[CSOptimizer] Look through OptionalEvaluationExprs when dealing with unapplied disjunctions

This improves performance for situations like `a?.b + 2` by
giving a high score to `b` which prioritizes it over an operator.

Author: xedin

lib/Sema/CSOptimizer.cpp

@@ -299,7 +299,13 @@ static void determineBestChoicesInContext(
     if (applicableFn.isNull()) {
       auto *locator = disjunction->getLocator();
       if (auto expr = getAsExpr(locator->getAnchor())) {
-        if (auto *parentExpr = cs.getParentExpr(expr)) {
+        auto *parentExpr = cs.getParentExpr(expr);
+        // Look through optional evaluation, so
+        // we can cover expressions like `a?.b + 2`.
+        if (isExpr<OptionalEvaluationExpr>(parentExpr))
+          parentExpr = cs.getParentExpr(parentExpr);
+
+        if (parentExpr) {
           // If this is a chained member reference or a direct operator
           // argument it could be prioritized since it helps to establish
           // context for other calls i.e. `(a.)b + 2` if `a` and/or `b`

validation-test/Sema/type_checker_perf/fast/swift_package_index_1.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -typecheck %s -solver-scope-threshold=1000
+// RUN: %target-swift-frontend -typecheck %s -solver-scope-threshold=500
 // REQUIRES: tools-release,no_asan
 
 public class Cookie {