[clang] Fix wrong -Wunused-local-typedef warning if use is a dependent qualified identifier

[krisb]

Summary

Attempt to fix Tom Smeding's example from https://bugs.llvm.org/show_bug.cgi?id=24883.

Given the case like this one:

struct A {
  void f() const {}
};

template <typename T>
void handler(const T &item) {
  using a_type_t = A;
  item.a_type_t::f();
}

int main() {
  handler(A());
}

there is no way to know whether the typedef is used or not before
the templated context is instantiated.

Having this the patch proposes deffering all the diagnostics for
typedefs defined within a dependent context.

[krisb]

Simplified diagnostic condition in TemplateDeclInstantiator::InstantiateTypedefNameDecl().

clang/lib/Sema/SemaDecl.cpp

static bool ShouldDiagnoseUnusedDecl(const NamedDecl *D) {
   if (D->hasAttr<UnusedAttr>() || D->hasAttr<ObjCPreciseLifetimeAttr>())
     return false;
 
   if (isa<LabelDecl>(D))
     return true;
 
   // Except for labels, we only care about unused decls that are local to
   // functions.
-  bool WithinFunction = D->getDeclContext()->isFunctionOrMethod();
-  if (const auto *R = dyn_cast<CXXRecordDecl>(D->getDeclContext()))
-    // For dependent types, the diagnostic is deferred.
-    WithinFunction =
-        WithinFunction || (R->isLocalClass() && !R->isDependentType());
+  auto *Context = D->getDeclContext();
+  bool WithinFunction = Context->isFunctionOrMethod();
+  if (const auto *R = dyn_cast<CXXRecordDecl>(Context))
+    WithinFunction = WithinFunction || R->isLocalClass();
   if (!WithinFunction)
     return false;
 
-  if (isa<TypedefNameDecl>(D))
+  if (const auto *TD = dyn_cast<TypedefNameDecl>(D)) {
+    // Defer warnings for typedefs within a dependent context.
+    if (Context->isDependentContext())
+      return false;
     return true;
+  }
 
   // White-list anything that isn't a local variable.
   if (!isa<VarDecl>(D) || isa<ParmVarDecl>(D) || isa<ImplicitParamDecl>(D))
     return false;
 
   // Types of valid local variables should be complete, so this should succeed.
   if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
 

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

Decl *TemplateDeclInstantiator::InstantiateTypedefNameDecl(TypedefNameDecl *D,
   SemaRef.InstantiateAttrs(TemplateArgs, D, Typedef);
 
   if (D->getUnderlyingType()->getAs<DependentNameType>())
     SemaRef.inferGslPointerAttribute(Typedef);
 
   Typedef->setAccess(D->getAccess());
   Typedef->setReferenced(D->isReferenced());
 
+  // Diagnose unused local typedefs.
+  if (!Typedef->isInvalidDecl() && (!RD || RD->isLocalClass()))
+    SemaRef.DiagnoseUnusedDecl(Typedef);
+
   return Typedef;
 }
 
 Decl *TemplateDeclInstantiator::VisitTypedefDecl(TypedefDecl *D) {
   Decl *Typedef = InstantiateTypedefNameDecl(D, /*IsTypeAlias=*/false);
   if (Typedef)
     Owner->addDecl(Typedef);
   return Typedef;
if (!D->isCXXClassMember())
       SemaRef.InstantiateClassMembers(D->getLocation(), Record, TemplateArgs,
                                       TSK_ImplicitInstantiation);
 
     // This class may have local implicit instantiations that need to be
     // performed within this scope.
     LocalInstantiations.perform();
   }
 
-  SemaRef.DiagnoseUnusedNestedTypedefs(Record);
-
   if (IsInjectedClassName)
     assert(Record->isInjectedClassName() && "Broken injected-class-name");
 
   return Record;
 }
 
 /// Adjust the given function type for an instantiation of the
 /// given declaration, to cope with modifications to the function's type that

clang/test/SemaCXX/warn-unused-local-typedef.cpp

 };
 
 void foo() {
   A<int> x;
   x.foo();
 }
 } // TypedefInLocalClassOfTemplateClassMember
 
+namespace TypedefInDependentQualifiedIdentifier {
+struct A { void f() const {} };
+
+template <typename T>
+void handler(const T &item) {
+  using a_type_t = A; // no-diag
+  using b_type_t = A; // expected-warning {{unused type alias 'b_type_t'}}
+  item.a_type_t::f();
+}
+
+void foo() {
+  handler(A());
+}
+} // TypedefInDependentQualifiedIdentifier
+
 // This should not disable any warnings:
 #pragma clang diagnostic ignored "-Wunused-local-typedef"