fix incorrect static parameter definedness check (#49097)

We use the specialized signature of a method for accurate analysis of
whether a static parameter is constrained or not. However, it turns out
that we can only use it when it doesn't contain any free type variables
(which it sometimes does, e.g., when the inference entry is given a
signature with a free type variable). In such cases, we should use the
method signature, which, by design, never contains any free type
variables.

base/compiler/inferencestate.jl

@@ -487,7 +487,17 @@ function sptypes_from_meth_instance(linfo::MethodInstance)
                 ty = UnionAll(tv, Type{tv})
             end
             @label ty_computed
-            undef = !constrains_param(v, linfo.specTypes, #=covariant=#true)
+            undef = !(let sig=sig
+                # if the specialized signature `linfo.specTypes` doesn't contain any free
+                # type variables, we can use it for a more accurate analysis of whether `v`
+                # is constrained or not, otherwise we should use `def.sig` which always
+                # doesn't contain any free type variables
+                if !has_free_typevars(linfo.specTypes)
+                    sig = linfo.specTypes
+                end
+                @assert !has_free_typevars(sig)
+                constrains_param(v, sig, #=covariant=#true)
+            end)
         elseif isvarargtype(v)
             ty = Int
             undef = false

test/compiler/inference.jl

@@ -4684,6 +4684,26 @@ unknown_sparam_nothrow2(x::Ref{Ref{T}}) where T = @isdefined(T) ? T::Type : noth
 @test only(Base.return_types(unknown_sparam_nothrow1, (Ref,))) === Type
 @test only(Base.return_types(unknown_sparam_nothrow2, (Ref{Ref{T}} where T,))) === Type
 
+struct Issue49027{Ty<:Number}
+    x::Ty
+end
+function issue49027(::Type{<:Issue49027{Ty}}) where Ty
+    if @isdefined Ty # should be false when `Ty` is given as a free type var.
+        return Ty::DataType
+    end
+    return nothing
+end
+@test only(Base.return_types(issue49027, (Type{Issue49027{TypeVar(:Ty)}},))) >: Nothing
+@test isnothing(issue49027(Issue49027{TypeVar(:Ty)}))
+function issue49027_integer(::Type{<:Issue49027{Ty}}) where Ty<:Integer
+    if @isdefined Ty # should be false when `Ty` is given as a free type var.
+        return Ty::DataType
+    end
+    nothing
+end
+@test only(Base.return_types(issue49027_integer, (Type{Issue49027{TypeVar(:Ty,Int)}},))) >: Nothing
+@test isnothing(issue49027_integer(Issue49027{TypeVar(:Ty,Int)}))
+
 function fapplicable end
 gapplicable() = Val(applicable(fapplicable))
 gapplicable(x) = Val(applicable(fapplicable; x))