[cxx-interop] Fix lazy member lookup for forward-declared structs inside of a namespace.

Author: zoecarver

lib/ClangImporter/SwiftLookupTable.cpp

@@ -1967,8 +1967,14 @@ void importer::addEntryToLookupTable(SwiftLookupTable &table,
         if (!alreadyAdded.insert(canonicalMember).second)
           continue;
 
-        if (auto namedMember = dyn_cast<clang::NamedDecl>(canonicalMember))
+        if (auto namedMember = dyn_cast<clang::NamedDecl>(canonicalMember)) {
+          // Make sure we're looking at the definition, otherwise, there won't
+          // be any members to add.
+          if (auto recordDecl = dyn_cast<clang::RecordDecl>(namedMember))
+            if (auto def = recordDecl->getDefinition())
+              namedMember = def;
           addEntryToLookupTable(table, namedMember, nameImporter);
+        }
       }
     }
   }

test/Interop/Cxx/class/Inputs/forward-declared-in-namespace.h

@@ -0,0 +1,14 @@
+#ifndef TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_IN_NAMESPACE_H
+#define TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_IN_NAMESPACE_H
+
+namespace Space {
+
+struct C;
+
+struct C {
+  void test() const;
+};
+
+}
+
+#endif // TEST_INTEROP_CXX_CLASS_INPUTS_FORWARD_DECLARED_IN_NAMESPACE_H

test/Interop/Cxx/class/Inputs/module.modulemap

@@ -105,5 +105,9 @@ module ForwardDeclaredCxxRecord {
 
 module ReturnsLargeClass {
   header "returns-large-class.h"
+}
+
+module ForwardDeclaredInNamespace {
+  header "forward-declared-in-namespace.h"
   requires cplusplus
 }

test/Interop/Cxx/class/forward-declared-in-namespace-typechecker.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-experimental-cxx-interop
+
+import ForwardDeclaredInNamespace
+
+public func test(c: Space.C) { c.test() }