produce a warning when unions are not auto-inferred

See #705.

spec/operator/or_spec.lua

@@ -11,7 +11,7 @@ describe("or", function()
       local m2: {string:Ty} = m1 or { foo = t }
    ]]))
 
-   it("record or record: need to be compatible", util.check_type_error([[
+   it("record or record: need to be compatible", util.check_warnings([[
       local record R1
          x: number
       end
@@ -21,7 +21,10 @@ describe("or", function()
       local r1: R1
       local r2: R2
       local r3 = r2 or r1
+      print(r3)
    ]], {
+      -- no warnings because `r2 or r1` is not a valid union
+   }, {
       { msg = "cannot use operator 'or' for types R2 and R1" }
    }))
 
@@ -97,11 +100,14 @@ describe("or", function()
       { msg = "cannot use operator 'or' for types boolean and {}" },
    }))
 
-   it("does not produce new unions if not asked to", util.check_type_error([[
+   it("does not produce new unions if not asked to", util.check_warnings([[
       local x: number | string
 
       local s = x is string and x .. "!" or x + 1
+      print(s)
    ]], {
+      { y = 3, msg = [[if a union type was intended, consider declaring it explicitly]] }
+   }, {
       { y = 3, msg = [[cannot use operator 'or' for types string and number]] },
    }))
 

tl.lua

@@ -5870,6 +5870,10 @@ tl.type_check = function(ast, opts)
    end
 
    local function is_valid_union(typ)
+      if typ.typename ~= "union" then
+         return false, nil
+      end
+
 
 
       local n_table_types = 0
@@ -10049,6 +10053,9 @@ tl.type_check = function(ast, opts)
                   node.type, meta_on_operator = check_metamethod(node, node.op.op, a, b)
                   if not node.type then
                      node_error(node, "cannot use operator '" .. node.op.op:gsub("%%", "%%%%") .. "' for types %s and %s", resolve_tuple(orig_a), resolve_tuple(orig_b))
+                     if node.op.op == "or" and is_valid_union(unite({ orig_a, orig_b })) then
+                        node_warning("hint", node, "if a union type was intended, consider declaring it explicitly")
+                     end
                   end
                end
 

tl.tl

@@ -5870,6 +5870,10 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
    end
 
    local function is_valid_union(typ: Type): boolean, string
+      if typ.typename ~= "union" then
+         return false, nil
+      end
+
       -- check for limitations in our union support
       -- due to codegen limitations (we only check with type() so far)
       local n_table_types = 0
@@ -10049,6 +10053,9 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
                   node.type, meta_on_operator = check_metamethod(node, node.op.op, a, b)
                   if not node.type then
                      node_error(node, "cannot use operator '" .. node.op.op:gsub("%%", "%%%%") .. "' for types %s and %s", resolve_tuple(orig_a), resolve_tuple(orig_b))
+                     if node.op.op == "or" and is_valid_union(unite({orig_a, orig_b})) then
+                        node_warning("hint", node, "if a union type was intended, consider declaring it explicitly")
+                     end
                   end
                end