SILGen: Pick UIApplicationMain by looking directly in the Clang module.

This saves us from having to do overload resolution on the overlay module, which fails if an overlay declaration exactly matches the imported signature of the original ObjC declaration. Fixes rdar://problem/42352695 harder.

Author: jckarter

lib/SILGen/SILGenFunction.cpp

@@ -438,30 +438,25 @@ void SILGenFunction::emitArtificialTopLevel(ClassDecl *mainClass) {
     // we're getting away with it because the types are guaranteed to already
     // be imported.
     ASTContext &ctx = getASTContext();
-    ModuleDecl *UIKit = ctx.getLoadedModule(ctx.getIdentifier("UIKit"));
+    
+    std::pair<Identifier, SourceLoc> UIKitName =
+      {ctx.getIdentifier("UIKit"), SourceLoc()};
+    
+    ModuleDecl *UIKit = ctx
+      .getClangModuleLoader()
+      ->loadModule(SourceLoc(), UIKitName);
+    assert(UIKit && "couldn't find UIKit objc module?!");
     SmallVector<ValueDecl *, 1> results;
     UIKit->lookupQualified(UIKit->getInterfaceType(),
                            ctx.getIdentifier("UIApplicationMain"),
                            NL_QualifiedDefault,
                            /*resolver*/nullptr,
                            results);
-    assert(!results.empty() && "couldn't find UIApplicationMain in UIKit");
-
-    // We want the original UIApplicationMain() declaration from Objective-C,
-    // not any overlay overloads.
-    ValueDecl *UIApplicationMainDecl = nullptr;
-    for (auto *result : results) {
-      if (result->hasClangNode()) {
-        assert(!UIApplicationMainDecl
-               && "more than one UIApplicationMain defined in ObjC?!");
-        UIApplicationMainDecl = result;
-#ifndef NDEBUG
-        break;
-#endif
-      }
-    }
-    
-    assert(UIApplicationMainDecl && "no UIApplicationMain defined in ObjC?!");
+    assert(results.size() == 1
+           && "couldn't find a unique UIApplicationMain in the UIKit ObjC "
+              "module?!");
+
+    ValueDecl *UIApplicationMainDecl = results.front();
 
     auto mainRef = SILDeclRef(UIApplicationMainDecl).asForeign();
     auto UIApplicationMainFn = SGM.M.getOrCreateFunction(mainClass, mainRef,

test/SILGen/Inputs/UIKit.swift

@@ -2,3 +2,9 @@ import Foundation
 @_exported import UIKit
 
 public func UIApplicationMain() {}
+public func UIApplicationMain(_ argc: Int32,
+                              _ argv: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!,
+                              _ principalClassName: String?,
+                              _ delegateClassName: String?) -> Int32 {
+  return 0
+}