Assignment to _ sets the binding type of _

[xal-0]

A syntax.jl test documents:

# `const` assignment to `_` drops the assignment effect,
# and the conversion, but not the rhs.

julia> _::Int = 1
1

julia> _::String = "a"
ERROR: cannot set type for global Main._. It already has a value or is already set to a different type.
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1

On both 1.11 and nightly, a set_binding_type!or globaldecl, respectively, sneaks through.

1.11:

julia> Meta.@lower _::Int = 1
:($(Expr(:thunk, CodeInfo(
    @ none within `top-level scope`
1 ─      global Main._
│   %2 = Int
│        Core.set_binding_type!(Main, :_, %2)
└──      return 1
))))

Nightly:

julia> Meta.@lower _::Int = 1
:($(Expr(:thunk, CodeInfo(
1 ─ %1 = Main.Int
│        $(Expr(:globaldecl, :(Main._), :(%1)))
│        $(Expr(:latestworld))
└──      return 1
))))

[xal-0]

Related: when :: appears on the left hand side of an assignment, it has different behaviour depending on whether it is attached to a variable name or another expression. x[]::T = y performs a typeassert on x[], disregarding the type of y.

julia> x = Ref{Any}(123)
Base.RefValue{Any}(123)

julia> x[]::Int = "hello"
"hello"

Lowering of x[]::Int = "hello":

1 ─ %1 = Main.x
│   %2 =   dynamic Base.getindex(%1)
│   %3 = Main.Int
│          dynamic Core.typeassert(%2, %3)
│   %5 = Main.x
│          dynamic Base.setindex!(%5, "hello")
└──      return "hello"

This also has the unfortunate side effect of making x::T, y::U = a, b different from (x, y)::Tuple{T, U} = a, b:

x::T, y::U = a, b:

1 ─ %1  = Main.a
│   %2  = Main.b
│   %3  = Main.T
│         $(Expr(:globaldecl, :(Main.x), :(%3)))
│         $(Expr(:latestworld))
│         $(Expr(:globaldecl, :(Main.x)))
│         $(Expr(:latestworld))
│   %8  =   builtin Core.get_binding_type(Main, :x)
│         #s1 = %1
│   %10 = #s1
│   %11 =   dynamic %10 isa %8
└──       goto #3 if not %11
2 ─       goto #4
3 ─ %14 = #s1
└──       #s1 = Base.convert(%8, %14)
4 ┄ %16 = #s1
│           dynamic Base.setglobal!(Main, :x, %16)
│   %18 = Main.U
│         $(Expr(:globaldecl, :(Main.y), :(%18)))
│         $(Expr(:latestworld))
│         $(Expr(:globaldecl, :(Main.y)))
│         $(Expr(:latestworld))
│   %23 =   builtin Core.get_binding_type(Main, :y)
│         @_2 = %2
│   %25 = @_2
│   %26 =   dynamic %25 isa %23
└──       goto #6 if not %26
5 ─       goto #7
6 ─ %29 = @_2
└──       @_2 = Base.convert(%23, %29)
7 ┄ %31 = @_2
│           dynamic Base.setglobal!(Main, :y, %31)
│   %33 =   dynamic Core.tuple(%1, %2)
└──       return %33

(x, y)::Tuple{T, U} = a, b:

1 ─ %1  = Main.x
│   %2  = Main.y
│   %3  =   dynamic Core.tuple(%1, %2)
│   %4  = Main.Tuple
│   %5  = Main.T
│   %6  = Main.U
│   %7  =   builtin Core.apply_type(%4, %5, %6)
│           dynamic Core.typeassert(%3, %7)
│         $(Expr(:globaldecl, :(Main.x)))
│         $(Expr(:latestworld))
│   %11 = Main.a
│   %12 =   builtin Core.get_binding_type(Main, :x)
│         #s1 = %11
│   %14 = #s1
│   %15 =   dynamic %14 isa %12
└──       goto #3 if not %15
2 ─       goto #4
3 ─ %18 = #s1
└──       #s1 = Base.convert(%12, %18)
4 ┄ %20 = #s1
│           dynamic Base.setglobal!(Main, :x, %20)
│         $(Expr(:globaldecl, :(Main.y)))
│         $(Expr(:latestworld))
│   %24 = Main.b
│   %25 =   builtin Core.get_binding_type(Main, :y)
│         @_2 = %24
│   %27 = @_2
│   %28 =   dynamic %27 isa %25
└──       goto #6 if not %28
5 ─       goto #7
6 ─ %31 = @_2
└──       @_2 = Base.convert(%25, %31)
7 ┄ %33 = @_2
│           dynamic Base.setglobal!(Main, :y, %33)
│   %35 = Main.a
│   %36 = Main.b
│   %37 =   dynamic Core.tuple(%35, %36)
└──       return %37