[clang][dataflow] Fix crash when operator= result type is not destination type. (#90898)

The existing code was full of comments about how we assume this is
always the
case, but it's not mandated by the standard, and there is code out there
that
returns a different type. So check that the result type is in fact the
same as
the destination type before attempting to copy to the result.

To make sure that we don't bail out in more cases than intended, I've
extended
existing tests to verify that in the common case, we do return the
destination
object (by reference or value, as the case may be).

Author: martinboehme

clang/lib/Analysis/FlowSensitive/Transfer.cpp

@@ -556,14 +556,23 @@ class TransferVisitor : public ConstStmtVisitor<TransferVisitor> {
 
       copyRecord(*LocSrc, *LocDst, Env);
 
-      // If the expr is a glvalue, we can reasonably assume the operator is
-      // returning T& and thus we can assign it `LocDst`.
-      if (S->isGLValue()) {
+      // The assignment operator can have an arbitrary return type. We model the
+      // return value only if the return type is the same as or a base class of
+      // the destination type.
+      if (S->getType().getCanonicalType().getUnqualifiedType() !=
+          LocDst->getType().getCanonicalType().getUnqualifiedType()) {
+        auto ReturnDecl = S->getType()->getAsCXXRecordDecl();
+        auto DstDecl = LocDst->getType()->getAsCXXRecordDecl();
+        if (ReturnDecl == nullptr || DstDecl == nullptr)
+          return;
+        if (!DstDecl->isDerivedFrom(ReturnDecl))
+          return;
+      }
+
+      if (S->isGLValue())
         Env.setStorageLocation(*S, *LocDst);
-      } else if (S->getType()->isRecordType()) {
-        // Assume that the assignment returns the assigned value.
+      else
         copyRecord(*LocDst, Env.getResultObjectLocation(*S), Env);
-      }
 
       return;
     }

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

@@ -2228,7 +2228,7 @@ TEST(TransferTest, AssignmentOperator) {
       A Foo = { 1 };
       A Bar = { 2 };
       // [[p1]]
-      Foo = Bar;
+      A &Rval = (Foo = Bar);
       // [[p2]]
       Foo.Baz = 3;
       // [[p3]]
@@ -2274,6 +2274,7 @@ TEST(TransferTest, AssignmentOperator) {
               cast<RecordStorageLocation>(Env2.getStorageLocation(*BarDecl));
 
           EXPECT_TRUE(recordsEqual(*FooLoc2, *BarLoc2, Env2));
+          EXPECT_EQ(&getLocForDecl(ASTCtx, Env2, "Rval"), FooLoc2);
 
           const auto *FooBazVal2 =
               cast<IntegerValue>(getFieldValue(FooLoc2, *BazDecl, Env2));
@@ -2441,7 +2442,75 @@ TEST(TransferTest, AssignmentOperatorReturnsByValue) {
   // This is a crash repro.
   std::string Code = R"(
     struct S {
-      S operator=(S&& other);
+      S operator=(const S&);
+      int i;
+    };
+    void target() {
+      S S1 = { 1 };
+      S S2 = { 2 };
+      S S3 = { 3 };
+      // [[before]]
+      // Test that the returned value is modeled by assigning to another value.
+      S1 = (S2 = S3);
+      (void)0;
+      // [[after]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const ValueDecl *S1Decl = findValueDecl(ASTCtx, "S1");
+        const ValueDecl *S2Decl = findValueDecl(ASTCtx, "S2");
+        const ValueDecl *S3Decl = findValueDecl(ASTCtx, "S3");
+
+        const Environment &EnvBefore =
+            getEnvironmentAtAnnotation(Results, "before");
+
+        EXPECT_FALSE(recordsEqual(
+            *EnvBefore.get<RecordStorageLocation>(*S1Decl),
+            *EnvBefore.get<RecordStorageLocation>(*S2Decl), EnvBefore));
+        EXPECT_FALSE(recordsEqual(
+            *EnvBefore.get<RecordStorageLocation>(*S2Decl),
+            *EnvBefore.get<RecordStorageLocation>(*S3Decl), EnvBefore));
+
+        const Environment &EnvAfter =
+            getEnvironmentAtAnnotation(Results, "after");
+
+        EXPECT_TRUE(recordsEqual(*EnvAfter.get<RecordStorageLocation>(*S1Decl),
+                                 *EnvAfter.get<RecordStorageLocation>(*S2Decl),
+                                 EnvAfter));
+        EXPECT_TRUE(recordsEqual(*EnvAfter.get<RecordStorageLocation>(*S2Decl),
+                                 *EnvAfter.get<RecordStorageLocation>(*S3Decl),
+                                 EnvAfter));
+      });
+}
+
+TEST(TransferTest, AssignmentOperatorReturnsDifferentTypeByRef) {
+  // This is a crash repro.
+  std::string Code = R"(
+    struct DifferentType {};
+    struct S {
+      DifferentType& operator=(const S&);
+    };
+    void target() {
+      S s;
+      s = S();
+      // [[p]]
+    }
+  )";
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {});
+}
+
+TEST(TransferTest, AssignmentOperatorReturnsDifferentTypeByValue) {
+  // This is a crash repro.
+  std::string Code = R"(
+    struct DifferentType {};
+    struct S {
+      DifferentType operator=(const S&);
     };
     void target() {
       S s;