Merge pull request #41594 from slavapestov/rqm-foreign-function-type-cycle

Sema: Fix cycle in structural type resolution via TypeChecker::diagnoseInvalidFunctionType()

Author: slavapestov

lib/Sema/TypeChecker.cpp

@@ -543,6 +543,12 @@ bool TypeChecker::diagnoseInvalidFunctionType(FunctionType *fnTy, SourceLoc loc,
                                               Optional<FunctionTypeRepr *>repr,
                                               DeclContext *dc,
                                               Optional<TypeResolutionStage> stage) {
+  // Some of the below checks trigger cycles if we don't have a generic
+  // signature yet; we'll run the checks again in
+  // TypeResolutionStage::Interface.
+  if (stage == TypeResolutionStage::Structural)
+    return false;
+
   // If the type has a placeholder, don't try to diagnose anything now since
   // we'll produce a better diagnostic when (if) the expression successfully
   // typechecks.
@@ -582,8 +588,7 @@ bool TypeChecker::diagnoseInvalidFunctionType(FunctionType *fnTy, SourceLoc loc,
 
   // `@differentiable` function types must return a differentiable type and have
   // differentiable (or `@noDerivative`) parameters.
-  if (extInfo.isDifferentiable() &&
-      stage != TypeResolutionStage::Structural) {
+  if (extInfo.isDifferentiable()) {
     auto result = fnTy->getResult();
     auto params = fnTy->getParams();
     auto diffKind = extInfo.getDifferentiabilityKind();

test/Generics/protocol_typealias_cycle_3.swift

@@ -0,0 +1,9 @@
+// RUN: %target-swift-frontend -typecheck %s -requirement-machine-protocol-signatures=on
+
+protocol P {
+  typealias MyFunction =
+      @convention(c) (UnsafeMutableRawPointer?,
+                      UnsafeMutableRawPointer?,
+                      UnsafeMutableRawPointer?) -> CInt
+}
+