deprecate using _ as an rvalue

NEWS.md

@@ -47,6 +47,9 @@ Language changes
   * `@.` is now parsed as `@__dot__`, and can be used to add dots to
     every function call, operator, and assignment in an expression ([#20321]).
 
+  * The identifier `_` can be assigned, but accessing its value is deprecated,
+    allowing this syntax to be used in the future for discarding values ([#9343], [#18251]).
+
 Breaking changes
 ----------------
 

base/inference.jl

@@ -582,7 +582,7 @@ function typeof_tfunc(t::ANY)
         elseif t === Any
             return DataType
         else
-            return Type{_} where _<:t
+            return Type{_T} where _T<:t
         end
     elseif isa(t, Union)
         a = widenconst(typeof_tfunc(t.a))
@@ -891,7 +891,7 @@ function fieldtype_tfunc(s0::ANY, name::ANY)
     if exact
         return Const(ft)
     end
-    return Type{_} where _<:ft
+    return Type{_T} where _T<:ft
 end
 add_tfunc(fieldtype, 2, 2, fieldtype_tfunc)
 
@@ -1002,17 +1002,17 @@ function apply_type_tfunc(headtypetype::ANY, args::ANY...)
     catch ex
         # type instantiation might fail if one of the type parameters
         # doesn't match, which could happen if a type estimate is too coarse
-        return Type{_} where _<:headtype
+        return Type{_T} where _T<:headtype
     end
     !uncertain && canconst && return Const(appl)
     if isvarargtype(headtype)
         return Type
     end
     if uncertain && type_too_complex(appl,0)
-        return Type{_} where _<:headtype
+        return Type{_T} where _T<:headtype
     end
     if istuple
-        return Type{_} where _<:appl
+        return Type{_T} where _T<:appl
     end
     ans = Type{appl}
     for i = length(outervars):-1:1

src/jlfrontend.scm

@@ -99,6 +99,8 @@
                   (or (memq (car e) '(toplevel line module import importall using export
                                                error incomplete))
                       (and (eq? (car e) 'global) (every symbol? (cdr e))))))
+         (if (eq? e '_)
+             (syntax-deprecation #f "_ as an rvalue" ""))
          e)
         (else
          (let ((last *in-expand*))

src/julia-syntax.scm

@@ -3314,15 +3314,20 @@ f(x) = yt(x)
     (define (compile e break-labels value tail)
       (if (or (not (pair? e)) (memq (car e) '(null ssavalue quote inert top core copyast the_exception $
                                                    globalref outerref cdecl stdcall fastcall thiscall llvmcall)))
-          (let ((e (if (and arg-map (symbol? e))
-                       (get arg-map e e)
-                       e)))
-            (cond (tail  (emit-return e))
-                  (value e)
-                  ((or (eq? e 'true) (eq? e 'false)) #f)
-                  ((symbol? e) (emit e) #f)  ;; keep symbols for undefined-var checking
-                  ((and (pair? e) (eq? (car e) 'outerref)) (emit e) #f)  ;; keep globals for undefined-var checking
-                  ((and (pair? e) (eq? (car e) 'globalref)) (emit e) #f) ;; keep globals for undefined-var checking
+          (let ((e1 (if (and arg-map (symbol? e))
+                        (get arg-map e e)
+                        e)))
+            (if (and value (or (eq? e '_)
+                               (and (pair? e) (or (eq? (car e) 'outerref)
+                                                  (eq? (car e) 'globalref))
+                                    (eq? (cadr e) '_))))
+                (syntax-deprecation #f "_ as an rvalue" ""))
+            (cond (tail  (emit-return e1))
+                  (value e1)
+                  ((or (eq? e1 'true) (eq? e1 'false)) #f)
+                  ((symbol? e1) (emit e1) #f)  ;; keep symbols for undefined-var checking
+                  ((and (pair? e1) (eq? (car e1) 'outerref)) (emit e1) #f)  ;; keep globals for undefined-var checking
+                  ((and (pair? e1) (eq? (car e1) 'globalref)) (emit e1) #f) ;; keep globals for undefined-var checking
                   (else #f)))
           (case (car e)
             ((call new foreigncall)

test/core.jl

@@ -3115,7 +3115,7 @@ typealias PossiblyInvalidUnion{T} Union{T,Int}
 # issue #13007
 call13007{T,N}(::Type{Array{T,N}}) = 0
 call13007(::Type{Array}) = 1
-@test length(Base._methods(call13007, Tuple{Type{_} where _<:Array}, 4, typemax(UInt))) == 2
+@test length(Base._methods(call13007, Tuple{Type{x} where x<:Array}, 4, typemax(UInt))) == 2
 
 # detecting cycles during type intersection, e.g. #1631
 cycle_in_solve_tvar_constraints{S}(::Type{Nullable{S}}, x::S) = 0

test/inference.jl

@@ -164,7 +164,7 @@ code_llvm(DevNull, f14009, (Int,))
 arithtype9232{T<:Real}(::Type{T},::Type{T}) = arithtype9232(T)
 result_type9232{T1<:Number,T2<:Number}(::Type{T1}, ::Type{T2}) = arithtype9232(T1, T2)
 # this gave a "type too large", but not reliably
-@test length(code_typed(result_type9232, Tuple{(Type{_} where _<:Union{Float32,Float64}), Type{T2} where T2<:Number})) == 1
+@test length(code_typed(result_type9232, Tuple{(Type{x} where x<:Union{Float32,Float64}), Type{T2} where T2<:Number})) == 1
 
 
 # issue #10878