Allow limit_type_depth to introduce more than one new TypeVar

Fixes #20615.

Author: martinholters

base/inference.jl

@@ -660,21 +660,21 @@ function type_depth(t::ANY)
     return 0
 end
 
-function limit_type_depth(t::ANY, d::Int, cov::Bool=true, var::Union{Void,TypeVar}=nothing)
+function limit_type_depth(t::ANY, d::Int, cov::Bool=true, vars::Vector{TypeVar}=TypeVar[])
     if isa(t,Union)
         if d > MAX_TYPE_DEPTH
             return Any
         end
-        return Union{map(x->limit_type_depth(x, d+1, cov, var), (t.a,t.b))...}
+        return Union{map(x->limit_type_depth(x, d+1, cov, vars), (t.a,t.b))...}
     elseif isa(t,UnionAll)
         v = t.var
         if v.ub === Any
             if v.lb === Bottom
-                return UnionAll(t.var, limit_type_depth(t.body, d, cov, var))
+                return UnionAll(t.var, limit_type_depth(t.body, d, cov, vars))
             end
             ub = Any
         else
-            ub = limit_type_depth(v.ub, d+1, true, nothing)
+            ub = limit_type_depth(v.ub, d+1, true)
         end
         if v.lb === Bottom || type_depth(v.lb) > MAX_TYPE_DEPTH
             # note: lower bounds need to be widened by making them lower
@@ -683,25 +683,22 @@ function limit_type_depth(t::ANY, d::Int, cov::Bool=true, var::Union{Void,TypeVa
             lb = v.lb
         end
         v2 = TypeVar(v.name, lb, ub)
-        return UnionAll(v2, limit_type_depth(t{v2}, d, cov, var))
+        return UnionAll(v2, limit_type_depth(t{v2}, d, cov, vars))
     elseif !isa(t,DataType)
         return t
     end
     P = t.parameters
     isempty(P) && return t
     if d > MAX_TYPE_DEPTH
         cov && return t.name.wrapper
-        # TODO mutating a TypeVar is not great style
-        var.ub = t.name.wrapper
+        var = TypeVar(gensym(), t.name.wrapper)
+        push!(vars, var)
         return var
     end
     stillcov = cov && (t.name === Tuple.name)
-    if cov && !stillcov
-        var = TypeVar(:_)
-    end
-    Q = map(x->limit_type_depth(x, d+1, stillcov, var), P)
+    Q = map(x->limit_type_depth(x, d+1, stillcov, vars), P)
     R = t.name.wrapper{Q...}
-    return (cov && !stillcov) ? UnionAll(var, R) : R
+    return (cov && !stillcov) ? foldr(UnionAll, R, vars) : R
 end
 
 const DataType_name_fieldindex = fieldindex(DataType, :name)

test/inference.jl

@@ -642,3 +642,9 @@ end
 # infinite type growth via lower bounds (formed by intersection)
 f20267(x::T20267{T}, y::T) where (T) = f20267(Any[1][1], x.inds)
 @test Base.return_types(f20267, (Any, Any)) == Any[Union{}]
+
+# issue #20615
+let A = 1:2, z = zip(A, A, A, A, A, A, A, A, A, A, A, A)
+    @test z isa Core.Inference.limit_type_depth(typeof(z), 0)
+    @test start(z) == (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, (1, 1)))))))))))
+end