Fix typeassert_tfunc for vararg PartialStructs

Author: martinholters

base/compiler/tfuncs.jl

@@ -555,17 +555,22 @@ function typeassert_type_instance(@nospecialize(v), @nospecialize(t))
         widev = widenconst(v)
         if widev <: t
             return v
-        elseif typeintersect(widev, t) === Bottom
+        end
+        tr = typeintersect(widev, t)
+        if tr === Bottom
             return Bottom
         end
-        @assert widev <: Tuple
-        new_fields = Vector{Any}(undef, length(v.fields))
+        @assert tr <: Tuple
+        new_fields = Vector{Any}(undef, length(tr.parameters))
         for i = 1:length(new_fields)
-            new_fields[i] = typeassert_type_instance(v.fields[i], getfield_tfunc(t, Const(i)))
+            new_fields[i] = typeassert_type_instance(getfield_tfunc(v, Const(i)), getfield_tfunc(tr, Const(i)))
             if new_fields[i] === Bottom
                 return Bottom
             end
         end
+        if isvatuple(tr)
+            new_fields[end] = Vararg{new_fields[end]}
+        end
         return tuple_tfunc(new_fields)
     elseif isa(v, Conditional)
         if !(Bool <: t)

test/compiler/inference.jl

@@ -2980,3 +2980,13 @@ f38888() = S38888(Base.inferencebarrier(3))
 @test f38888() isa S38888
 g38888() = S38888(Base.inferencebarrier(3), nothing)
 @test g38888() isa S38888
+
+# issue #38971
+f28971() = (1, [2,3]...)::Tuple{Int,Int,Int}
+@test @inferred(f28971()) == (1, 2, 3)
+g28971_1() = (1, [2,3]...)::Tuple{Vararg{Int}}
+@test @inferred(Tuple{Int, Vararg{Int}}, g28971_1()) == (1, 2, 3)
+g28971_2() = (1, [2,3]...)::Tuple{Int, Vararg{Int}}
+@test @inferred(Tuple{Int, Vararg{Int}}, g28971_2()) == (1, 2, 3)
+g28971_3() = (1, [2,3]...)::Tuple{Int, Int, Vararg{Int}}
+@test @inferred(Tuple{Int, Int, Vararg{Int}}, g28971_3()) == (1, 2, 3)