Fix typo in >= 3 arg fastmath method (#54630)

Author: Zentrik

base/fastmath.jl

@@ -286,7 +286,7 @@ for op in (:+, :*, :min, :max)
         # definitions down to avoid losing type information.
         # type promotion
         $op_fast(a::Number, b::Number, c::Number, xs::Number...) =
-        $op_fast(promote(x,y,c,xs...)...)
+            $op_fast(promote(a,b,c,xs...)...)
         # fall-back implementation that applies after promotion
         $op_fast(a::T, b::T, c::T, xs::T...) where {T<:Number} = (@inline; afoldl($op_fast, ($op_fast)(($op_fast)(a,b),c), xs...))
     end

test/fastmath.jl

@@ -343,3 +343,9 @@ end
     x = @fastmath maximum(Float16[1,2,3]; init = Float16(0))
     @test x == Float16(3)
 end
+
+@testset "Test promotion of >=3 arg fastmath" begin
+    # Bug caught in https://github.com/JuliaLang/julia/pull/54513#discussion_r1620553369
+    x = @fastmath 1. + 1. + 1f0
+    @test x == 3.0
+end