Protocol existentials do not trigger warnings if they will never be executed

[chrisballinger]

Description

Early returns do not trigger warnings for protocol existentials, see example below

Reproduction

protocol SomeProtocol {
    func doTheThing()
}

func output(someProtocol: SomeProtocol) -> String {
    return "test"
    someProtocol.doTheThing() // this should be a warning
}

Expected behavior

early return should cause a compiler warning on the code that will never be executed

Environment

$ swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

Additional information

using a generic brings the warning back, e.g.

func output(someProtocol: some SomeProtocol) -> String {
    return "test"
    someProtocol.doTheThing() // this shows a warning
}

[jamieQ]

looked into this a bit out of curiosity. it seems to me likely that this behavior is due to hitting this branch in the SILGen code. it appears that, by default, diagnosing 'implicit' expressions (i.e. those that are not explicitly-written in source code) is suppressed, but there are a handful of carve-outs for things that should be exempted from this rule (e.g. optional chaining). in the reproduction, the relevant portion of the AST is something like:

(open_existential_expr implicit type='()' location=/app/example.swift:12:18 range=[/app/example.swift:12:5 - line:12:29]
  (opaque_value_expr implicit type='any SomeProtocol' location=/app/example.swift:12:5 range=[/app/example.swift:12:5 - line:12:5] @ 0x5a20baa22810)
  (declref_expr type='any SomeProtocol' location=/app/example.swift:12:5 range=[/app/example.swift:12:5 - line:12:5] decl=output.(file).output(someProtocol:maybeSP:someClass:).someProtocol@/app/example.swift:9:13 function_ref=unapplied)
  (call_expr type='()' location=/app/example.swift:12:18 range=[/app/example.swift:12:5 - line:12:29] nothrow isolationCrossing=none
    (dot_syntax_call_expr type='() -> ()' location=/app/example.swift:12:18 range=[/app/example.swift:12:5 - line:12:18] nothrow isolationCrossing=none
      (declref_expr type='(any SomeProtocol) -> () -> ()' location=/app/example.swift:12:18 range=[/app/example.swift:12:18 - line:12:18] decl=output.(file).SomeProtocol.doTheThing()@/app/example.swift:2:10 [with (substitution_map generic_signature=<Self where Self : SomeProtocol> (substitution Self -> @opened("7A730076-1388-11EF-ABE0-81AF1B259855", any SomeProtocol) Self))] function_ref=single)
      (argument_list implicit
        (argument
          (opaque_value_expr implicit type='any SomeProtocol' location=/app/example.swift:12:5 range=[/app/example.swift:12:5 - line:12:5] @ 0x5a20baa22810))))
    (argument_list))))))

so i'd assume adding some special handling for that particular structure (i.e. OpenExistentialExpr), as is done for the optional chaining case, would likely be one way to address the issue. or perhaps there's a way to generalize what the special cases have in common, e.g. a top-level implicit expression which internally contains a non-implicit expression that should be diagnosed.