proposal: go/types: add Func.RecvBase method #81715

@ianlancetaylor recently pointed out that third-party analyzer writers often need to copy a number of widely used helper internal functions from x/tools, and spurred me to think about which ones should be published.

One of the most commonly used go/types helpers (18 uses within x/tools) is RecvBase, which returns the "receiver base type" of a method. In other words, given N.M or (*N).M, it returns N, and additionally a boolean indicating the presence of the pointer.

We propose to add it to go/types as a method of *types.Func.

package types // "go/types"

// RecvBase returns the receiver base type (if any) of the method fn,
// and reports whether the receiver is a pointer to it.
//
// The spec says of a method declaration: the receiver's "type must be
// a defined type T or a pointer to a defined type T, possibly followed
// by a list of type parameter names [P1, P2, …] enclosed in square
// brackets. T is called the receiver base type."
//
// The named result is nil if fn is not a method, or if its receiver
// is an anonymous interface or struct type, as in interface{ f() }.
func (fn *Func) RecvBase() (isPtr bool, named *types.Named)

BTW, the boolean result appears first to avoid misinterpretation as an ok bool, and because that's the natural
reading order in *Named.