Default implementation of similar_type for FieldArray (#731)

This should free users from defining similar_type in many cases, though
they'll still need to do so when their type is parametric on the eltype.
(There's no general way for us to know how to reparameterize such
user-defined types.)

Author: c42f

src/FieldArray.jl

@@ -6,7 +6,15 @@ will automatically define `getindex` and `setindex!` appropriately. An immutable
 `FieldArray` will be as performant as an `SArray` of similar length and element type,
 while a mutable `FieldArray` will behave similarly to an `MArray`.
 
-For example:
+Note that you must define the fields of any `FieldArray` subtype in column major order. If you
+want to use an alternative ordering you will need to pay special attention in providing your
+own definitions of `getindex`, `setindex!` and tuple conversion.
+
+If you define a `FieldArray` which is parametric on the element type you should
+consider defining `similar_type` as in the `FieldVector` example.
+
+
+# Example
 
     struct Stiffness <: FieldArray{Tuple{2,2,2,2}, Float64, 4}
         xxxx::Float64
@@ -26,10 +34,6 @@ For example:
         xyyy::Float64
         yyyy::Float64
     end
-
- Note that you must define the fields of any `FieldArray` subtype in column major order. If you
- want to use an alternative ordering you will need to pay special attention in providing your
- own definitions of `getindex`, `setindex!` and tuple conversion.
 """
 abstract type FieldArray{N, T, D} <: StaticArray{N, T, D} end
 
@@ -41,7 +45,13 @@ will automatically define `getindex` and `setindex!` appropriately. An immutable
 `FieldMatrix` will be as performant as an `SMatrix` of similar length and element type,
 while a mutable `FieldMatrix` will behave similarly to an `MMatrix`.
 
-For example:
+Note that the fields of any subtype of `FieldMatrix` must be defined in column
+major order unless you are willing to implement your own `getindex`.
+
+If you define a `FieldMatrix` which is parametric on the element type you
+should consider defining `similar_type` as in the `FieldVector` example.
+
+# Example
 
     struct Stress <: FieldMatrix{3, 3, Float64}
         xx::Float64
@@ -67,13 +77,12 @@ For example:
      2.0  5.0  8.0
      3.0  6.0  9.0
 
-
 will give you the transpose of what the multi-argument formatting suggests. For clarity,
 you may consider using the alternative
 
-    sigma = Stress(@SArray[1.0 2.0 3.0;
-                           4.0 5.0 6.0;
-                           7.0 8.0 9.0])
+    sigma = Stress(SA[1.0 2.0 3.0;
+                      4.0 5.0 6.0;
+                      7.0 8.0 9.0])
 """
 abstract type FieldMatrix{N1, N2, T} <: FieldArray{Tuple{N1, N2}, T, 2} end
 
@@ -85,13 +94,19 @@ will automatically define `getindex` and `setindex!` appropriately. An immutable
 `FieldVector` will be as performant as an `SVector` of similar length and element type,
 while a mutable `FieldVector` will behave similarly to an `MVector`.
 
-For example:
+If you define a `FieldVector` which is parametric on the element type you
+should consider defining `similar_type` to preserve your array type through
+array operations as in the example below.
+
+# Example
 
-    struct Point3D <: FieldVector{3, Float64}
-        x::Float64
-        y::Float64
-        z::Float64
+    struct Vec3D{T} <: FieldVector{3, T}
+        x::T
+        y::T
+        z::T
     end
+
+    StaticArrays.similar_type(::Type{<:Vec3D}, ::Type{T}, s::Size{(3,)}) where {T} = Vec3D{T}
 """
 abstract type FieldVector{N, T} <: FieldArray{Tuple{N}, T, 1} end
 
@@ -109,3 +124,15 @@ end
 Base.cconvert(::Type{<:Ptr}, a::FieldArray) = Base.RefValue(a)
 Base.unsafe_convert(::Type{Ptr{T}}, m::Base.RefValue{FA}) where {N,T,D,FA<:FieldArray{N,T,D}} =
     Ptr{T}(Base.unsafe_convert(Ptr{FA}, m))
+
+# We can automatically preserve FieldArrays in array operations which do not
+# change their eltype or Size. This should cover all non-parametric FieldArray,
+# but for those which are parametric on the eltype the user will still need to
+# overload similar_type themselves.
+similar_type(::Type{A}, ::Type{T}, S::Size) where {N, T, A<:FieldArray{N, T}} =
+    _fieldarray_similar_type(A, T, S, Size(A))
+
+# Extra layer of dispatch to match NewSize and OldSize
+_fieldarray_similar_type(A, T, NewSize::S, OldSize::S) where {S} = A
+_fieldarray_similar_type(A, T, NewSize, OldSize) =
+    default_similar_type(T, NewSize, length_val(NewSize))

test/FieldMatrix.jl

@@ -13,7 +13,7 @@
                 zz::Float64
             end
 
-            StaticArrays.similar_type(::Type{Tensor3x3}, ::Type{Float64}, s::Size{(3,3)}) = Tensor3x3
+            # No need to define similar_type for non-parametric FieldMatrix (#792)
         end)
 
         p = Tensor3x3(1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0)

test/FieldVector.jl

@@ -7,7 +7,7 @@
                 z::Float64
             end
 
-            StaticArrays.similar_type(::Type{Point3D}, ::Type{Float64}, s::Size{(3,)}) = Point3D
+            # No need to define similar_type for non-parametric FieldVector (#792)
         end)
 
         p = Point3D(1.0, 2.0, 3.0)
@@ -29,6 +29,8 @@
                       0.0 0.0 2.0]
 
         @test @inferred(m*p) === Point3D(2.0, 4.0, 6.0)
+        @test @inferred(SA[2.0 0.0 0.0;
+                           0.0 2.0 0.0]*p) === SVector((2.0, 4.0))
 
         @test @inferred(similar_type(Point3D)) == Point3D
         @test @inferred(similar_type(Point3D, Float64)) == Point3D
@@ -92,4 +94,20 @@
         @test length(x[1]) == 2
         @test x.x == (1, 2)
     end
+
+    @testset "FieldVector with parametric eltype and without similar_type" begin
+        eval(quote
+            struct FVT{T} <: FieldVector{2, T}
+                x::T
+                y::T
+            end
+
+            # No similar_type defined - test fallback codepath
+        end)
+
+        @test @inferred(similar_type(FVT{Float64}, Float32)) == SVector{2,Float32} # Fallback code path
+        @test @inferred(similar_type(FVT{Float64}, Size(2))) == FVT{Float64}
+        @test @inferred(similar_type(FVT{Float64}, Size(3))) == SVector{3,Float64}
+        @test @inferred(similar_type(FVT{Float64}, Float32, Size(3))) == SVector{3,Float32}
+    end
 end