Improve inferability of shape::Dims for cat (#39294)

`cat` is often called with Varargs or heterogenous inputs,
and inference almost always fails. Even when all the arrays
are of the same type, if the number of varargs isn't known
inference typically fails. The culprit is probably #36454.

This reduces the number of failures considerably, by avoiding
creation of vararg length tuples in the shape-inference pipeline.

base/abstractarray.jl

@@ -1580,6 +1580,7 @@ cat_indices(A::AbstractArray, d) = axes(A, d)
 cat_similar(A, ::Type{T}, shape) where T = Array{T}(undef, shape)
 cat_similar(A::AbstractArray, ::Type{T}, shape) where T = similar(A, T, shape)
 
+# These are for backwards compatibility (even though internal)
 cat_shape(dims, shape::Tuple{Vararg{Int}}) = shape
 function cat_shape(dims, shapes::Tuple)
     out_shape = ()
@@ -1588,6 +1589,11 @@ function cat_shape(dims, shapes::Tuple)
     end
     return out_shape
 end
+# The new way to compute the shape (more inferrable than combining cat_size & cat_shape, due to Varargs + issue#36454)
+cat_size_shape(dims) = ntuple(zero, Val(length(dims)))
+@inline cat_size_shape(dims, X, tail...) = _cat_size_shape(dims, _cshp(1, dims, (), cat_size(X)), tail...)
+_cat_size_shape(dims, shape) = shape
+@inline _cat_size_shape(dims, shape, X, tail...) = _cat_size_shape(dims, _cshp(1, dims, shape, cat_size(X)), tail...)
 
 _cshp(ndim::Int, ::Tuple{}, ::Tuple{}, ::Tuple{}) = ()
 _cshp(ndim::Int, ::Tuple{}, ::Tuple{}, nshape) = nshape
@@ -1631,7 +1637,7 @@ _cat(dims, X...) = cat_t(promote_eltypeof(X...), X...; dims=dims)
 @inline cat_t(::Type{T}, X...; dims) where {T} = _cat_t(dims, T, X...)
 @inline function _cat_t(dims, ::Type{T}, X...) where {T}
     catdims = dims2cat(dims)
-    shape = cat_shape(catdims, map(cat_size, X))
+    shape = cat_size_shape(catdims, X...)
     A = cat_similar(X[1], T, shape)
     if count(!iszero, catdims)::Int > 1
         fill!(A, zero(T))

test/abstractarray.jl

@@ -692,6 +692,12 @@ function test_cat(::Type{TestAbstractArray})
     # 36041
     @test_throws MethodError cat(["a"], ["b"], dims=[1, 2])
     @test cat([1], [1], dims=[1, 2]) == I(2)
+
+    # inferrability
+    As = [zeros(2, 2) for _ = 1:2]
+    @test @inferred(cat(As...; dims=Val(3))) == zeros(2, 2, 2)
+    cat3v(As) = cat(As...; dims=Val(3))
+    @test @inferred(cat3v(As)) == zeros(2, 2, 2)
 end
 
 function test_ind2sub(::Type{TestAbstractArray})