[analyzer] Fix note for member reference (#68691)

In the following code:
```cpp
int main() {
    struct Wrapper {char c; int &ref; };
    Wrapper w = {.c = 'a', .ref = *(int *)0 };
    w.ref = 1;
}
```

The clang static analyzer will produce the following warnings and notes:
```
test.cpp:12:11: warning: Dereference of null pointer [core.NullDereference]
   12 |     w.ref = 1;
      |     ~~~~~~^~~
test.cpp:11:5: note: 'w' initialized here
   11 |     Wrapper w = {.c = 'a', .ref = *(int *)0 };
      |     ^~~~~~~~~
test.cpp:12:11: note: Dereference of null pointer
   12 |     w.ref = 1;
      |     ~~~~~~^~~
1 warning generated.
```
In the line where `w` is created, the note gives information about the
initialization of `w` instead of `w.ref`. Let's compare it to a similar
case where a null pointer dereference happens to a pointer member:

```cpp
int main() {
     struct Wrapper {char c; int *ptr; };
     Wrapper w = {.c = 'a', .ptr = nullptr };
     *w.ptr = 1;
}
```

Here the following error and notes are seen:
```
test.cpp:18:12: warning: Dereference of null pointer (loaded from field 'ptr') [core.NullDereference]
   18 |     *w.ptr = 1;
      |        ~~~ ^
test.cpp:17:5: note: 'w.ptr' initialized to a null pointer value
   17 |     Wrapper w = {.c = 'a', .ptr = nullptr };
      |     ^~~~~~~~~
test.cpp:18:12: note: Dereference of null pointer (loaded from field 'ptr')
   18 |     *w.ptr = 1;
      |        ~~~ ^
1 warning generated.
```
Here the note that shows the initialization the initialization of
`w.ptr` in shown instead of `w`.

This commit is here to achieve similar notes for member reference as the
notes of member pointers, so the report looks like the following:

```
test.cpp:12:11: warning: Dereference of null pointer [core.NullDereference]
   12 |     w.ref = 1;
      |     ~~~~~~^~~
test.cpp:11:5: note: 'w.ref' initialized to a null pointer value
   11 |     Wrapper w = {.c = 'a', .ref = *(int *)0 };
      |     ^~~~~~~~~
test.cpp:12:11: note: Dereference of null pointer
   12 |     w.ref = 1;
      |     ~~~~~~^~~
1 warning generated.
```
Here the initialization of `w.ref` is shown instead of `w`.

---------

Authored-by: Gábor Spaits <gabor.spaits@ericsson.com>
Reviewed-by: Donát Nagy <donat.nagy@ericsson.com>

Author: spaits

clang/lib/StaticAnalyzer/Core/BugReporterVisitors.cpp

@@ -132,6 +132,16 @@ const Expr *bugreporter::getDerefExpr(const Stmt *S) {
     }
     // Pattern match for a few useful cases: a[0], p->f, *p etc.
     else if (const auto *ME = dyn_cast<MemberExpr>(E)) {
+      // This handles the case when the dereferencing of a member reference
+      // happens. This is needed, because the AST for dereferencing a
+      // member reference looks like the following:
+      // |-MemberExpr
+      //  `-DeclRefExpr
+      // Without this special case the notes would refer to the whole object
+      // (struct, class or union variable) instead of just the relevant member.
+
+      if (ME->getMemberDecl()->getType()->isReferenceType())
+        break;
       E = ME->getBase();
     } else if (const auto *IvarRef = dyn_cast<ObjCIvarRefExpr>(E)) {
       E = IvarRef->getBase();
@@ -157,26 +167,42 @@ const Expr *bugreporter::getDerefExpr(const Stmt *S) {
   return E;
 }
 
