implement splatting in new calls

NEWS.md

@@ -4,8 +4,8 @@ Julia v1.2 Release Notes
 New language features
 ---------------------
 
-* The `extrema` function now accepts a function argument in the same manner as `minimum` and
-  `maximum` ([#30323]).
+  * Argument splatting (`x...`) can now be used in calls to the `new` pseudo-function in
+    constructors ([#30577]).
 
 Multi-threading changes
 -----------------------
@@ -25,11 +25,14 @@ Command-line option changes
 New library functions
 ---------------------
 
-* `getipaddrs()` function returns all the IP addresses of the local machine ([#30349])
+  * `getipaddrs()` function returns all the IP addresses of the local machine ([#30349])
 
 Standard library changes
 ------------------------
 
+  * The `extrema` function now accepts a function argument in the same manner as `minimum` and
+    `maximum` ([#30323]).
+
 #### LinearAlgebra
 
 * Added keyword arguments `rtol`, `atol` to `pinv` and `nullspace` ([#29998]).
@@ -44,7 +47,7 @@ Standard library changes
 
 #### Dates
 
-* Fixed `repr` such that it displays `DateTime` as it would be entered in Julia ([#30200]).
+  * Fixed `repr` such that it displays `DateTime` as it would be entered in Julia ([#30200]).
 
 #### Miscellaneous
 

base/essentials.jl

@@ -280,6 +280,13 @@ convert(::Type{Tuple{Vararg{V}}}, x::Tuple{Vararg{V}}) where {V} = x
 convert(T::Type{Tuple{Vararg{V}}}, x::Tuple) where {V} =
     (convert(tuple_type_head(T), x[1]), convert(T, tail(x))...)
 
+# used for splatting in `new`
+convert_prefix(::Type{Tuple{}}, x::Tuple) = x
+convert_prefix(::Type{<:AtLeast1}, x::Tuple{}) = x
+convert_prefix(::Type{T}, x::T) where {T<:AtLeast1} = x
+convert_prefix(::Type{T}, x::AtLeast1) where {T<:AtLeast1} =
+    (convert(tuple_type_head(T), x[1]), convert_prefix(tuple_type_tail(T), tail(x))...)
+
 # TODO: the following definitions are equivalent (behaviorally) to the above method
 # I think they may be faster / more efficient for inference,
 # if we could enable them, but are they?

src/julia-syntax.scm

@@ -696,8 +696,6 @@
                 (call (curly ,name ,@params) ,@field-names)))))
 
 (define (new-call Tname type-params params args field-names field-types)
-  (if (any vararg? args)
-      (error "... is not supported inside \"new\""))
   (if (any kwarg? args)
       (error "\"new\" does not accept keyword arguments"))
   (if (length> params (length type-params))
@@ -706,8 +704,22 @@
                    `(outerref ,Tname)
                    `(curly (outerref ,Tname)
                            ,@type-params))))
-    (cond ((length> args (length field-names))
-           `(call (top error) "new: too many arguments"))
+    (cond ((length> (filter (lambda (a) (not (vararg? a))) args) (length field-names))
+           `(call (core throw) (call (top ArgumentError)
+                                     ,(string "new: too many arguments (expected " (length field-names) ")"))))
+          ((any vararg? args)
+           (if (every (lambda (ty) (equal? ty '(core Any)))
+                      field-types)
+               `(splatnew ,Texpr (call (core tuple) ,@args))
+               (let ((tn (make-ssavalue)))
+                 `(block
+                   (= ,tn ,Texpr)
+                   (splatnew ,tn (call (top convert_prefix)
+                                       (curly (core Tuple)
+                                              ,@(map (lambda (fld)
+                                                       `(call (core fieldtype) ,tn (quote ,fld)))
+                                                     field-names))
+                                       (call (core tuple) ,@args)))))))
           (else
            (if (equal? type-params params)
                `(new ,Texpr ,@(map (lambda (fty val)

test/core.jl

@@ -6816,3 +6816,33 @@ f29828() = 2::String
 g29828() = 2::Any[String][1]
 @test_throws TypeError(:typeassert, String, 2) f29828()
 @test_throws TypeError(:typeassert, String, 2) g29828()
+
+# splatting in `new`
+struct SplatNew{T}
+    x::Int8
+    y::T
+    SplatNew{T}(args...) where {T} = new(0,args...,1)
+    SplatNew(args...) = new{Float32}(args...)
+    SplatNew{Any}(args...) = new(args...)
+    SplatNew{Tuple{Int16}}(args...) = new([2]..., args...)
+    SplatNew{Int8}() = new(1,2,3)
+end
+let x = SplatNew{Int16}()
+    @test x.x === Int8(0)
+    @test x.y === Int16(1)
+end
+@test_throws ArgumentError SplatNew{Int16}(1)
+let x = SplatNew(3,2)
+    @test x.x === Int8(3)
+    @test x.y === 2.0f0
+end
+@test_throws ArgumentError SplatNew(1,2,3)
+let x = SplatNew{Any}(1)
+    @test x.x === Int8(1)
+    @test !isdefined(x, :y)
+end
+let x = SplatNew{Tuple{Int16}}((1,))
+    @test x.x === Int8(2)
+    @test x.y === (Int16(1),)
+end
+@test_throws ArgumentError SplatNew{Int8}()