Revisit PR10177: don't instantiate a variable if it's only referenced in a

dependent context and can't be used in a constant expression.

Per C++ [temp.inst]p2, "the instantiation of a static data member does not
occur unless the static data member is used in a way that requires the
definition to exist".

This doesn't /quite/ match that, as we still instantiate static data members
that are usable in constant expressions even if the use doesn't require a
definition. A followup patch will fix that for both variables and functions.

llvm-svn: 291295

clang/include/clang/Sema/Sema.h

@@ -27,6 +27,7 @@
 #include "clang/AST/NSAPI.h"
 #include "clang/AST/PrettyPrinter.h"
 #include "clang/AST/TypeLoc.h"
+#include "clang/AST/TypeOrdering.h"
 #include "clang/Basic/ExpressionTraits.h"
 #include "clang/Basic/LangOptions.h"
 #include "clang/Basic/Module.h"
@@ -3801,6 +3802,9 @@ public:
   /// variable will have in the given scope.
   QualType getCapturedDeclRefType(VarDecl *Var, SourceLocation Loc);
 
+  /// Mark all of the declarations referenced within a particular AST node as
+  /// referenced. Used when template instantiation instantiates a non-dependent
+  /// type -- entities referenced by the type are now referenced.
   void MarkDeclarationsReferencedInType(SourceLocation Loc, QualType T);
   void MarkDeclarationsReferencedInExpr(Expr *E,
                                         bool SkipLocalVariables = false);
@@ -6877,6 +6881,10 @@ public:
   /// Specializations whose definitions are currently being instantiated.
   llvm::DenseSet<std::pair<Decl *, unsigned>> InstantiatingSpecializations;
 
+  /// Non-dependent types used in templates that have already been instantiated
+  /// by some template instantiation.
+  llvm::DenseSet<QualType> InstantiatedNonDependentTypes;
+
   /// \brief Extra modules inspected when performing a lookup during a template
   /// instantiation. Computed lazily.
   SmallVector<Module*, 16> ActiveTemplateInstantiationLookupModules;

clang/lib/Sema/SemaExpr.cpp

@@ -14122,48 +14122,13 @@ static void DoMarkVarDeclReferenced(Sema &SemaRef, SourceLocation Loc,
          "Invalid Expr argument to DoMarkVarDeclReferenced");
   Var->setReferenced();
 
+  if (SemaRef.isUnevaluatedContext())
+    return;
+
   TemplateSpecializationKind TSK = Var->getTemplateSpecializationKind();
-  bool MarkODRUsed = true;
-
-  // If the context is not potentially evaluated, this is not an odr-use and
-  // does not trigger instantiation.
-  if (!IsPotentiallyEvaluatedContext(SemaRef)) {
-    if (SemaRef.isUnevaluatedContext())
-      return;
-
-    // If we don't yet know whether this context is going to end up being an
-    // evaluated context, and we're referencing a variable from an enclosing
-    // scope, add a potential capture.
-    //
-    // FIXME: Is this necessary? These contexts are only used for default
-    // arguments, where local variables can't be used.
-    const bool RefersToEnclosingScope =
-        (SemaRef.CurContext != Var->getDeclContext() &&
-         Var->getDeclContext()->isFunctionOrMethod() && Var->hasLocalStorage());
-    if (RefersToEnclosingScope) {
-      if (LambdaScopeInfo *const LSI =
-              SemaRef.getCurLambda(/*IgnoreCapturedRegions=*/true)) {
-        // If a variable could potentially be odr-used, defer marking it so
-        // until we finish analyzing the full expression for any
-        // lvalue-to-rvalue
-        // or discarded value conversions that would obviate odr-use.
-        // Add it to the list of potential captures that will be analyzed
-        // later (ActOnFinishFullExpr) for eventual capture and odr-use marking
-        // unless the variable is a reference that was initialized by a constant
-        // expression (this will never need to be captured or odr-used).
-        assert(E && "Capture variable should be used in an expression.");
-        if (!Var->getType()->isReferenceType() ||
-            !IsVariableNonDependentAndAConstantExpression(Var, SemaRef.Context))
-          LSI->addPotentialCapture(E->IgnoreParens());
-      }
-    }
-
-    if (!isTemplateInstantiation(TSK))
-      return;
-
-    // Instantiate, but do not mark as odr-used, variable templates.
-    MarkODRUsed = false;
-  }
+  bool MarkODRUsed = IsPotentiallyEvaluatedContext(SemaRef);
+  bool NeedDefinition =
+      MarkODRUsed || Var->isUsableInConstantExpressions(SemaRef.Context);
 
