Make T.instance inferable (#30086)

Author: Keno

base/compiler/tfuncs.jl

@@ -31,6 +31,7 @@ const DATATYPE_PARAMETERS_FIELDINDEX = fieldindex(DataType, :parameters)
 const DATATYPE_TYPES_FIELDINDEX = fieldindex(DataType, :types)
 const DATATYPE_SUPER_FIELDINDEX = fieldindex(DataType, :super)
 const DATATYPE_MUTABLE_FIELDINDEX = fieldindex(DataType, :mutable)
+const DATATYPE_INSTANCE_FIELDINDEX = fieldindex(DataType, :instance)
 
 const TYPENAME_NAME_FIELDINDEX = fieldindex(Core.TypeName, :name)
 const TYPENAME_MODULE_FIELDINDEX = fieldindex(Core.TypeName, :module)
@@ -292,8 +293,11 @@ function isdefined_tfunc(@nospecialize(args...))
                 return Const(false)
             elseif !isvatuple(a1) && isbitstype(fieldtype(a1, idx))
                 return Const(true)
-            elseif isa(arg1, Const) && isimmutable((arg1::Const).val)
-                return Const(isdefined((arg1::Const).val, idx))
+            elseif isa(arg1, Const)
+                arg1v = (arg1::Const).val
+                if isimmutable(arg1v) || isdefined(arg1v, idx) || (isa(arg1v, DataType) && is_dt_const_field(idx))
+                    return Const(isdefined(arg1v, idx))
+                end
             end
         end
     end
@@ -518,13 +522,19 @@ function subtype_tfunc(@nospecialize(a), @nospecialize(b))
 end
 add_tfunc(<:, 2, 2, subtype_tfunc, 0)
 
+is_dt_const_field(fld::Int) = (
+     fld == DATATYPE_NAME_FIELDINDEX ||
+     fld == DATATYPE_PARAMETERS_FIELDINDEX ||
+     fld == DATATYPE_TYPES_FIELDINDEX ||
+     fld == DATATYPE_SUPER_FIELDINDEX ||
+     fld == DATATYPE_MUTABLE_FIELDINDEX ||
+     fld == DATATYPE_INSTANCE_FIELDINDEX
+    )
 function const_datatype_getfield_tfunc(@nospecialize(sv), fld::Int)
-    if (fld == DATATYPE_NAME_FIELDINDEX ||
-            fld == DATATYPE_PARAMETERS_FIELDINDEX ||
-            fld == DATATYPE_TYPES_FIELDINDEX ||
-            fld == DATATYPE_SUPER_FIELDINDEX ||
-            fld == DATATYPE_MUTABLE_FIELDINDEX)
-        return AbstractEvalConstant(getfield(sv, fld))
+    if fld == DATATYPE_INSTANCE_FIELDINDEX
+        return isdefined(sv, fld) ? Const(getfield(sv, fld)) : Union{}
+    elseif is_dt_const_field(fld)
+        return Const(getfield(sv, fld))
     end
     return nothing
 end

test/compiler/compiler.jl

@@ -2130,3 +2130,7 @@ let ci = code_typed(bar_inlining_apply, Tuple{})[1].first
     @test length(ci.code) == 2
     @test ci.code[1].head == :foreigncall
 end
+
+# Test that inference can infer .instance of types
+f_instance(::Type{T}) where {T} = T.instance
+@test @inferred(f_instance(Nothing)) === nothing