[CSOptimizer] Literal arguments should cause score reset only for operators

Resetting score to `0.1` is intended to make sure that the
solver picks the outermost disjunction in literal chains like
`1 + 2 + 3 ...` because that would provide context to the
inner choices.

Resolves: #78371
Resolves: rdar://142105691

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -958,7 +958,7 @@ static void determineBestChoicesInContext(
           // Preferring outer disjunction first works better in situations
           // when contextual type for the whole chain becomes available at
           // some point during solving at it would allow for faster pruning.
-          if (score > 0 && onlyLiteralCandidates)
+          if (score > 0 && onlyLiteralCandidates && decl->isOperator())
             score = 0.1;
 
           // If one of the result types matches exactly, that's a good

validation-test/Sema/issue78371.swift

@@ -0,0 +1,22 @@
+// RUN: %target-typecheck-verify-swift
+
+// https://github.com/swiftlang/swift/issues/78371
+
+// Note that the test has to use a standard operator because
+// custom ones are not optimized.
+
+struct Scalar : Equatable {
+    init(_: UInt8) {}
+    init?(_: Int) {}
+}
+
+func ==(_: Scalar, _: Scalar) -> Bool { }
+
+extension Optional where Wrapped == Scalar {
+    static func ==(_: Wrapped?, _: Wrapped?) -> Wrapped { }
+}
+
+func test(a: Scalar) {
+    let result = a == Scalar(0x07FD)
+    let _: Scalar = result // Ok
+}