[Sema] Fix crash in unused-lambda-capture warning for VLAs

[malcolm.parsons]

Summary

Clang was crashing when diagnosing an unused-lambda-capture for a VLA because
From.getVariable() is null for the capture of a VLA bound.
Warning about the VLA bound capture is not helpful, so only warn for the VLA
itself.

Fixes: PR35555

[dim]

For the rest, LGTM. It fixes the segfault, for both the full original test case, and my reduced version.

lib/Sema/SemaLambda.cpp

1491

Just for sanity's sake, I would still put an assert here, that getVariable() does not return nullptr. That might catch similar issues in the future. (And it is better than a segfault... :) )

[aaron.ballman]

LGTM!

[malcolm.parsons]

Add assert.

[malcolm.parsons]

lib/Sema/SemaLambda.cpp

1491

I'm adding an assert to getVariable() instead.

include/clang/Sema/ScopeInfo.h

public:
     bool isNested() const {
       return VarAndNestedAndThis.getInt() & IsNestedCapture;
     }
     bool isODRUsed() const { return ODRUsed; }
     bool isNonODRUsed() const { return NonODRUsed; }
     void markUsed(bool IsODRUse) { (IsODRUse ? ODRUsed : NonODRUsed) = true; }
 
     VarDecl *getVariable() const {
+      assert(isVariableCapture());
       return VarAndNestedAndThis.getPointer();
     }
     
     /// \brief Retrieve the location at which this variable was captured.
     SourceLocation getLocation() const { return Loc; }
     
     /// \brief Retrieve the source location of the ellipsis, whose presence
     /// indicates that the capture is a pack expansion.

lib/Sema/SemaLambda.cpp

bool Sema::CaptureHasSideEffects(const LambdaScopeInfo::Capture &From) {
 
   return false;
 }
 
 void Sema::DiagnoseUnusedLambdaCapture(const LambdaScopeInfo::Capture &From) {
   if (CaptureHasSideEffects(From))
     return;
 
+  if (From.isVLATypeCapture())
+    return;
+
   auto diag = Diag(From.getLocation(), diag::warn_unused_lambda_capture);
   if (From.isThisCapture())
     diag << "'this'";
   else
     diag << From.getVariable();
   diag << From.isNonODRUsed();
 }
 
 ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
                                  LambdaScopeInfo *LSI) {
   // Collect information from the lambda scope.
   SmallVector<LambdaCapture, 4> Captures;
   SmallVector<Expr *, 4> CaptureInits;

test/SemaCXX/warn-unused-lambda-capture.cpp

void test_templated() {
 
   volatile int v;
   auto explicit_by_value_volatile = [v] {};
 }
 
 void test_use_template() {
   test_templated<int>(); // expected-note{{in instantiation of function template specialization 'test_templated<int>' requested here}}
 }
+
+namespace pr35555 {
+int a;
+void b() {
+  int c[a];
+  auto vla_used = [&c] { return c[0]; };
+  auto vla_unused = [&c] {}; // expected-warning{{lambda capture 'c' is not used}}
+}
+} // namespace pr35555