feat: type narrow types with literal fields

changelog.md

@@ -6,6 +6,7 @@
 * `NEW` Infer function parameter types when overriding the same-named class function in an instance of that class [#2158](https://github.com/LuaLS/lua-language-server/issues/2158)
 * `FIX` Eliminate floating point error in test benchmark output
 * `FIX` Remove luamake install from make scripts
+* `NEW` Types with literal fields can be narrowed.
 
 ## 3.10.6
 `2024-9-10`

script/vm/tracer.lua

@@ -256,6 +256,44 @@ function mt:fastWardCasts(pos, node)
     return node
 end
 
+--- Return types of source which have a field with the value of literal.
+--- @param uri uri
+--- @param source parser.object
+--- @param fieldName string
+--- @param literal parser.object
+--- @return string[]?
+local function getNodeTypesWithLiteralField(uri, source, fieldName, literal)
+    local loc = vm.getVariable(source)
+    if not loc then
+        return
+    end
+
+    local tys
+
+    for _, c in ipairs(vm.compileNode(loc)) do
+        if c.cate == 'type' then
+            for _, set in ipairs(c:getSets(uri)) do
+                if set.type == 'doc.class' then
+                    for _, f in ipairs(set.fields) do
+                        if f.field[1] == fieldName then
+                            for _, t in ipairs(f.extends.types) do
+                                if t[1] == literal[1] then
+                                  tys = tys or {}
+                                  table.insert(tys, set.class[1])
+                                  break
+                                end
+                            end
+                            break
+                        end
+                    end
+                end
+            end
+        end
+    end
+
+    return tys
+end
+
 local lookIntoChild = util.switch()
     : case 'getlocal'
     : case 'getglobal'
@@ -637,6 +675,27 @@ local lookIntoChild = util.switch()
                         end
                     end
                 end
+            elseif handler.type == 'getfield'
+            and    handler.node.type == 'getlocal' then
+                local tys = getNodeTypesWithLiteralField(
+                    tracer.uri, handler.node, handler.field[1], checker)
+
+                -- TODO: handle more types
+                if tys and #tys == 1 then
+                    local ty = tys[1]
+                    topNode = topNode:copy()
+                    if action.op.type == '==' then
+                        topNode:narrow(tracer.uri, ty)
+                        if outNode then
+                            outNode:remove(ty)
+                        end
+                    else
+                        topNode:remove(ty)
+                        if outNode then
+                            outNode:narrow(tracer.uri, ty)
+                        end
+                    end
+                end
             elseif handler.type == 'call'
             and    checker.type == 'string'
             and    handler.node.special == 'type'

test/type_inference/common.lua

@@ -4453,3 +4453,53 @@ function A:func(x) end
 local a = {}
 function a:func(<?x?>) end
 ]]
+
+TEST 'A' [[
+---@class A
+---@field type 'a'
+---@field field1 integer
+
+---@class B
+---@field type 'b'
+
+local obj --- @type A|B
+
+if obj.type == 'a' and obj.field1 > 0 then
+    local <?r?> = obj
+end
+]]
+
+TEST 'B' [[
+---@class A
+---@field type 'a'
+
+---@class B
+---@field type 'b'
+
+local obj --- @type A|B
+
+if obj.type == 'a' then
+    ---
+else
+    local <?r?> = obj
+end
+]]
+
+TEST 'A' [[
+---@class A
+---@field type 'a'
+
+---@class B
+---@field type 'b'
+
+---@class C
+---@field type 'c'
+
+---@alias AB A|B
+
+local obj --- @type C|AB
+
+if obj.type == 'a' then
+    local <?r?> = obj
+end
+]]