teach inference that T::Type when ::Type{T} is an argument

Author: JeffBezanson

base/compiler/inferencestate.jl

@@ -145,6 +145,24 @@ function sptypes_from_meth_instance(linfo::MethodInstance)
             end
             if Any <: ub
                 ty = Any
+                # if this parameter came from arg::Type{T}, we know that T::Type
+                sig = linfo.def.sig
+                temp = sig
+                for j = 1:i-1
+                    temp = temp.body
+                end
+                Pi = temp.var
+                while temp isa UnionAll
+                    temp = temp.body
+                end
+                sigtypes = temp.parameters
+                for j = 1:length(sigtypes)
+                    tj = sigtypes[j]
+                    if isType(tj) && tj.parameters[1] === Pi
+                        ty = Type
+                        break
+                    end
+                end
             else
                 lb = v.lb
                 while lb isa TypeVar

test/compiler/inference.jl

@@ -2204,3 +2204,6 @@ _rttf_test(::Int128) = 0
 _call_rttf_test() = Core.Compiler.return_type(_rttf_test, Tuple{Any})
 @test Core.Compiler.return_type(_rttf_test, Tuple{Any}) === Int
 @test _call_rttf_test() === Int
+
+f_with_Type_arg(::Type{T}) where {T} = T
+@test Base.return_types(f_with_Type_arg, (Any,)) == Any[Type]