Solve all problems with extra indirection

The `getindex` and `setindex!` methods for `Trues` are limited
while also risking  ambiguities. This replaces those definitions
with a specialization for `to_indices` that avoids such problems.

Closes #162

src/trues.jl

@@ -22,33 +22,5 @@ const Falses = Zeros{Bool, N, Axes} where {N, Axes}
 
 
 # y[mask] = x when mask isa Trues (cf y[:] = x)
-# Supported here only for arrays with standard OneTo axes.
-function Base.setindex!(y::AbstractArray{T,N}, x, mask::Trues{N, NTuple{N,Base.OneTo{Int}}}) where {T,N}
-    if axes(x) isa NTuple{N,Base.OneTo{Int}} && axes(y) isa NTuple{N,Base.OneTo{Int}}
-        @boundscheck size(y) == size(mask) || throw(BoundsError(y, mask))
-        @boundscheck size(x) == size(mask) || throw(DimensionMismatch(
-            "tried to assign $(length(x)) elements to $(length(y)) destinations"))
-        @boundscheck checkbounds(y, mask)
-        return copyto!(y, x)
-    end
-    return setindex!(y, x, trues(size(mask))) # fall back on usual setindex!
-end
-
-# x[mask] when mask isa Trues (cf x[trues(size(x))] or x[:])
-# Supported here only for arrays with standard OneTo axes.
-function Base.getindex(x::AbstractArray{T,N}, mask::Trues{N, NTuple{N,Base.OneTo{Int}}}) where {T,N}
-    if axes(x) isa NTuple{N,Base.OneTo{Int}}
-       @boundscheck size(x) == size(mask) || throw(BoundsError(x, mask))
-       return vec(x)
-    end
-    return x[trues(size(x))] # else revert to usual getindex method
-end
-
-# https://github.com/JuliaArrays/FillArrays.jl/issues/148 and 150
-function Base.getindex(
-    a::AbstractFill{T, N, Tuple{Vararg{Base.OneTo{Int}, N}}},
-    b::Trues{N, Tuple{Vararg{Base.OneTo{Int}, N}}},
-) where {T, N}
-    @boundscheck size(a) == size(b) || throw(BoundsError(a, b))
-    return Fill(getindex_value(a), length(a))
-end
+
+Base.to_indices(A::AbstractArray{T,N}, I::Trues{N}) where {T,N} = LinearIndices(A)