Index: clang/lib/Analysis/FlowSensitive/Transfer.cpp
===================================================================
--- clang/lib/Analysis/FlowSensitive/Transfer.cpp
+++ clang/lib/Analysis/FlowSensitive/Transfer.cpp
@@ -561,7 +561,9 @@
     assert(ConstructorDecl != nullptr);
 
     if (ConstructorDecl->isCopyOrMoveConstructor()) {
-      assert(S->getNumArgs() == 1);
+      // It is permissible for a copy/move constructor to have additional
+      // parameters as long as they have default arguments defined for them.
+      assert(S->getNumArgs());
 
       const Expr *Arg = S->getArg(0);
       assert(Arg != nullptr);
Index: clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
===================================================================
--- clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
+++ clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
@@ -2102,6 +2102,54 @@
       });
 }
 
+TEST(TransferTest, CopyConstructorWithDefaultArgument) {
+  std::string Code = R"(
+    struct A {
+      int Baz;
+      A() = default;
+      A(const A& a, bool def = true) { Baz = a.Baz; }
+    };
+
+    void target() {
+      A Foo;
+      (void)Foo.Baz;
+      A Bar = Foo;
+      // [[p]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        ASSERT_THAT(Results.keys(), UnorderedElementsAre("p"));
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        const ValueDecl *FooDecl = findValueDecl(ASTCtx, "Foo");
+        ASSERT_THAT(FooDecl, NotNull());
+
+        const ValueDecl *BarDecl = findValueDecl(ASTCtx, "Bar");
+        ASSERT_THAT(BarDecl, NotNull());
+
+        const ValueDecl *BazDecl = findValueDecl(ASTCtx, "Baz");
+        ASSERT_THAT(BazDecl, NotNull());
+
+        const auto *FooLoc = cast<AggregateStorageLocation>(
+            Env.getStorageLocation(*FooDecl, SkipPast::None));
+        const auto *BarLoc = cast<AggregateStorageLocation>(
+            Env.getStorageLocation(*BarDecl, SkipPast::None));
+
+        const auto *FooVal = cast<StructValue>(Env.getValue(*FooLoc));
+        const auto *BarVal = cast<StructValue>(Env.getValue(*BarLoc));
+        EXPECT_EQ(FooVal, BarVal);
+
+        const auto *FooBazVal =
+            cast<IntegerValue>(Env.getValue(FooLoc->getChild(*BazDecl)));
+        const auto *BarBazVal =
+            cast<IntegerValue>(Env.getValue(BarLoc->getChild(*BazDecl)));
+        EXPECT_EQ(FooBazVal, BarBazVal);
+      });
+}
+
 TEST(TransferTest, CopyConstructorWithParens) {
   std::string Code = R"(
     struct A {