Add @kw_only macro

`@kw_only` behaves the same as `@with_kw` but does not declare a default
constructor when no inner constructor is found.

Author: Frankie Robertson

src/Parameters.jl

@@ -45,7 +45,7 @@ import Base: @__doc__
 import OrderedCollections: OrderedDict
 using UnPack: @unpack, @pack!
 
-export @with_kw, @with_kw_noshow, type2dict, reconstruct, @unpack, @pack!, @pack, @consts
+export @with_kw, @kw_only, @with_kw_noshow, type2dict, reconstruct, @unpack, @pack!, @pack, @consts
 
 ## Parser helpers
 #################
@@ -326,7 +326,7 @@ macro pack_MM(varname)
 end
 ```
 """
-function with_kw(typedef, mod::Module, withshow=true)
+function with_kw(typedef, mod::Module, withshow=true, allow_default=true)
     if typedef.head==:tuple # named-tuple
         withshow==false && error("`@with_kw_noshow` not supported for named tuples")
         return with_kw_nt(typedef, mod)
@@ -486,18 +486,27 @@ function with_kw(typedef, mod::Module, withshow=true)
         push!(args, k)
         push!(kwargs.args, Expr(:kw,k,w))
     end
-    if length(typparas)>0
-        tps = stripsubtypes(typparas)
-        innerc = :( $tn{$(tps...)}($kwargs) where {$(tps...)} = $tn{$(tps...)}($(args...)))
+    if allow_default
+        if length(typparas)>0
+            tps = stripsubtypes(typparas)
+            innerc = :( $tn{$(tps...)}($kwargs) where {$(tps...)} = $tn{$(tps...)}($(args...)))
+        else
+            innerc = :($tn($kwargs) = $tn($(args...)) )
+        end
     else
-        innerc = :($tn($kwargs) = $tn($(args...)) )
+        if length(typparas)>0
+            tps = stripsubtypes(typparas)
+            innerc = :( $tn{$(tps...)}($kwargs) where {$(tps...)} = new{$(tps...)}($(args...)))
+        else
+            innerc = :($tn($kwargs) = new($(args...)) )
+        end
     end
     push!(typ.args[3].args, innerc)
 
     # Inner positional constructor: only make it if no inner
     # constructors are user-defined.  If one or several are defined,
     # assume that one has the standard positional signature.
-    if length(inner_constructors)==0
+    if length(inner_constructors)==0 && allow_default
         if length(typparas)>0
             tps = stripsubtypes(typparas)
             innerc2 = :( $tn{$(tps...)}($(args...)) where {$(tps...)} = new{$(tps...)}($(args...)) )
@@ -679,6 +688,14 @@ macro with_kw(args...)
           """)
 end
 
+"""
+As `@with_kw` but does not declare a default constructor when no inner
+constructor is found.
+"""
+macro kw_only(typedef)
+    return esc(with_kw(typedef, __module__, true, false))
+end
+
 """
 As `@with_kw` but does not define a `show` method to avoid annoying
 redefinition warnings.

test/runtests.jl

@@ -81,6 +81,34 @@ const TMT1_3 = MT1_3{Int} # Julia bug https://github.com/JuliaLang/julia/issues/
 @test "Field r Default: 4\n" == Markdown.plain(REPL.fielddoc(TMT1_3, :r))
 @test "A field Default: sdaf\n" == Markdown.plain(REPL.fielddoc(TMT1_3, :c))
 
+# no positional inner constructor
+@kw_only struct KWO
+    foo
+    bar
+end
+
+function KWO(args...)
+    global outer_constructor_called = true
+    KWO(; foo="foo", bar="bar")
+end
+
+outer_constructor_called = false
+kwo = KWO(42)
+@test outer_constructor_called == true
+@test kwo.foo == "foo"
+@test kwo.bar == "bar"
+
+outer_constructor_called = false
+kwo2 = KWO(42, 43)
+@test outer_constructor_called == true
+@test kwo2.foo == "foo"
+@test kwo2.bar == "bar"
+
+outer_constructor_called = false
+kwo3 = KWO(; foo=42, bar=43)
+@test outer_constructor_called == false
+@test kwo3.foo == 42
+@test kwo3.bar == 43
 
 # parameter-less
 @with_kw_noshow mutable struct MT2