improve apply_type_tfunc accuracy in rare case (JuliaLang#49069)

In the unlikely event this call fails, we can either confidently
conclude the result will always fail and stop inference immediately
there. Or we can at least conclude that the base type is confidently
known, which can potentially improve ml-matches performance later by
excluding Union{} or other subtypes.

Author: vtjnash

base/compiler/tfuncs.jl

@@ -1628,6 +1628,7 @@ function apply_type_nothrow(𝕃::AbstractLattice, argtypes::Vector{Any}, @nospe
     (headtype === Union) && return true
     isa(rt, Const) && return true
     u = headtype
+    # TODO: implement optimization for isvarargtype(u) and istuple occurences (which are valid but are not UnionAll)
     for i = 2:length(argtypes)
         isa(u, UnionAll) || return false
         ai = widenconditional(argtypes[i])
@@ -1747,6 +1748,9 @@ const _tvarnames = Symbol[:_A, :_B, :_C, :_D, :_E, :_F, :_G, :_H, :_I, :_J, :_K,
         end
         ua = ua.body
     end
+    if largs > outer_start && isa(headtype, UnionAll) # e.g. !isvarargtype(ua) && !istuple
+        return Bottom # too many arguments
+    end
     outer_start = outer_start - largs + 1
 
     varnamectr = 1
@@ -1815,19 +1819,40 @@ const _tvarnames = Symbol[:_A, :_B, :_C, :_D, :_E, :_F, :_G, :_H, :_I, :_J, :_K,
                 push!(outervars, v)
             end
         end
-        if isa(ua, UnionAll)
+        if ua isa UnionAll
             ua = ua.body
-        else
-            ua = nothing
+            #otherwise, sometimes ua isa Vararg (Core.TypeofVararg) or Tuple (DataType)
         end
     end
     local appl
     try
         appl = apply_type(headtype, tparams...)
     catch ex
-        # type instantiation might fail if one of the type parameters
-        # doesn't match, which could happen if a type estimate is too coarse
-        return isvarargtype(headtype) ? TypeofVararg : Type{<:headtype}
+        # type instantiation might fail if one of the type parameters doesn't
+        # match, which could happen only if a type estimate is too coarse
+        # and might guess a concrete value while the actual type for it is Bottom
+        if !uncertain
+            return Union{}
+        end
+        canconst = false
+        uncertain = true
+        empty!(outervars)
+        outer_start = 1
+        # FIXME: if these vars are substituted with TypeVar here, the result
+        # might be wider than the input, so should we use the `.name.wrapper`
+        # object here instead, to replace all of these outervars with
+        # unconstrained ones? Note that this code is nearly unreachable though,
+        # and possibly should simply return Union{} here also, since
+        # `apply_type` is already quite conservative about detecting and
+        # throwing errors.
+        appl = headtype
+        if isa(appl, UnionAll)
+            for _ = 1:largs
+                appl = appl::UnionAll
+                push!(outervars, appl.var)
+                appl = appl.body
+            end
+        end
     end
     !uncertain && canconst && return Const(appl)
     if isvarargtype(appl)

test/compiler/inference.jl

@@ -2648,10 +2648,14 @@ end |> only === Int
 # https://github.com/JuliaLang/julia/issues/47089
 import Core: Const
 import Core.Compiler: apply_type_tfunc
-struct Issue47089{A,B} end
+struct Issue47089{A<:Number,B<:Number} end
 let 𝕃 = Core.Compiler.fallback_lattice
     A = Type{<:Integer}
     @test apply_type_tfunc(𝕃, Const(Issue47089), A, A) <: (Type{Issue47089{A,B}} where {A<:Integer, B<:Integer})
+    @test apply_type_tfunc(𝕃, Const(Issue47089), Const(Int), Const(Int), Const(Int)) === Union{}
+    @test apply_type_tfunc(𝕃, Const(Issue47089), Const(String)) === Union{}
+    @test apply_type_tfunc(𝕃, Const(Issue47089), Const(AbstractString)) === Union{}
+    @test apply_type_tfunc(𝕃, Const(Issue47089), Type{Ptr}, Type{Ptr{T}} where T) === Base.rewrap_unionall(Type{Issue47089.body.body}, Issue47089)
 end
 @test only(Base.return_types(keys, (Dict{String},))) == Base.KeySet{String, T} where T<:(Dict{String})
 @test only(Base.return_types((r)->similar(Array{typeof(r[])}, 1), (Base.RefValue{Array{Int}},))) == Vector{<:Array{Int}}