@alwaysThrows doesn't play nice with case_block_not_terminated

[nex3]

The analyzer won't allow a function annotated with @alwaysThrows as the final statement in a case. However, it also considers code after it to be dead. For example:

import 'package:meta/meta.dart';

@alwaysThrows
void thrower() {
  throw "dang";
}

void doIt() {
  switch ("foo") {
    case "foo":
      thrower();

    case "bar":
      thrower();
      return null;
  }
}

This produces one error and one hint:

Analyzing test.dart...
  error • The last statement of the 'case' should be 'break', 'continue', 'rethrow', 'return' or 'throw' at test.dart:10:5 • case_block_not_terminated                                                 
  hint • Dead code at test.dart:15:7 • dead_code
1 error and 1 hint found.

[lrhn]

The analyzer does not have any options here, the compile-time error is required by the specification.
The specification currently says about case e_k: s_k:

Let $s$ be the last statement of the statement sequence $s_k$.
If $s$ is a non-empty block statement, let $s$ instead be the last statement of the block statement.
It is a compile-time error if $s$ is not a \BREAK{}, \CONTINUE{}, \RETHROW{} or \RETURN{} statement
or an expression statement where the expression is a \THROW{} expression.

(I'm still all for removing that error completely).

[nex3]

Consider this a +1 for both changing the spec to allow the analyzer to behave more gracefully here (@leafpetersen) and for silencing the "dead code" error for return null (@srawlins) at least until the language allows the more graceful alternative.

[srawlins]

Yuck. Awkward case. I'm not sure how we can mark some statements after an @alwaysThrows function call as dead, but some not. (99% of the time, you want this dead code highlighted.)

[leafpetersen]

Presumably return thrower() is a workaround?

@lrhn is the claim that this error is not useful in general? It does seem more likely to be an error than not, not?

If we had a true bottom type, then we could allow anything of type bottom as a terminator.

[srawlins]

(Language issue: https://github.com/dart-lang/sdk/issues/26704)

[srawlins]

(Quick tie in, this was brought up #30259 (comment) as well; a solution might get tagged with that bug)

[srawlins]

Oh ho! There is already a trick in the analyzer for this. You need to break instead of return null:

sdk/pkg/analyzer/lib/src/generated/resolver.dart

Lines 2529 to 2535 in 7c77ed0

	/**
	* Given some list of [statements], loop through the list searching for dead
	* statements. If [allowMandated] is true, then allow dead statements that are
	* mandated by the language spec. This allows for a final break, continue,
	* return, or throw statement at the end of a switch case, that are mandated
	* by the language spec.
	*/

sdk/pkg/analyzer/lib/src/generated/resolver.dart

Lines 2554 to 2557 in 7c77ed0

	// If mandated statements are allowed, and only the last statement is
	// dead, and it's a BreakStatement, then assume it is a statement
	// mandated by the language spec, there to avoid a
	// CASE_BLOCK_NOT_TERMINATED error.