Ignore async/sync mismatch on main

The main function is different from other function resolutions. It isn't
being called from a synchronous or asynchronous context, but defines
whether the program starts in a synchronous or async context. As a
result, we should not prefer one over the other, so the scoring
mechanism shouldn't involve the async/sync score when resolving the main
function.

This patch adds a constraint solver flag to ignore async/sync context
mismatches so that we do not favor one over the other, but otherwise use
the normal resolution behavior.

Author: etcwilde

include/swift/Sema/ConstraintSystem.h

@@ -1515,6 +1515,9 @@ enum class ConstraintSystemFlags {
   /// calling conventions, say due to Clang attributes such as
   /// `__attribute__((ns_consumed))`.
   UseClangFunctionTypes = 0x80,
+
+  /// When set, ignore async/sync mismatches
+  IgnoreAsyncSyncMismatch = 0x100,
 };
 
 /// Options that affect the constraint system as a whole.

lib/Sema/ConstraintSystem.cpp

@@ -3304,7 +3304,8 @@ void ConstraintSystem::resolveOverload(ConstraintLocator *locator,
     // If we're choosing an asynchronous declaration within a synchronous
     // context, or vice-versa, increase the async/async mismatch score.
     if (auto func = dyn_cast<AbstractFunctionDecl>(decl)) {
-      if (!func->hasPolymorphicEffect(EffectKind::Async) &&
+      if (!Options.contains(ConstraintSystemFlags::IgnoreAsyncSyncMismatch) &&
+          !func->hasPolymorphicEffect(EffectKind::Async) &&
           func->isAsyncContext() != isAsynchronousContext(useDC)) {
         increaseScore(
             func->isAsyncContext() ? SK_AsyncInSyncMismatch : SK_SyncInAsync);

lib/Sema/TypeCheckAttr.cpp

@@ -2172,7 +2172,8 @@ SynthesizeMainFunctionRequest::evaluate(Evaluator &evaluator,
   // type-checking the call to mainType.main().
 
   auto resolution = resolveValueMember(
-      *declContext, nominal->getInterfaceType(), context.Id_main);
+      *declContext, nominal->getInterfaceType(), context.Id_main,
+      constraints::ConstraintSystemFlags::IgnoreAsyncSyncMismatch);
   FuncDecl *mainFunction =
       resolveMainFunctionDecl(declContext, resolution, context);
   if (!mainFunction) {