rework type parameter stripping (#78)

Fixes #69

Author: nsajko

src/FixedSizeArray.jl

@@ -68,7 +68,9 @@ end
 Base.size(a::FixedSizeArray) = a.size
 
 function Base.similar(::T, ::Type{E}, size::NTuple{N,Int}) where {T<:FixedSizeArray,E,N}
-    with_replaced_parameters(DenseArray, T, Val(E), Val(N))(undef, size)
+    spec = TypeParametersElementTypeAndDimensionality()
+    S = val_parameter(with_stripped_type_parameters(spec, T)){E, N}
+    S(undef, size)
 end
 
 Base.isassigned(a::FixedSizeArray, i::Int) = isassigned(a.mem, i)
@@ -106,7 +108,8 @@ end
 # broadcasting
 
 function Base.BroadcastStyle(::Type{T}) where {T<:FixedSizeArray}
-    Broadcast.ArrayStyle{stripped_type(DenseArray, T)}()
+    spec = TypeParametersElementTypeAndDimensionality()
+    Broadcast.ArrayStyle{val_parameter(with_stripped_type_parameters(spec, T))}()
 end
 
 function Base.similar(
@@ -118,30 +121,58 @@ end
 
 # helper functions
 
-normalized_type(::Type{T}) where {T} = T
+val_parameter(::Val{P}) where {P} = P
 
-function stripped_type_unchecked(::Type{DenseVector}, ::Type{<:GenericMemory{K,<:Any,AS}}) where {K,AS}
-    GenericMemory{K,<:Any,AS}
-end
+struct TypeParametersElementType end
+struct TypeParametersElementTypeAndDimensionality end
+
+"""
+    with_stripped_type_parameters_unchecked(spec, t::Type)::Val{s}
 
-Base.@assume_effects :consistent function stripped_type_unchecked(
-    ::Type{DenseArray}, ::Type{<:FixedSizeArray{<:Any,<:Any,V}},
-) where {V}
-    U = stripped_type(DenseVector, V)
-    FixedSizeArray{E,N,U{E}} where {E,N}
+An implementation detail of [`with_stripped_type_parameters`](@ref). Don't call
+directly.
+"""
+function with_stripped_type_parameters_unchecked end
+
+function with_stripped_type_parameters_unchecked(::TypeParametersElementType, ::Type{<:(GenericMemory{K, T, AS} where {T})}) where {K, AS}
+    s = GenericMemory{K, T, AS} where {T}
+    Val{s}()
 end
 
-function stripped_type(::Type{T}, ::Type{S}) where {T,S<:T}
-    ret = stripped_type_unchecked(T, S)::Type{<:T}::UnionAll
-    S::Type{<:ret}
-    normalized_type(ret)  # ensure `UnionAll` type variable order is normalized
+# `Base.@assume_effects :consistent` is a workaround for:
+# https://github.com/JuliaLang/julia/issues/56966
+Base.@assume_effects :consistent function with_stripped_type_parameters_unchecked(::TypeParametersElementTypeAndDimensionality, ::Type{<:(FixedSizeArray{T, N, Mem} where {T, N})}) where {Mem}
+    spec_mem = TypeParametersElementType()
+    mem_v = with_stripped_type_parameters(spec_mem, Mem)
+    mem = val_parameter(mem_v)
+    s = FixedSizeArray{T, N, mem{T}} where {T, N}
+    Val{s}()
 end
 
-function with_replaced_parameters(::Type{T}, ::Type{S}, ::Val{P1}, ::Val{P2}) where {T,S<:T,P1,P2}
-    t = T{P1,P2}::Type{<:T}
-    s = stripped_type(T, S)
-    S::Type{<:s}
-    s{P1,P2}::Type{<:s}::Type{<:T}::Type{<:t}
+"""
+    with_stripped_type_parameters(spec, t::Type)::Val{s}
+
+The type `s` is a `UnionAll` supertype of `t`:
+
+```julia
+(s isa UnionAll) && (t <: s)
+```
+
+Furthermore, `s` has type variables in place of the type parameters specified
+via `spec`.
+
+NB: `Val{s}()` is returned instead of `s` so the method would be *consistent*
+from the point of view of Julia's effect inference, enabling constant folding.
+
+NB: this function is supposed to only have the one method. To add
+functionality, add methods to [`with_stripped_type_parameters_unchecked`](@ref).
+"""
+function with_stripped_type_parameters(spec, t::Type)
+    ret = with_stripped_type_parameters_unchecked(spec, t)
+    s = val_parameter(ret)
+    s = s::UnionAll
+    s = s::(Type{T} where {T>:t})
+    Val{s}()
 end
 
 dimension_count_of(::Base.SizeUnknown) = 1