IRGen: Relax an assertion

The result type mismatch comes up when we're calling a covariant
override of a class method resiliently as well.

Author: slavapestov

lib/IRGen/GenCall.cpp

@@ -1510,11 +1510,14 @@ void CallEmission::emitToUnmappedExplosion(Explosion &out) {
 
   // For ABI reasons the result type of the call might not actually match the
   // expected result type.
+  //
+  // This can happen when calling C functions, or class method dispatch thunks
+  // for methods that have covariant ABI-compatible overrides.
   auto expectedNativeResultType = nativeSchema.getExpandedType(IGF.IGM);
   if (result->getType() != expectedNativeResultType) {
-    // This should only be needed when we call C functions.
-    assert(getCallee().getOrigFunctionType()->getLanguage() ==
-           SILFunctionLanguage::C);
+    auto origFnType = getCallee().getOrigFunctionType();
+    assert(origFnType->getLanguage() == SILFunctionLanguage::C ||
+           origFnType->getRepresentation() == SILFunctionTypeRepresentation::Method);
     result =
         IGF.coerceValue(result, expectedNativeResultType, IGF.IGM.DataLayout);
   }

test/IRGen/class_resilience_thunks.swift

@@ -55,3 +55,7 @@ public func testDispatchThunkMyOverride(d: MyDerived, o: Object) {
 
   // CHECK: ret void
 }
+
+public func testDispatchThunkCast(d: Derived) {
+  _ = d.returnsSuperclass()
+}

test/Inputs/resilient_class_thunks.swift

@@ -2,12 +2,18 @@ public class Object {
   public init() {}
 }
 
+public class Subclass : Object {}
+
 open class Base<T> {
   open func takesT(_: T) {}
 
   open func takesInt(_: Int) {}
 
   open func takesReference(_: Object) {}
+
+  open func returnsSuperclass() -> Object {
+    fatalError()
+  }
 }
 
 open class Derived : Base<Int> {
@@ -19,4 +25,9 @@ open class Derived : Base<Int> {
 
   // Override has different formal type but is ABI-compatible
   open override func takesReference(_: Object?) {}
+
+  // Override has a more specific return type but is ABI-compatible
+  open override func returnsSuperclass() -> Subclass {
+    fatalError()
+  }
 }