fix #31803, covariance handling in fieldtype_tfunc

Author: JeffBezanson

base/compiler/tfuncs.jl

@@ -939,7 +939,7 @@ function _fieldtype_tfunc(@nospecialize(s), exact::Bool, @nospecialize(name))
         ft = ftypes[fld]
     end
 
-    exactft = exact || !has_free_typevars(ft)
+    exactft = exact || (!has_free_typevars(ft) && u.name !== Tuple.name)
     ft = rewrap_unionall(ft, s)
     if exactft
         if issingletontype(ft)

test/compiler/inference.jl

@@ -621,6 +621,7 @@ let fieldtype_tfunc = Core.Compiler.fieldtype_tfunc,
     @test fieldtype_tfunc(Union{Type{Base.RefValue{<:Real}}, Type{Int32}}, Const(:x)) == Const(Real)
     @test fieldtype_tfunc(Const(Union{Base.RefValue{<:Real}, Type{Int32}}), Const(:x)) == Type
     @test fieldtype_tfunc(Type{Union{Base.RefValue{T}, Type{Int32}}} where {T<:Real}, Const(:x)) == Type
+    @test fieldtype_tfunc(Type{<:Tuple}, Const(1)) == Type
     @test fieldtype_nothrow(Type{Base.RefValue{<:Real}}, Const(:x))
     @test !fieldtype_nothrow(Type{Union{}}, Const(:x))
     @test !fieldtype_nothrow(Union{Type{Base.RefValue{T}}, Int32} where {T<:Real}, Const(:x))