Fix fargs handling for form_partially_defined_struct

Compiler/src/abstractinterpretation.jl

@@ -2690,11 +2690,13 @@ function abstract_call_known(interp::AbstractInterpreter, @nospecialize(f),
             elseif f === setfield! && length(argtypes) == 4 && isa(argtypes[3], Const)
                 # from there on we know that the struct field will never be undefined,
                 # so we try to encode that information with a `PartialStruct`
-                farg2 = ssa_def_slot(fargs[2], sv)
-                if farg2 isa SlotNumber
-                    refined = form_partially_defined_struct(𝕃ᵢ, argtypes[2], argtypes[3])
-                    if refined !== nothing
-                        refinements = SlotRefinement(farg2, refined)
+                if rt !== Bottom && isa(fargs, Vector{Any})
+                    farg2 = ssa_def_slot(fargs[2], sv)
+                    if farg2 isa SlotNumber
+                        refined = form_partially_defined_struct(𝕃ᵢ, argtypes[2], argtypes[3])
+                        if refined !== nothing
+                            refinements = SlotRefinement(farg2, refined)
+                        end
                     end
                 end
             end

Compiler/test/inference.jl

@@ -6393,4 +6393,19 @@ end
     a
 end === Union{} # `setfield!` tfunc should be able to figure out this object is runtime invalid
 
+# only refine with `PartialStruct` on `setfield!` when we have full argument type information
+let src = code_typed1((Base.RefValue{String}, String)) do x, val
+        setfield!(x, :x, val)
+        isdefined(x, :x)
+    end
+    retval = src.code[end].val
+    @test retval === true
+    src = code_typed1((Base.RefValue{String}, String)) do x, args...
+        setfield!(x, :x, args...)
+        isdefined(x, :x)
+    end
+    retval = src.code[end].val
+    @test isa(retval, Core.SSAValue)
+end
+
 end # module inference