Only allow logical indexing with vectors or...

a single index that matches the size of the array it indexes into

base/abstractarray.jl

@@ -104,7 +104,7 @@ function checkbounds(::Type{Bool}, sz::Integer, r::Range)
     @_propagate_inbounds_meta
     isempty(r) | (checkbounds(Bool, sz, first(r)) & checkbounds(Bool, sz, last(r)))
 end
-checkbounds(::Type{Bool}, sz::Integer, I::AbstractArray{Bool}) = length(I) == sz
+checkbounds{N}(::Type{Bool}, sz::Integer, I::AbstractArray{Bool,N}) = N == 1 && length(I) == sz
 function checkbounds(::Type{Bool}, sz::Integer, I::AbstractArray)
     @_inline_meta
     b = true

base/multidimensional.jl

@@ -192,7 +192,7 @@ using .IteratorsMD
 # improvement over the general definitions in abstractarray.jl
 for N = 1:5
     args = [:($(symbol(:I, d))) for d = 1:N]
-    targs = [:($(symbol(:I, d))::Union{Colon,Number,AbstractVector}) for d = 1:N]  # prevent co-opting the CartesianIndex version
+    targs = [:($(symbol(:I, d))::Union{Colon,Number,AbstractArray}) for d = 1:N]  # prevent co-opting the CartesianIndex version
     exs = [:(checkbounds(Bool, size(A, $d), $(args[d]))) for d = 1:N]
     cbexpr = exs[1]
     for d = 2:N

doc/manual/arrays.rst

@@ -264,7 +264,7 @@ where each ``I_k`` may be:
 2. A ``Range`` of the form ``a:b``, or ``a:b:c``
 3. A ``:`` or ``Colon()`` to select entire dimensions
 4. An arbitrary integer array, including the empty array ``[]``
-5. A boolean array to select elements at its ``true`` indices
+5. A boolean array to select a vector of elements at its ``true`` indices
 
 If all the indices are scalars, then the result ``X`` is a single element from
 the array ``A``. Otherwise, ``X`` is an array with the same number of
@@ -282,11 +282,14 @@ indexed with scalars are dropped. For example, the result of ``A[2, I, 3]`` is
 an array with size ``size(I)``. Its ``i``\ th element is populated by
 ``A[2, I[i], 3]``.
 
-Boolean arrays must be the same length as the dimension they are indexing into.
-Indexing by a boolean array ``B`` is the same as indexing by the vector that is
-returned by ``find(B)``; the size of a dimension indexed by a boolean array will
-be the number of true values in the vector. It is generally more efficient to
-use boolean arrays as indices directly instead of first calling ``find``.
+Indexing by a boolean array ``B`` is effectively the same as indexing by the
+vector that is returned by :func:`find(B) <find>`. Often referred to as logical
+indexing, this selects elements at the indices where the values are ``true``,
+akin to a mask. A logical index must be a vector of the same length as the
+dimension it indexes into, or it must be the only index provided and match the
+size and dimensionality of the array it indexes into. It is generally more
+efficient to use boolean arrays as indices directly instead of first calling
+:func:`find`.
 
 Additionally, single elements of a multidimensional array can be indexed as
 ``x = A[I]``, where ``I`` is a ``CartesianIndex``. It effectively behaves like