RFC: Allow indexing with non-static ranges to produce static arrays

src/indexing.jl

@@ -75,7 +75,7 @@ end
 @inline index_size(::Size, ::Int) = Size()
 @inline index_size(::Size, a::StaticArray) = Size(a)
 @inline index_size(s::Size, ::Colon) = s
-@inline index_size(s::Size, a::SOneTo{n}) where n = Size(n,)
+@inline index_size(::Size, a::AbstractRange{<:Integer}) = Size(length(a),)
 
 @inline index_sizes(::S, inds...) where {S<:Size} = map(index_size, unpack_size(S), inds)
 
@@ -92,9 +92,9 @@ linear_index_size(ind_sizes::Type{<:Size}...) = _linear_index_size((), ind_sizes
 @inline _linear_index_size(t::Tuple, ::Type{Size{S}}, ind_sizes...) where {S} = _linear_index_size((t..., prod(S)), ind_sizes...)
 
 _ind(i::Int, ::Int, ::Type{Int}) = :(inds[$i])
-_ind(i::Int, j::Int, ::Type{<:StaticArray}) = :(inds[$i][$j])
 _ind(i::Int, j::Int, ::Type{Colon}) = j
 _ind(i::Int, j::Int, ::Type{<:SOneTo}) = j
+_ind(i::Int, j::Int, ::Type{<:AbstractArray}) = :(inds[$i][$j])
 
 ################################
 ## Non-scalar linear indexing ##
@@ -215,7 +215,7 @@ end
 
 # getindex
 
-@propagate_inbounds function getindex(a::StaticArray, inds::Union{Int, StaticArray{<:Tuple, Int}, SOneTo, Colon}...)
+@propagate_inbounds function getindex(a::StaticArray, inds::Union{Int, StaticArray{<:Tuple, Int}, AbstractRange, Colon}...)
     _getindex(a, index_sizes(Size(a), inds...), inds)
 end
 

test/abstractarray.jl

@@ -85,7 +85,7 @@ using StaticArrays, Test, LinearAlgebra
         @test similar(v, SOneTo(3), SOneTo(4)) isa MMatrix{3,4,Int}
         @test similar(v, 3, SOneTo(4)) isa Matrix
 
-        @test m[:, 1:2] isa Matrix
+        @test m[:, 1:2] isa SMatrix{2, 2, Int}
         @test m[:, [true, false, false]] isa Matrix
         @test m[:, SOneTo(2)] isa SMatrix{2, 2, Int}
         @test m[:, :] isa SMatrix{2, 3, Int}

test/indexing.jl

@@ -223,4 +223,34 @@ using StaticArrays, Test
         @test eltype(Bvv) == Int
         @test Bvv[:] == [B[1,2,3,4], B[1,1,3,4]]
     end
+
+    @testset "Indexing with constants" begin
+        function SVector_UnitRange()
+            x = SA[1, 2, 3]
+            x[2:end]
+        end
+        @test SVector_UnitRange() === SA[2, 3]
+        VERSION ≥ v"1.2" && @test_const_fold SVector_UnitRange()
+
+        function SVector_StepRange()
+            x = SA[1, 2, 3, 4]
+            x[1:2:end]
+        end
+        @test SVector_StepRange() === SA[1, 3]
+        VERSION ≥ v"1.2" && @test_const_fold SVector_StepRange()
+
+        function SMatrix_UnitRange_UnitRange()
+            x = SA[1 2 3; 4 5 6]
+            x[1:2, 2:end]
+        end
+        @test SMatrix_UnitRange_UnitRange() === SA[2 3; 5 6]
+        VERSION ≥ v"1.2" && @test_const_fold SMatrix_UnitRange_UnitRange()
+
+        function SMatrix_StepRange_StepRange()
+            x = SA[1 2 3; 4 5 6]
+            x[1:1:2, 1:2:end]
+        end
+        @test SMatrix_StepRange_StepRange() === SA[1 3; 4 6]
+        VERSION ≥ v"1.2" && @test_const_fold SMatrix_StepRange_StepRange()
+    end
 end

test/testutil.jl

@@ -96,6 +96,45 @@ should_not_be_inlined(x) = _should_not_be_inlined(x)
 end
 
 
+"""
+    @test_const_fold f(args...)
+
+Test that constant folding works with a function call `f(args...)`.
+"""
+macro test_const_fold(ex)
+    quote
+        ci, = $(esc(:($InteractiveUtils.@code_typed optimize = true $ex)))
+        @test $(esc(ex)) == constant_return(ci)
+    end
+end
+
+struct NonConstantValue end
+
+function constant_return(ci)
+    if :rettype in fieldnames(typeof(ci))
+        ci.rettype isa Core.Compiler.Const && return ci.rettype.val
+        return NonConstantValue()
+    else
+        # for julia < 1.2
+        ex = ci.code[end]
+        Meta.isexpr(ex, :return) || return NonConstantValue()
+        val = ex.args[1]
+        return val isa QuoteNode ? val.value : val
+    end
+end
+
+@testset "@test_const_fold" begin
+    should_const_fold() = (1, 2, 3)
+    @test_const_fold should_const_fold()
+
+    x = Ref(1)
+    should_not_const_fold() = x[]
+    ts = @testset ErrorCounterTestSet "" begin
+        @test_const_fold should_not_const_fold()
+    end
+    @test ts.errorcount == 0 && ts.failcount == 1 && ts.passcount == 0
+end
+
 """
     @inferred_maybe_allow allow ex