[NFC] Add/clarify comments.

Author: dan-zheng

lib/Sema/CSSimplify.cpp

@@ -4432,6 +4432,9 @@ ConstraintSystem::simplifyApplicableFnConstraint(
   ConstraintLocatorBuilder outerLocator =
     getConstraintLocator(anchor, parts, locator.getSummaryFlags());
 
+  // Before stripping optional types, save original type for handling
+  // @dynamicCallable applications. This supports the fringe case where
+  // `Optional` itself is extended with @dynamicCallable functionality.
   auto origType2 = desugar2;
   unsigned unwrapCount = 0;
   if (shouldAttemptFixes()) {

test/attr/attr_dynamic_callable.swift

@@ -201,8 +201,8 @@ func testExistential(
   c: CallableStruct, d: CallableClass
 ) -> Int {
   // Types that define only the `withKeywordsArguments` method can be called
-  // with no arguments.
-  _ = b() + d()
+  // with no argument labels.
+  _ = b() + b(1, 2) + d() + d(1, 2)
   return a(1, 2, 3) + b(label1: 1, 2, label2: 3) + c(1, 2, 3) + d(label1: 1, 2, 3)
 }
 
@@ -243,7 +243,7 @@ func testRefinedProtocolType<P : FloatingPoint>(
 extension Optional : KeywordCallableProtocol {}
 extension Array : KeywordCallableProtocol {}
 
-func testOptionalExtension() {
+func testExtensions() {
   let x: Int? = 3
   // Test `Optional` extension.
   print(x())