   VarTemplateSpecializationDecl *VarSpec =
       dyn_cast<VarTemplateSpecializationDecl>(Var);
@@ -14173,14 +14138,15 @@ static void DoMarkVarDeclReferenced(Sema &SemaRef, SourceLocation Loc,
   // If this might be a member specialization of a static data member, check
   // the specialization is visible. We already did the checks for variable
   // template specializations when we created them.
-  if (TSK != TSK_Undeclared && !isa<VarTemplateSpecializationDecl>(Var))
+  if (NeedDefinition && TSK != TSK_Undeclared &&
+      !isa<VarTemplateSpecializationDecl>(Var))
     SemaRef.checkSpecializationVisibility(Loc, Var);
 
   // Perform implicit instantiation of static data members, static data member
   // templates of class templates, and variable template specializations. Delay
   // instantiations of variable templates, except for those that could be used
   // in a constant expression.
-  if (isTemplateInstantiation(TSK)) {
+  if (NeedDefinition && isTemplateInstantiation(TSK)) {
     bool TryInstantiating = TSK == TSK_ImplicitInstantiation;
 
     if (TryInstantiating && !isa<VarTemplateSpecializationDecl>(Var)) {
@@ -14219,9 +14185,6 @@ static void DoMarkVarDeclReferenced(Sema &SemaRef, SourceLocation Loc,
     }
   }
 
-  if (!MarkODRUsed)
-    return;
-
   // Per C++11 [basic.def.odr], a variable is odr-used "unless it satisfies
   // the requirements for appearing in a constant expression (5.19) and, if
   // it is an object, the lvalue-to-rvalue conversion (4.1)
@@ -14230,14 +14193,39 @@ static void DoMarkVarDeclReferenced(Sema &SemaRef, SourceLocation Loc,
   // Note that we use the C++11 definition everywhere because nothing in
   // C++03 depends on whether we get the C++03 version correct. The second
   // part does not apply to references, since they are not objects.
-  if (E && IsVariableAConstantExpression(Var, SemaRef.Context)) {
+  if (MarkODRUsed && E && IsVariableAConstantExpression(Var, SemaRef.Context)) {
     // A reference initialized by a constant expression can never be
     // odr-used, so simply ignore it.
     if (!Var->getType()->isReferenceType())
       SemaRef.MaybeODRUseExprs.insert(E);
-  } else
+  } else if (MarkODRUsed) {
     MarkVarDeclODRUsed(Var, Loc, SemaRef,
                        /*MaxFunctionScopeIndex ptr*/ nullptr);
+  } else {
+    // If we don't yet know whether this context is going to end up being an
+    // evaluated context, and we're referencing a variable from an enclosing
+    // scope, add a potential capture.
+    const bool RefersToEnclosingScope =
+        (SemaRef.CurContext != Var->getDeclContext() &&
+         Var->getDeclContext()->isFunctionOrMethod() && Var->hasLocalStorage());
+    if (RefersToEnclosingScope) {
+      if (LambdaScopeInfo *const LSI =
+              SemaRef.getCurLambda(/*IgnoreCapturedRegions=*/true)) {
+        // If a variable could potentially be odr-used, defer marking it so
+        // until we finish analyzing the full expression for any
+        // lvalue-to-rvalue
+        // or discarded value conversions that would obviate odr-use.
+        // Add it to the list of potential captures that will be analyzed
+        // later (ActOnFinishFullExpr) for eventual capture and odr-use marking
+        // unless the variable is a reference that was initialized by a constant
+        // expression (this will never need to be captured or odr-used).
+        assert(E && "Capture variable should be used in an expression.");
+        if (!Var->getType()->isReferenceType() ||
+            !IsVariableNonDependentAndAConstantExpression(Var, SemaRef.Context))
+          LSI->addPotentialCapture(E->IgnoreParens());
+      }
+    }
+  }
 }
 
