fix #38888, pessimistic sparam inference with concrete upper bound (#38899)

Author: JeffBezanson

base/compiler/utilities.jl

@@ -139,15 +139,45 @@ end
 const nonfunction_mt = typename(SimpleVector).mt
 
 function get_compileable_sig(method::Method, @nospecialize(atypes), sparams::SimpleVector)
-    isa(atypes, DataType) || return Nothing
+    isa(atypes, DataType) || return nothing
     mt = ccall(:jl_method_table_for, Any, (Any,), atypes)
     mt === nothing && return nothing
     return ccall(:jl_normalize_to_compilable_sig, Any, (Any, Any, Any, Any),
         mt, atypes, sparams, method)
 end
 
+# eliminate UnionAll vars that might be degenerate due to having identical bounds,
+# or a concrete upper bound and appearing covariantly.
+function subst_trivial_bounds(@nospecialize(atypes))
+    if !isa(atypes, UnionAll)
+        return atypes
+    end
+    v = atypes.var
+    if isconcretetype(v.ub) || v.lb === v.ub
+        return subst_trivial_bounds(atypes{v.ub})
+    end
+    return UnionAll(v, subst_trivial_bounds(atypes.body))
+end
+
+# If removing trivial vars from atypes results in an equivalent type, use that
+# instead. Otherwise we can get a case like issue #38888, where a signature like
+#   f(x::S) where S<:Int
+# gets cached and matches a concrete dispatch case.
+function normalize_typevars(method::Method, @nospecialize(atypes), sparams::SimpleVector)
+    at2 = subst_trivial_bounds(atypes)
+    if at2 !== atypes && at2 == atypes
+        atypes = at2
+        sp_ = ccall(:jl_type_intersection_with_env, Any, (Any, Any), at2, method.sig)::SimpleVector
+        sparams = sp_[2]::SimpleVector
+    end
+    return atypes, sparams
+end
+
 # get a handle to the unique specialization object representing a particular instantiation of a call
 function specialize_method(method::Method, @nospecialize(atypes), sparams::SimpleVector, preexisting::Bool=false, compilesig::Bool=false)
+    if isa(atypes, UnionAll)
+        atypes, sparams = normalize_typevars(method, atypes, sparams)
+    end
     if compilesig
         new_atypes = get_compileable_sig(method, atypes, sparams)
         new_atypes === nothing && return nothing

test/compiler/inference.jl

@@ -2979,3 +2979,13 @@ gVarargInt(x::Int) = 1
 gVarargInt(x) = 2
 fVarargInt(::Tuple{Vararg{Int, N}}) where {N} = Val{gVarargInt(N)}()
 @test only(Base.return_types(fVarargInt, Tuple{Tuple{Vararg{Int}}})) == Val{1}
+
+# issue #38888
+struct S38888{T}
+    S38888(x::S) where {S<:Int} = new{S}()
+    S38888(x::S, y) where {S2<:Int,S<:S2} = new{S}()
+end
+f38888() = S38888(Base.inferencebarrier(3))
+@test f38888() isa S38888
+g38888() = S38888(Base.inferencebarrier(3), nothing)
+@test g38888() isa S38888