Add squeeze(f, A, dims) for reductions to drop dims

This simple definition makes it easier to write reductions that drops the dimensions over which they reduce. Fixes JuliaLang#16606, addresses part of the root issue in JuliaLang#22000.

base/abstractarraymath.jl

@@ -84,8 +84,16 @@ function _dropdims(A::AbstractArray, dims::Dims)
     end
     reshape(A, d::typeof(_sub(axes(A), dims)))
 end
+
 _dropdims(A::AbstractArray, dim::Integer) = _dropdims(A, (Int(dim),))
 
+"""
+    squeeze(f, A, dims)
+
+Compute reduction `f` over dimensions `dims` in array `A` and drop those dimensions from the result.
+"""
+squeeze(f, A::AbstractArray, dims::Union{Dims, Integer}) = squeeze(f(A, dims), dims)
+
 ## Unary operators ##
 
 conj(x::AbstractArray{<:Real}) = x

test/arrayops.jl

@@ -303,6 +303,17 @@ end
     @test_throws ArgumentError dropdims(a, dims=4)
     @test_throws ArgumentError dropdims(a, dims=6)
 
+    @test @inferred(squeeze(sum, a, 1)) == @inferred(squeeze(sum, a, (1,))) == reshape(sum(a, 1), (1, 8, 8, 1))
+    @test @inferred(squeeze(sum, a, 3)) == @inferred(squeeze(sum, a, (3,))) == reshape(sum(a, 3), (1, 1, 8, 1))
+    @test @inferred(squeeze(sum, a, 4)) == @inferred(squeeze(sum, a, (4,))) == reshape(sum(a, 4), (1, 1, 8, 1))
+    @test @inferred(squeeze(sum, a, (1, 5))) == squeeze(sum, a, (5, 1)) == reshape(sum(a, (5, 1)), (1, 8, 8))
+    @test @inferred(squeeze(sum, a, (1, 2, 5))) == squeeze(sum, a, (5, 2, 1)) == reshape(sum(a, (5, 2, 1)), (8, 8))
+    @test_throws ArgumentError squeeze(sum, a, 0)
+    @test_throws ArgumentError squeeze(sum, a, (1, 1))
+    @test_throws ArgumentError squeeze(sum, a, (1, 2, 1))
+    @test_throws ArgumentError squeeze(sum, a, (1, 1, 2))
+    @test_throws ArgumentError squeeze(sum, a, 6)
+
     sz = (5,8,7)
     A = reshape(1:prod(sz),sz...)
     @test A[2:6] == [2:6;]