[ClangImporter] Account for synthesized types when filtering by module

We treat redeclarable Clang declarations as present in every module
where they're declared, except for struct/enum/union declarations
where we only count full definitions. This logic requires going from
the imported Swift declaration back to the Clang declaration,
something that's not really possible for "synthesized declarations"
today. The only top-level synthesized declarations we have right now
are the structs we make to wrap error code enums.

The 100% correct thing to do would be to account for people defining
error code enums consistently across multiple modules. In practice,
though, error code enums are used with Objective-C (the importer's
treatment of them is tied to NSError), where redefining existing types
is very unusual. Therefore, this fix just ignores redeclarations of
error code enums, whether they're definitions or not.

rdar://problem/45414271

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -2028,7 +2028,26 @@ static bool isVisibleFromModule(const ClangModuleUnit *ModuleFilter,
     return false;
 
   auto ClangNode = VD->getClangNode();
-  assert(ClangNode);
+  if (!ClangNode) {
+    // If we synthesized a ValueDecl, it won't have a Clang node. But so far
+    // all the situations where we synthesize top-level declarations are
+    // situations where we don't have to worry about C redeclarations.
+    // We should only consider the declaration visible from its owning module.
+    auto *SynthesizedTypeAttr =
+        VD->getAttrs().getAttribute<ClangImporterSynthesizedTypeAttr>();
+    assert(SynthesizedTypeAttr);
+
+    // When adding new ClangImporterSynthesizedTypeAttr::Kinds, make sure that
+    // the above statement still holds: "we don't want to allow these
+    // declarations to be treated as present in multiple modules".
+    switch (SynthesizedTypeAttr->getKind()) {
+    case ClangImporterSynthesizedTypeAttr::Kind::NSErrorWrapper:
+    case ClangImporterSynthesizedTypeAttr::Kind::NSErrorWrapperAnon:
+      break;
+    }
+
+    return false;
+  }
 
   auto &ClangASTContext = ModuleFilter->getClangASTContext();
   auto OwningClangModule = getClangOwningModule(ClangNode, ClangASTContext);

test/ClangImporter/Inputs/custom-modules/RedeclaredErrorEnum/Base.h

@@ -0,0 +1,9 @@
+@import Foundation;
+
+extern NSString * const SomeErrorDomain;
+// typedef NS_ERROR_ENUM(SomeErrorDomain, SomeErrorCode) { ... }
+typedef enum SomeErrorCode : long SomeErrorCode;
+enum __attribute__((ns_error_domain(SomeErrorDomain))) SomeErrorCode : long {
+  SomeErrorX,
+  SomeErrorY
+};

test/ClangImporter/Inputs/custom-modules/RedeclaredErrorEnum/Redeclared.h

@@ -0,0 +1,7 @@
+@import Foundation;
+@import Base;
+
+extern NSString * const SomeErrorDomain;
+// typedef NS_ERROR_ENUM(SomeErrorDomain, SomeErrorCode);
+typedef enum SomeErrorCode : long SomeErrorCode;
+enum __attribute__((ns_error_domain(SomeErrorDomain))) SomeErrorCode : long;

test/ClangImporter/Inputs/custom-modules/RedeclaredErrorEnum/module.modulemap

@@ -0,0 +1,8 @@
+module Base {
+  header "Base.h"
+}
+
+module Redeclared {
+  header "Redeclared.h"
+  export *
+}

test/ClangImporter/enum-error-redeclared.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck %s -verify -enable-objc-interop -I %S/Inputs/custom-modules/RedeclaredErrorEnum
+
+import Redeclared
+
+// Referencing this error type (defined in Base, redeclared in Redeclared)
+// used to cause a compiler crash (rdar://problem/45414271).
+_ = SomeError.self
+_ = SomeError.Code.self
+
+_ = Redeclared.SomeError.self
+_ = Base.SomeError.self