         end,
         map(collect(info.ssa_rdata_ref_ids)) do (k, id)
-            (id, new_inst(Expr(:call, __make_ref, _type(info.ssa_insts[k].type))))
+            (id, new_inst(Expr(:call, __make_ref, CC.widenconst(info.ssa_insts[k].type))))
         end,
     )
 end
@@ -462,7 +463,7 @@ function make_ad_stmts!(stmt::PiNode, line::ID, info::ADInfo)
         P = get_primal_type(info, line)
         val_rdata_ref_id = get_rev_data_id(info, stmt.val)
         output_rdata_ref_id = get_rev_data_id(info, line)
-        fwds = PiNode(__inc(stmt.val), fcodual_type(_type(stmt.typ)))
+        fwds = PiNode(__inc(stmt.val), fcodual_type(CC.widenconst(stmt.typ)))
         rvs = Expr(:call, __pi_rvs!, P, val_rdata_ref_id, output_rdata_ref_id)
     else
         # If the value of the PiNode is a constant / QuoteNode etc, then there is nothing to
@@ -470,7 +471,7 @@ function make_ad_stmts!(stmt::PiNode, line::ID, info::ADInfo)
         const_id = ID()
         fwds = [
             (const_id, new_inst(const_codual_stmt(stmt.val, info))),
-            (line, new_inst(PiNode(const_id, fcodual_type(_type(stmt.typ))))),
+            (line, new_inst(PiNode(const_id, fcodual_type(CC.widenconst(stmt.typ))))),
         ]
         rvs = nothing
     end
@@ -935,7 +936,7 @@ function rule_type(interp::MooncakeInterpreter{C}, sig_or_mi; debug_mode) where
     Treturn = Base.Experimental.compute_ir_rettype(ir)
     isva, _ = is_vararg_and_sparam_names(sig_or_mi)
 
-    arg_types = map(_type, ir.argtypes)
+    arg_types = map(CC.widenconst, ir.argtypes)
     sig = Tuple{arg_types...}
     arg_fwds_types = Tuple{map(fcodual_type, arg_types)...}
     arg_rvs_types = Tuple{map(rdata_type ∘ tangent_type, arg_types)...}
@@ -1281,7 +1282,7 @@ function forwards_pass_ir(
     end
 
     # Create and return the `BBCode` for the forwards-pass.
-    arg_types = vcat(Tshared_data, map(fcodual_type ∘ _type, ir.argtypes))
+    arg_types = vcat(Tshared_data, map(fcodual_type ∘ CC.widenconst, ir.argtypes))
     ir = BBCode(vcat(entry_block, blocks), arg_types, ir.sptypes, ir.linetable, ir.meta)
     return remove_unreachable_blocks!(ir)
 end
diff --git a/test/front_matter.jl b/test/front_matter.jl
index be56bdfe9..636684a10 100644
--- a/test/front_matter.jl
+++ b/test/front_matter.jl
@@ -63,8 +63,7 @@ using Mooncake:
     verify_rdata_value,
     is_primitive,
     MinimalCtx,
-    stmt,
-    _type
+    stmt
 
 using .TestUtils:
     test_rule,
diff --git a/test/interpreter/abstract_interpretation.jl b/test/interpreter/abstract_interpretation.jl
index 59a2a7792..3f29702d3 100644
--- a/test/interpreter/abstract_interpretation.jl
+++ b/test/interpreter/abstract_interpretation.jl
@@ -70,8 +70,4 @@ contains_primitive_behind_call(x) = @inline contains_primitive(x)
             @test stmt(ad_ir.stmts)[invoke_line].args[2] == GlobalRef(Main, :a_primitive)
         end
     end
-    @testset "_type" begin
-        @test _type(CC.Const(5.0)) === Float64
-        @test _type(CC.PartialTypeVar(TypeVar(:a, Union{}, Any), true, true)) === TypeVar
-    end
 end