Add a keyword name checking method to hasmethod (#30712)

With this change, `hasmethod` now accepts a tuple of symbols
corresponding to keyword argument names to check for when finding a
matching method.

NEWS.md

@@ -6,6 +6,7 @@ New language features
 
 * The `extrema` function now accepts a function argument in the same manner as `minimum` and
   `maximum` ([#30323]).
+* `hasmethod` can now check for matching keyword argument names ([#30712]).
 
 Multi-threading changes
 -----------------------

base/reflection.jl

@@ -1084,17 +1084,37 @@ function parentmodule(@nospecialize(f), @nospecialize(types))
 end
 
 """
-    hasmethod(f, Tuple type; world = typemax(UInt)) -> Bool
+    hasmethod(f, t::Type{<:Tuple}[, kwnames]; world=typemax(UInt)) -> Bool
 
 Determine whether the given generic function has a method matching the given
 `Tuple` of argument types with the upper bound of world age given by `world`.
 
+If a tuple of keyword argument names `kwnames` is provided, this also checks
+whether the method of `f` matching `t` has the given keyword argument names.
+If the matching method accepts a variable number of keyword arguments, e.g.
+with `kwargs...`, any names given in `kwnames` are considered valid. Otherwise
+the provided names must be a subset of the method's keyword arguments.
+
 See also [`applicable`](@ref).
 
+!!! compat "Julia 1.2"
+    Providing keyword argument names requires Julia 1.2 or later.
+
 # Examples
 ```jldoctest
 julia> hasmethod(length, Tuple{Array})
 true
+
+julia> hasmethod(sum, Tuple{Function, Array}, (:dims,))
+true
+
+julia> hasmethod(sum, Tuple{Function, Array}, (:apples, :bananas))
+false
+
+julia> g(; xs...) = 4;
+
+julia> hasmethod(g, Tuple{}, (:a, :b, :c, :d))  # g accepts arbitrary kwargs
+true
 ```
 """
 function hasmethod(@nospecialize(f), @nospecialize(t); world = typemax(UInt))
@@ -1103,6 +1123,18 @@ function hasmethod(@nospecialize(f), @nospecialize(t); world = typemax(UInt))
     return ccall(:jl_method_exists, Cint, (Any, Any, UInt), typeof(f).name.mt, t, world) != 0
 end
 
+function hasmethod(@nospecialize(f), @nospecialize(t), kwnames::Tuple{Vararg{Symbol}}; world=typemax(UInt))
+    hasmethod(f, t, world=world) || return false
+    isempty(kwnames) && return true
+    m = which(f, t)
+    max_world(m) <= world || return false
+    kws = kwarg_decl(m, Core.kwftype(typeof(f)))
+    for kw in kws
+        endswith(String(kw), "...") && return true
+    end
+    issubset(kwnames, kws)
+end
+
 """
     isambiguous(m1, m2; ambiguous_bottom=false) -> Bool
 

test/reflection.jl

@@ -796,6 +796,27 @@ Base.delete_method(m)
 
 end
 
+module HasmethodKwargs
+using Test
+f(x::Int; y=3) = x + y
+@test hasmethod(f, Tuple{Int})
+@test hasmethod(f, Tuple{Int}, ())
+@test hasmethod(f, Tuple{Int}, (:y,))
+@test !hasmethod(f, Tuple{Int}, (:jeff,))
+@test !hasmethod(f, Tuple{Int}, (:y,), world=typemin(UInt))
+g(; b, c, a) = a + b + c
+h(; kwargs...) = 4
+for gh = (g, h)
+    @test hasmethod(gh, Tuple{})
+    @test hasmethod(gh, Tuple{}, ())
+    @test hasmethod(gh, Tuple{}, (:a,))
+    @test hasmethod(gh, Tuple{}, (:a, :b))
+    @test hasmethod(gh, Tuple{}, (:a, :b, :c))
+end
+@test !hasmethod(g, Tuple{}, (:a, :b, :c, :d))
+@test hasmethod(h, Tuple{}, (:a, :b, :c, :d))
+end
+
 # issue #26267
 module M26267
 import Test