Reduce number of getindex(::Type, ...) methods (#44127)

Previously, there were special cases for `T[]`, `T[a]`, `T[a,b]` and
`T[a,b,c]`. Together with the general case for more elements, that meant
five methods to consider in cases like `T[x...]` where the length of `x`
was not known at compile time. That was beyond the inference limit and
such a call would be inferred as `Any`. So this change gets rid of all
the special cases.

The loop-based general case worked well if all arguments were of the
same type, but otherwise suffered from type-instability inside the loop.
Without the special cases for low element count this would be hit more
often, so for the non-homogeneous case, the loop is replaced with a call
to `afoldl` that basically unrolls the loop for up to 32 elements.

base/array.jl

@@ -402,17 +402,20 @@ julia> getindex(Int8, 1, 2, 3)
 """
 function getindex(::Type{T}, vals...) where T
     a = Vector{T}(undef, length(vals))
-    @inbounds for i = 1:length(vals)
-        a[i] = vals[i]
+    if vals isa NTuple
+        @inbounds for i in 1:length(vals)
+            a[i] = vals[i]
+        end
+    else
+        # use afoldl to avoid type instability inside loop
+        afoldl(1, vals...) do i, v
+            @inbounds a[i] = v
+            return i + 1
+        end
     end
     return a
 end
 
-getindex(::Type{T}) where {T} = (@inline; Vector{T}())
-getindex(::Type{T}, x) where {T} = (@inline; a = Vector{T}(undef, 1); @inbounds a[1] = x; a)
-getindex(::Type{T}, x, y) where {T} = (@inline; a = Vector{T}(undef, 2); @inbounds (a[1] = x; a[2] = y); a)
-getindex(::Type{T}, x, y, z) where {T} = (@inline; a = Vector{T}(undef, 3); @inbounds (a[1] = x; a[2] = y; a[3] = z); a)
-
 function getindex(::Type{Any}, @nospecialize vals...)
     a = Vector{Any}(undef, length(vals))
     @inbounds for i = 1:length(vals)

test/arrayops.jl

@@ -2318,10 +2318,12 @@ let A = zeros(Int, 2, 2), B = zeros(Float64, 2, 2)
     f40() = Float64[A A]
     f41() = [A B]
     f42() = Int[A B]
+    f43() = Int[A...]
+    f44() = Float64[A..., B...]
 
     for f in [f1, f2, f3, f4, f5, f6, f7, f8, f9, f10, f11, f12, f13, f14, f15, f16,
               f17, f18, f19, f20, f21, f22, f23, f24, f25, f26, f27, f28, f29, f30,
-              f31, f32, f33, f34, f35, f36, f37, f38, f39, f40, f41, f42]
+              f31, f32, f33, f34, f35, f36, f37, f38, f39, f40, f41, f42, f43, f44]
         @test isconcretetype(Base.return_types(f, ())[1])
     end
 end