proposal: use Go 1.27 generic methods for typed Context helpers (PathParam/QueryParam/FormParam/Bind)

まず、既存の v4 の型付きヘルパーセットと v5 Context API を確認してください。ジェネリックメソッドに必要な Go の最小バージョンを確認し、提案された Context メソッドと既存のパッケージレベルのヘルパーをどのように共存させるべきかを決定してください。ヘルパーセット、互換性の動作、および Go バージョン要件について合意し、プロジェクトのテストでカバーできた時点で作業は完了です。

Go 1.27 has been released with support for generic methods (release notes, proposal golang/go#77273). This removes the language limitation mentioned in #2856 — "structs can not have generic methods, only generic functions are allowed" — that forced the typed helpers in v4 to be package-level functions like echo.PathParam[int](c, "id").

Now that v5's Context is a struct and the language supports it, the helpers can become real methods:

Before (v4 style):

id, err := echo.PathParam[int](c, "id")
page, err := echo.QueryParam[int](c, "page")

After (generic methods):

id, err := c.PathParam[int]("id")
page, err := c.QueryParam[int]("page")

Suggested additions (mirroring the v4 helper set):

func (c *Context) PathParam[T any](name string) (T, error)
func (c *Context) QueryParam[T any](name string) (T, error)
func (c *Context) FormParam[T any](name string) (T, error)
func (c *Context) PathParamOr[T any](name string, def T) (T, error)
func (c *Context) QueryParamOr[T any](name string, def T) (T, error)
func (c *Context) FormParamOr[T any](name string, def T) (T, error)
func (c *Context) QueryParams[T any](name string) ([]T, error)
func (c *Context) FormParams[T any](name string) ([]T, error)

Note: since Context is no longer an interface in v5, adding methods is not a breaking change for users — existing code calling c.PathParam("id") keeps compiling, and the existing package-level generic functions can be kept (or deprecated) as thin wrappers.

The only blocker is the minimum Go version: generic methods require go 1.27 in go.mod. Filing this now so it's on the radar for when Echo bumps its minimum supported Go version to 1.27.