Thread safety analysis: Peel away NoOp implicit casts in initializers

[aaronpuchert]

Summary

This happens when someone initializes a variable with guaranteed copy
elision and an added const qualifier. Fixes PR43826.

[aaron.ballman]

LGTM!

clang/lib/Analysis/ThreadSafety.cpp

if (auto *VD = dyn_cast_or_null<VarDecl>(D)) {
       Expr *E = VD->getInit();
       if (!E)
         continue;
       E = E->IgnoreParens();
 
       // handle constructors that involve temporaries
       if (auto *EWC = dyn_cast<ExprWithCleanups>(E))
         E = EWC->getSubExpr();
+      if (auto *ICE = dyn_cast<ImplicitCastExpr>(E))
+        if (ICE->getCastKind() == CK_NoOp)
+          E = ICE->getSubExpr();
       if (auto *BTE = dyn_cast<CXXBindTemporaryExpr>(E))
         E = BTE->getSubExpr();
 
       if (const auto *CE = dyn_cast<CXXConstructExpr>(E)) {
         const auto *CtorD = dyn_cast_or_null<NamedDecl>(CE->getConstructor());
         if (!CtorD || !CtorD->hasAttrs())
           continue;
         handleCall(E, CtorD, VD);

clang/test/SemaCXX/warn-thread-safety-analysis.cpp

 void Foo::test() {
   {
     ReaderMutexLock lock(getMutexPtr().get());
     int b = a;
   }
   int b = a;  // expected-warning {{reading variable 'a' requires holding mutex 'getMutexPtr()'}}
 }
 
+#ifdef __cpp_guaranteed_copy_elision
+
+void guaranteed_copy_elision() {
+  MutexLock lock = MutexLock{&sls_mu};
+  sls_guard_var = 0;
+}
+
+void guaranteed_copy_elision_const() {
+  const MutexLock lock = MutexLock{&sls_mu};
+  sls_guard_var = 0;
+}
+
+#endif
+
 } // end namespace TemporaryCleanupExpr
 
 
 
 namespace SmartPointerTests {
 
 class Foo {
 public: