SILGen: Fix assertion failure when emitting foreign-to-native thunk for a method with inout 'self'

We import a C function taking a non-const pointer 'self' parameter
as a mutating method.

The getParameterTypes() helper method in SILGenBridging.cpp would
assert upon encountering the inout 'self' parameter, even though
emitForeignToNativeThunk() would still emit correct code as long
as the 'self' type was not bridged.

Relax the assertion a bit to hopefully still catch bugs where
other parameters are unexpectedly 'inout', but allow it on 'self',
and add a test.

Fixes <rdar://problem/70346482>.

Author: slavapestov

lib/SILGen/SILGenBridging.cpp

@@ -336,10 +336,17 @@ static ManagedValue emitManagedParameter(SILGenFunction &SGF, SILLocation loc,
 
 /// Get the type of each parameter, filtering out empty tuples.
 static SmallVector<CanType, 8>
-getParameterTypes(AnyFunctionType::CanParamArrayRef params) {
+getParameterTypes(AnyFunctionType::CanParamArrayRef params,
+                  bool hasSelfParam=false) {
   SmallVector<CanType, 8> results;
-  for (auto param : params) {
-    assert(!param.isInOut() && !param.isVariadic());
+  for (auto n : indices(params)) {
+    bool isSelf = (hasSelfParam ? n == params.size() - 1 : false);
+
+    const auto &param = params[n];
+    assert(isSelf || !param.isInOut() &&
+           "Only the 'self' parameter can be inout in a bridging thunk");
+    assert(!param.isVariadic());
+
     if (param.getPlainType()->isVoid())
       continue;
     results.push_back(param.getPlainType());
@@ -1723,10 +1730,11 @@ void SILGenFunction::emitForeignToNativeThunk(SILDeclRef thunk) {
     };
 
     {
+      bool hasSelfParam = fd->hasImplicitSelfDecl();
       auto foreignFormalParams =
-        getParameterTypes(foreignCI.LoweredType.getParams());
+        getParameterTypes(foreignCI.LoweredType.getParams(), hasSelfParam);
       auto nativeFormalParams =
-        getParameterTypes(nativeCI.LoweredType.getParams());
+        getParameterTypes(nativeCI.LoweredType.getParams(), hasSelfParam);
 
       for (unsigned nativeParamIndex : indices(params)) {
         // Bring the parameter to +1.
@@ -1762,7 +1770,7 @@ void SILGenFunction::emitForeignToNativeThunk(SILDeclRef thunk) {
 
         maybeAddForeignErrorArg();
 
-        bool isSelf = nativeParamIndex == params.size() - 1;
+        bool isSelf = (hasSelfParam && nativeParamIndex == params.size() - 1);
 
         if (memberStatus.isInstance()) {
           // Leave space for `self` to be filled in later.
@@ -1787,6 +1795,12 @@ void SILGenFunction::emitForeignToNativeThunk(SILDeclRef thunk) {
           F.mapTypeIntoContext(foreignFormalParams[nativeParamIndex])
             ->getCanonicalType();
 
+        if (isSelf) {
+          assert(!nativeCI.LoweredType.getParams()[nativeParamIndex].isInOut() ||
+                 nativeFormalType == foreignFormalType &&
+                 "Cannot bridge 'self' parameter if passed inout");
+        }
+
         auto foreignParam = foreignFnTy->getParameters()[foreignArgIndex++];
         SILType foreignLoweredTy =
             F.mapTypeIntoContext(foreignParam.getSILStorageType(

test/SILGen/Inputs/foreign_to_native_inout_self_helper.h

@@ -0,0 +1,3 @@
+typedef struct {} MyIterator __attribute__((swift_name("MyIterator")));
+
+void MyIteratorNext(MyIterator *self) __attribute__((swift_name("MyIterator.next(self:)")));

test/SILGen/foreign_to_native_inout_self.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-frontend -emit-silgen %s -import-objc-header %S/Inputs/foreign_to_native_inout_self_helper.h | %FileCheck %s
+
+protocol FakeIterator {
+  mutating func next()
+}
+
+extension MyIterator : FakeIterator {}
+
+// CHECK-LABEL: sil shared [serializable] [thunk] [ossa] @$sSo10MyIteratora4nextyyFTO : $@convention(method) (@inout MyIterator) -> () {
+// CHECK: bb0(%0 : $*MyIterator):
+// CHECK: [[FN:%.*]] = function_ref @MyIteratorNext : $@convention(c) (@inout MyIterator) -> ()
+// CHECK:   apply [[FN]](%0) : $@convention(c) (@inout MyIterator) -> ()
+// CHECK:   [[RESULT:%.*]] = tuple ()
+// CHECK:   return [[RESULT]] : $()
+// CHECK: }