[analyzer] Consider array subscripts to be interesting lvalues.

Static analyzer has a mechanism of clearing redundant nodes when
analysis hits a certain threshold with a number of nodes in exploded
graph (default is 1000). It is similar to GC and aims removing nodes
not useful for analysis. Unfortunately nodes corresponding to array
subscript expressions (that actively participate in data propagation)
get removed during the cleanup. This might prevent the analyzer from
generating useful notes about where it thinks the data came from.

This fix is pretty much consistent with the way analysis works
already. Lvalue "interestingness" stands for the analyzer's
possibility of tracking values through them.

Differential Revision: https://reviews.llvm.org/D78638

clang/lib/StaticAnalyzer/Core/ExplodedGraph.cpp

@@ -50,9 +50,8 @@ ExplodedGraph::~ExplodedGraph() = default;
 bool ExplodedGraph::isInterestingLValueExpr(const Expr *Ex) {
   if (!Ex->isLValue())
     return false;
-  return isa<DeclRefExpr>(Ex) ||
-         isa<MemberExpr>(Ex) ||
-         isa<ObjCIvarRefExpr>(Ex);
+  return isa<DeclRefExpr>(Ex) || isa<MemberExpr>(Ex) ||
+         isa<ObjCIvarRefExpr>(Ex) || isa<ArraySubscriptExpr>(Ex);
 }
 
 bool ExplodedGraph::shouldCollect(const ExplodedNode *node) {

clang/test/Analysis/CheckThatArraySubsciptNodeIsNotCollected.cpp

@@ -0,0 +1,40 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core -analyzer-output=text -verify %s
+
+class A {
+public:
+  int method();
+};
+
+A *foo();
+void bar(A *);
+
+int index;
+
+// We want to check here that the notes about the origins of the null pointer
+// (array[index] = foo()) will get to the final report.
+//
+// The analyzer used to drop exploded nodes for array subscripts when it was
+// time to collect redundant nodes. This GC-like mechanism kicks in only when
+// the exploded graph is large enough (>1K nodes). For this reason, 'index'
+// is a global variable, and the sink point is inside of a loop.
+
+void test() {
+  A *array[42];
+  A *found;
+
+  for (index = 0; (array[index] = foo()); ++index) { // expected-note {{Loop condition is false. Execution continues on line 34}}
+    // expected-note@-1 {{Value assigned to 'index'}}
+    // expected-note@-2 {{Assigning value}}
+    // expected-note@-3 {{Assuming pointer value is null}}
+    if (array[0])
+      break;
+  }
+
+  do {
+    found = array[index]; // expected-note {{Null pointer value stored to 'found'}}
+
+    if (found->method()) // expected-warning {{Called C++ object pointer is null [core.CallAndMessage]}}
+      // expected-note@-1 {{Called C++ object pointer is null}}
+      bar(found);
+  } while (--index);
+}