inference: handle Vararg in abstract_call_unionall for argtypes computed by abstract_apply (#51393)

This commit adds special handling for `Vararg` types that may appear at
the end of `argtypes`, as computed by `abstract_apply`. Even though PR

within the abstract state, they can still be part of `argtypes`. As a
result, this kind of special handling is still necessary. It remains an
open question whether we can refactor `abstract_apply` to prevent
`Vararg`s from appearing in `argtypes` in the first place.

Author: aviatesk

base/compiler/abstractinterpretation.jl

@@ -1874,7 +1874,12 @@ function abstract_call_unionall(interp::AbstractInterpreter, argtypes::Vector{An
         a2 = argtypes[2]
         a3 = argtypes[3]
         ⊑ᵢ = ⊑(typeinf_lattice(interp))
-        nothrow = a2 ⊑ᵢ TypeVar && (a3 ⊑ᵢ Type || a3 ⊑ᵢ TypeVar)
+        if isvarargtype(a3)
+            a3 = unwrapva(a3)
+            nothrow = false
+        else
+            nothrow = a2 ⊑ᵢ TypeVar && (a3 ⊑ᵢ Type || a3 ⊑ᵢ TypeVar)
+        end
         if isa(a3, Const)
             body = a3.val
         elseif isType(a3)

test/compiler/inference.jl

@@ -5,7 +5,7 @@ import Core.Compiler: Const, Conditional, ⊑, ReturnNode, GotoIfNot
 isdispatchelem(@nospecialize x) = !isa(x, Type) || Core.Compiler.isdispatchelem(x)
 
 using Random, Core.IR
-using InteractiveUtils: code_llvm
+using InteractiveUtils
 
 include("irutils.jl")
 
@@ -5101,3 +5101,8 @@ end |> only === Val{5}
 @test fully_eliminated() do
     length(continue_const_prop(1, 5))
 end
+
+# https://github.com/JuliaLang/julia/issues/51310
+@test code_typed() do
+    b{c} = d...
+end |> only |> first isa Core.CodeInfo