+static const VarDecl *getVarDeclForExpression(const Expr *E) {
+  if (const auto *DR = dyn_cast<DeclRefExpr>(E))
+    return dyn_cast<VarDecl>(DR->getDecl());
+  return nullptr;
+}
+
 static const MemRegion *
 getLocationRegionIfReference(const Expr *E, const ExplodedNode *N,
                              bool LookingForReference = true) {
-  if (const auto *DR = dyn_cast<DeclRefExpr>(E)) {
-    if (const auto *VD = dyn_cast<VarDecl>(DR->getDecl())) {
-      if (LookingForReference && !VD->getType()->isReferenceType())
-        return nullptr;
-      return N->getState()
-          ->getLValue(VD, N->getLocationContext())
-          .getAsRegion();
+  if (const auto *ME = dyn_cast<MemberExpr>(E)) {
+    // This handles null references from FieldRegions, for example:
+    //   struct Wrapper { int &ref; };
+    //   Wrapper w = { *(int *)0 };
+    //   w.ref = 1;
+    const Expr *Base = ME->getBase();
+    const VarDecl *VD = getVarDeclForExpression(Base);
+    if (!VD)
+      return nullptr;
+
+    const auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());
+    if (!FD)
+      return nullptr;
+
+    if (FD->getType()->isReferenceType()) {
+      SVal StructSVal = N->getState()->getLValue(VD, N->getLocationContext());
+      return N->getState()->getLValue(FD, StructSVal).getAsRegion();
     }
+    return nullptr;
   }
 
-  // FIXME: This does not handle other kinds of null references,
-  // for example, references from FieldRegions:
-  //   struct Wrapper { int &ref; };
-  //   Wrapper w = { *(int *)0 };
-  //   w.ref = 1;
-
-  return nullptr;
+  const VarDecl *VD = getVarDeclForExpression(E);
+  if (!VD)
+    return nullptr;
+  if (LookingForReference && !VD->getType()->isReferenceType())
+    return nullptr;
+  return N->getState()->getLValue(VD, N->getLocationContext()).getAsRegion();
 }
 
 /// Comparing internal representations of symbolic values (via

clang/test/Analysis/diagnostics/deref-track-symbolic-region.cpp

@@ -41,3 +41,34 @@ int testRefToNullPtr2() {
   return *p2;         //expected-warning {{Dereference of null pointer}}
                       // expected-note@-1{{Dereference of null pointer}}
 }
+
+void testMemberNullPointerDeref() {
+  struct Wrapper {char c; int *ptr; };  
+  Wrapper w = {'a', nullptr};           // expected-note {{'w.ptr' initialized to a null pointer value}}
+  *w.ptr = 1;                           //expected-warning {{Dereference of null pointer}}
+                                        // expected-note@-1{{Dereference of null pointer}}
+}
+
+void testMemberNullReferenceDeref() {
+  struct Wrapper {char c; int &ref; };
+  Wrapper w = {.c = 'a', .ref = *(int *)0 }; // expected-note {{'w.ref' initialized to a null pointer value}}
+                                             // expected-warning@-1 {{binding dereferenced null pointer to reference has undefined behavior}}
+  w.ref = 1;                                 //expected-warning {{Dereference of null pointer}}
+                                             // expected-note@-1{{Dereference of null pointer}}
+}
+
+void testReferenceToPointerWithNullptr() {
+  int *i = nullptr;                   // expected-note {{'i' initialized to a null pointer value}}
+  struct Wrapper {char c; int *&a;};
+  Wrapper w {'c', i};                 // expected-note{{'w.a' initialized here}}
+  *(w.a) = 25;                        // expected-warning {{Dereference of null pointer}}
+                                      // expected-note@-1 {{Dereference of null pointer}}
+}
+
+void testNullReferenceToPointer() {
+  struct Wrapper {char c; int *&a;};
+  Wrapper w {'c', *(int **)0 };           // expected-note{{'w.a' initialized to a null pointer value}}
+                                          // expected-warning@-1 {{binding dereferenced null pointer to reference has undefined behavior}}
+  w.a = nullptr;                          // expected-warning {{Dereference of null pointer}}
+                                          // expected-note@-1 {{Dereference of null pointer}}
+}