Disfavor the TestPredicate-returning == overloads

[mansbernhardt]

Problem

The two public == operators in ModelTester.swift that return TestPredicate exist so that expect { a == b } failures can print both sides. Every file that imports SwiftModel can see them, though, and outside a builder they can win. A downstream app wrote let immediate = self.outcome == .needsNewPlayer in non-test code, it inferred TestPredicate, and it needed an explicit : Bool to compile.

Fix

• Both == overloads are now @_disfavoredOverload, so an untyped comparison infers Bool. No other operator overloads return TestPredicate.
• Keeping the builder's rich messages: disfavoring the operators alone caused a regression. The builder's buildExpression(Bool) was already disfavored, so each path carried one disfavored declaration, and the solver broke the tie toward the concrete synthesized ==. As a result, expect { model.outcome == .needsNewPlayer } lost its − Outcome.needsNewPlayer / + Outcome.pending diff. The fix removes AssertBuilder.buildExpression(Bool). A plain Bool now reaches the existing disfavored buildExpression(Bool?) through a value-to-optional conversion, and that extra cost keeps the tie going to TestPredicate. A comment at the overload explains why no Bool overload exists.

Verification

• Two new builder … == … snapshots in OutputSnapshotTests cover the builder path with no type annotation. Before this PR, no snapshot covered it; the existing one used let pred: TestPredicate = …. The shapes are optional Int? == Int and an enum compared against an implicit member. I recorded both on main first, and the output with this PR is byte-identical on Swift 6.3.3 (Xcode 26) and 6.4 (Xcode 27). With only the operators disfavored, the enum snapshot fails.
• Int == literal and String == literal resolve differently per toolchain, so they have no snapshot. On 6.3 they now show the rich diff, where main printed only SimpleCounter.count == 3. On 6.4 they still resolve to the concrete Int.==, as on main. Neither toolchain regresses. The first CI run failed only because I had pinned the 6.4 output for these two shapes.
• TestPredicateOverloadTests uses a plain import SwiftModel. It assigns x == y to an untyped let and passes the result to a function that only accepts Bool, covering enum implicit-member, optional, struct and generic comparisons. It also checks type(of:) == Bool.self. The file fails to compile without the fix.
• swift build --build-tests is warning-free. scripts/test passes: 721 main tests, 126 snapshot tests and the macro tests. The CounterFact, Onboarding, Search and TodoList examples build for testing. Standups fails inside the third-party swiftui-navigation checkout with the local toolchain, which is unrelated to this change.

🤖 Generated with Claude Code