Swift 6.4: ternary with a doubly-labelled tuple result fails to type-check

[strike19]

Description

Swift 6.4: ternary with a doubly-labelled tuple result fails to type-check

Filed against: Swift compiler (Xcode 27.0 beta)
Date: 2026-08-25
Reporter: OrcusKern project

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Summary

In Swift 6.4 (Xcode 27.0 beta), a ternary conditional expression whose
result type is a tuple with labels at both the outer and the inner
level fails to type-check. The compiler cannot produce a diagnostic and
emits its internal fallback message:

error: failed to produce diagnostic for expression;
       please submit a bug report (https://swift.org/contributing/#reporting-bugs)

The same code compiles without error or warning in Swift 6.2
(Xcode 26.6).

Environment

Failing	Xcode 27.0 beta, build 27A5237l — Apple Swift version 6.4 (swiftlang-6.4.0.30.4 clang-2100.3.30.1)
Working	Xcode 26.6, build 17F113 — SDK MacOSX26.5
Host	macOS 26.6.2 (build 25G83), Apple Silicon (arm64)
Command	xcrun swiftc -typecheck reproducer.swift

Reproducer

Six lines, no imports, no generics, no map:

func f() {
    let a = (x: Int64(0), y: Int64(0))
    let b = (x: Int64(1), y: Int64(1))
    let r: (p: (x: Int64, y: Int64), q: (x: Int64, y: Int64)) =
        (a.x, a.y) <= (b.x, b.y) ? (a, b) : (b, a)
    _ = r
}

Expected: compiles, as it does in Swift 6.2.
Actual: error: failed to produce diagnostic for expression on the
line holding the ternary.

Reduction — what switches the bug off

Each row changes exactly one thing against the reproducer above.
All five variants compile under Swift 6.2; only two still fail under 6.4.

Variant	Change	Swift 6.2	Swift 6.4
Reproducer	—	ok	error
A	inner labels removed: (p: (Int64, Int64), q: (Int64, Int64))	ok	ok
B	outer labels removed: ((x: Int64, y: Int64), (x: Int64, y: Int64))	ok	ok
C	if/else instead of the ternary	ok	ok
D	Int instead of Int64	ok	error

So the trigger is labels on both nesting levels, combined with the
ternary conditional. Removing the labels on either level, or replacing
the ternary with an if/else, makes it compile. The element type is
irrelevant.

Workaround

Replace the ternary with an if/else:

let r: (p: (x: Int64, y: Int64), q: (x: Int64, y: Int64))
if (a.x, a.y) <= (b.x, b.y) { r = (a, b) } else { r = (b, a) }

Related but distinct

failed to produce diagnostic for expression is the type checker's
generic fallback, so it covers many unrelated causes. I checked the
eleven issues filed against swiftlang/swift since June 2026 carrying
that exact message (as of 2026-08-25); none matches this trigger:

Issue	Its trigger
#91612	typed throws, error type differs only in generic arguments
#91587	comparison between Character and String.Index
#91518	mixed-type array literal passed to Set()
#91215	@ViewBuilder func with switch over Result
#91156	optional-chained Int? assigned to Int64?
#90805	bad ?? expression given to a CChar initialiser
#90780	closure with disagreeing inferred return types
#90733	invalid key path in constrained dynamic member lookup
#90492	type(of:) referenced as a first-class function
#90322	FRT from an array cast as AnyObject
#85133	no reproducer given

#90780 is the nearest — also a closure whose return types cannot be
reconciled. This report is distinct: the minimal reproducer contains no
closure at all, no optionals, no generics and no map. It is a plain
let binding with an explicitly written tuple type and a ternary. The
original real-world occurrence did involve a map closure, which is why
the two can look alike at first glance.

Real-world impact

Found while building an existing, previously clean Swift 6 package
against the Xcode 27 beta toolchain. One expression in the whole
package triggers it, in a closure returning a pair of coordinate
tuples:

let paare = segmente.map { s -> (p: (x: Int64, y: Int64), q: (x: Int64, y: Int64)) in
    let a = ganz(s.a), b = ganz(s.b)
    return (a.x, a.y) <= (b.x, b.y) ? (a, b) : (b, a)
}

The build stops there; nothing else in the package fails. Since the
compiler emits no usable diagnostic, the cause is not discoverable from
the message alone — it took bisecting the expression to find it.

Reproduction

// Swift 6.4 (Xcode 27.0 beta 27A5237l):
//   error: failed to produce diagnostic for expression
// Swift 6.2 (Xcode 26.6): compiles cleanly.
//
//   xcrun swiftc -typecheck 2026-08-25-reproducer.swift

func f() {
    let a = (x: Int64(0), y: Int64(0))
    let b = (x: Int64(1), y: Int64(1))
    let r: (p: (x: Int64, y: Int64), q: (x: Int64, y: Int64)) =
        (a.x, a.y) <= (b.x, b.y) ? (a, b) : (b, a)
    _ = r
}

Expected behavior

Hopefully compiles without error for 27 version once rolled out.

Environment

| Failing | Xcode 27.0 beta, build 27A5237l — Apple Swift version 6.4 (swiftlang-6.4.0.30.4 clang-2100.3.30.1) |
| Working | Xcode 26.6, build 17F113 — SDK MacOSX26.5 |
| Host | macOS 26.6.2 (build 25G83), Apple Silicon (arm64) |
| Command | xcrun swiftc -typecheck reproducer.swift |

Additional information