[analyzer] Don't crash with assertion failure on structured bindings

[george.karpenkov]

Summary

Proper modeling still remains to be done.
Note that BindingDecl#getHoldingVar() is almost always null, and this should probably be handled by dealing with DecompositionDecl beforehand.

rdar://36852163

[NoQ]

It seems that semantically this construct creates a copy of s - let's refer to it as _s (it could be represented as a VarRegion because DecompositionDecl is a sub-class of VarDecl) - and the lvalue of a would be a sub-region within _s based on the BindingDecl which would be represented via a completely new kind of DeclRegion (because BindingDecl is not a sub-class of FieldDecl, which is good, because otherwise we would have jumped into the pointer-to-member handling code, which is not correct).

As far as i understand, decompositions may also be user-defined, and in this case they might require different regions to represent the result (and also call expressions in the CFG to inline user-defined implementations).

test/Analysis/structured_bindings.cc

7–8

You can add clang_analyzer_eval or clang_analyzer_warnIfReached that demonstrates the current incorrect behavior that should be fixed by addressing the FIXME. Eg., we don't understand that a is equal to 1, so clang_analyzer_eval(a == 1) would be UNKNOWN, and clang_analyzer_warnIfReached() on the else-branch would warn.

lib/StaticAnalyzer/Core/ExprEngine.cpp

     // Note how this expression is lvalue, however pointer-to-member is NonLoc.
     SVal V = svalBuilder.conjureSymbolVal(Ex, LCtx, getContext().VoidPtrTy,
                                           currBldrCtx->blockCount());
     state = state->assume(V.castAs<DefinedOrUnknownSVal>(), true);
     Bldr.generateNode(Ex, Pred, state->BindExpr(Ex, LCtx, V), nullptr,
           ProgramPoint::PostLValueKind);
     return;
   }
+  if (const auto* BD = dyn_cast<BindingDecl>(D)) {
+    // FIXME: proper support for bound declarations.
+    // For now, let's just prevent crashing.
+    return;
+  }
 
   llvm_unreachable("Support for this Decl not implemented.");
 }
 
 /// VisitArraySubscriptExpr - Transfer function for array accesses
 void ExprEngine::VisitArraySubscriptExpr(const ArraySubscriptExpr *A,
                                              ExplodedNode *Pred,
                                              ExplodedNodeSet &Dst){

test/Analysis/structured_bindings.cc

+// RUN: %clang_analyze_cc1 -std=c++17 -analyzer-checker=core -verify %s
+
+// expected-no-diagnostics
+struct s { int a; };
+int foo() {
+    auto[a] = s{1}; // FIXME: proper modelling
+    if (a) {
+    }
+}
+