SILGen: Set context generic params for bridging thunks.

So that we do the right thing with thunks for generic ObjC methods. Partially fixes rdar://problem/27869089.

Author: jckarter

lib/SILGen/SILGenBridging.cpp

@@ -957,6 +957,9 @@ void SILGenFunction::emitNativeToForeignThunk(SILDeclRef thunk) {
     SGM.M, SGM.M.getSwiftModule(), subs);
   SILType substSILTy = SILType::getPrimitiveObjectType(substTy);
 
+  // Use the same context generic params as the native entry point.
+  F.setContextGenericParams(nativeInfo.ContextGenericParams);
+
   auto loc = thunk.getAsRegularLocation();
   loc.markAutoGenerated();
   Scope scope(Cleanups, CleanupLocation::get(loc));
@@ -1116,6 +1119,9 @@ void SILGenFunction::emitForeignToNativeThunk(SILDeclRef thunk) {
   auto nativeFnTy = F.getLoweredFunctionType();
   assert(nativeFnTy == nativeCI.SILFnType);
 
+  // Use the same context generic params as the native entry point.
+  F.setContextGenericParams(nativeCI.ContextGenericParams);
+  
   // Find the foreign error convention and 'self' parameter index.
   Optional<ForeignErrorConvention> foreignError;
   if (nativeFnTy->hasErrorResult()) {

test/SILGen/Inputs/objc_bridged_generic_conformance.h

@@ -0,0 +1,5 @@
+@import Foundation;
+
+@interface Thingy<T>: NSObject
+
+@end

test/SILGen/objc_bridged_generic_conformance.swift

@@ -0,0 +1,8 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-silgen %s -import-objc-header %S/Inputs/objc_bridged_generic_conformance.h -verify
+// REQUIRES: objc_interop
+
+protocol P { func test() }
+
+extension Thingy: P {
+  func test() {}
+}