provide specific error message for redefinition of polys

Author: hishamhm

spec/declaration/record_function_spec.lua

@@ -44,6 +44,19 @@ describe("record function", function()
          { y = 8, msg = "type signature of 'do_x' does not match its declaration in Y: argument 2: got string, expected integer" },
       }))
 
+      it("cannot implement a polymorphic function via redeclaration", util.check_type_error([[
+         local record Y
+            do_x: function(integer, integer): integer
+            do_x: function(integer): integer
+         end
+
+         function Y.do_x(a: integer, b: string): integer
+             return a + math.tointeger(b)
+         end
+      ]], {
+         { y = 6, msg = "type signature does not match declaration: field has multiple function definitions" },
+      }))
+
       it("a consistent redeclaration produces a warning", util.check_warnings([[
          local record Y
          end

spec/declaration/record_method_spec.lua

@@ -474,6 +474,19 @@ describe("record method", function()
          { y = 5, msg = "type signature of 'do_x' does not match its declaration in Y: argument 2: got string, expected integer" },
       }))
 
+      it("cannot implement a polymorphic method via redeclaration", util.check_type_error([[
+         local record Y
+            do_x: function(Y, integer, integer): integer
+            do_x: function(Y, integer): integer
+         end
+
+         function Y:do_x(a: integer, b: string): integer
+             return a + math.tointeger(b)
+         end
+      ]], {
+         { y = 6, msg = "type signature does not match declaration: field has multiple function definitions" },
+      }))
+
       it("an inconsistent type in redeclaration produces an error", util.check_type_error([[
          local record Y
          end

tl.lua

@@ -9557,6 +9557,11 @@ tl.type_check = function(ast, opts)
 
                local ok, err = same_type(fn_type, rfieldtype)
                if not ok then
+                  if rfieldtype.typename == "poly" then
+                     add_errs_prefixing(node, err, errors, "type signature does not match declaration: field has multiple function definitions (such polymorphic declarations are intended for Lua module interoperability)")
+                     return
+                  end
+
                   local shortname = node.fn_owner.type.typename == "nominal" and
                   show_type(node.fn_owner.type) or
                   show_fn_owner(node)

tl.tl

@@ -9557,6 +9557,11 @@ tl.type_check = function(ast: Node, opts: TypeCheckOptions): Result, string
 
                local ok, err = same_type(fn_type, rfieldtype)
                if not ok then
+                  if rfieldtype.typename == "poly" then
+                     add_errs_prefixing(node, err, errors, "type signature does not match declaration: field has multiple function definitions (such polymorphic declarations are intended for Lua module interoperability)")
+                     return
+                  end
+
                   local shortname = node.fn_owner.type.typename == "nominal"
                                     and show_type(node.fn_owner.type)
                                     or show_fn_owner(node)