Need diagnostic for mismatched Either branches in result builder

[dabrahams]

Description

Not getting this right yields: error: failed to produce diagnostic for expression; please submit a bug report (https://swift.org/contributing/#reporting-bugs)

Reproduction

@resultBuilder
struct ASTBuilder {
  static func buildEither<T>(first x: T) -> (T.Type, T) { (T.self, x) }
  static func buildEither<T>(second x: T) -> (T, T.Type) { (x, T.self) }
  static func buildBlock(_ x: Any...) -> [Any] { x }
}

@ASTBuilder
var x: Any {
  if false { "3" }
  else { 30 }
}

Expected behavior

No request to file a bug; good diagnostic. An explanation of the rules in the docs.

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: x86_64-apple-macosx15.0

Additional information

The exact rules are weird; there's probably a lot of type deduction going on.

For example, this works:

@resultBuilder
struct ASTBuilder {
  static func buildEither<T>(first x: T) -> T { x }
  static func buildEither<T>(second x: T) -> T { x }
  static func buildBlock(_ x: Any...) -> [Any] { x }
}

@ASTBuilder
var x: Any {
  if false { "3" }
  else { 30 }
}

This of course fails (understandable):

@resultBuilder
struct ASTBuilder {
  static func buildEither<T>(first x: T) -> T { x }
  static func buildEither<T>(second x: T) -> T { x }
  static func buildBlock<T>(_ x: T...) -> [T] { x }
}

var a = 1
@ASTBuilder
var x: Any {
  if a == 0 { "3" }
  else { 30 }
}

One might expect this to work but it fails the same way:

@resultBuilder
struct ASTBuilder {
  static func buildEither<T>(first x: T) -> T { x }
  static func buildEither<T>(second x: T) -> T { x }
  static func buildBlock(_ x: Any...) -> [Any] { x }
  static func buildPartialBlock<T>(first x: T) -> T { x }
  static func buildPartialBlock<T, U>(accumulated: T, next: U) -> (T, U) { (accumulated, next) }
}

var a = 1
@ASTBuilder
var x: Any {
  if a == 0 { "3" }
  else { 30 }
}