 /// \brief Mark a variable referenced, and check whether it is odr-used
@@ -14346,33 +14334,28 @@ namespace {
     MarkReferencedDecls(Sema &S, SourceLocation Loc) : S(S), Loc(Loc) { }
 
     bool TraverseTemplateArgument(const TemplateArgument &Arg);
-    bool TraverseRecordType(RecordType *T);
   };
 }
 
 bool MarkReferencedDecls::TraverseTemplateArgument(
     const TemplateArgument &Arg) {
-  if (Arg.getKind() == TemplateArgument::Declaration) {
-    if (Decl *D = Arg.getAsDecl())
-      S.MarkAnyDeclReferenced(Loc, D, true);
+  {
+    // A non-type template argument is a constant-evaluated context.
+    EnterExpressionEvaluationContext Evaluated(S, Sema::ConstantEvaluated);
+    if (Arg.getKind() == TemplateArgument::Declaration) {
+      if (Decl *D = Arg.getAsDecl())
+        S.MarkAnyDeclReferenced(Loc, D, true);
+    } else if (Arg.getKind() == TemplateArgument::Expression) {
+      S.MarkDeclarationsReferencedInExpr(Arg.getAsExpr(), false);
+    }
   }
 
   return Inherited::TraverseTemplateArgument(Arg);
 }
 
-bool MarkReferencedDecls::TraverseRecordType(RecordType *T) {
-  if (ClassTemplateSpecializationDecl *Spec
-                  = dyn_cast<ClassTemplateSpecializationDecl>(T->getDecl())) {
-    const TemplateArgumentList &Args = Spec->getTemplateArgs();
-    return TraverseTemplateArguments(Args.data(), Args.size());
-  }
-
-  return true;
-}
-
 void Sema::MarkDeclarationsReferencedInType(SourceLocation Loc, QualType T) {
   MarkReferencedDecls Marker(*this, Loc);
-  Marker.TraverseType(Context.getCanonicalType(T));
+  Marker.TraverseType(T);
 }
 
 namespace {

clang/lib/Sema/SemaTemplate.cpp

@@ -5158,6 +5158,11 @@ ExprResult Sema::CheckTemplateArgument(NonTypeTemplateParmDecl *Param,
     return Arg;
   }
 
+  // The initialization of the parameter from the argument is
+  // a constant-evaluated context.
+  EnterExpressionEvaluationContext ConstantEvaluated(*this,
+                                                     Sema::ConstantEvaluated);
+
   if (getLangOpts().CPlusPlus1z) {
     // C++1z [temp.arg.nontype]p1:
     //   A template-argument for a non-type template parameter shall be

clang/test/SemaCXX/PR10177.cpp

@@ -24,6 +24,13 @@ void f() {
   (void)class_ref<int, int&, U<2>::a>(); // expected-note {{here}}
 };
 
+template<typename T>
+void not_instantiated() {
+  // These cases (arguably) do not require instantiation of U<i>::a.
+  (void)alias_ref<int, int&, U<3>::a>();
+  (void)func_ref<int, int&, U<4>::a>();
+  (void)class_ref<int, int&, U<5>::a>();
+};
 
 template<int N>
 void fi() {
@@ -33,7 +40,7 @@ void fi() {
 };
 
 int main() {
-  f<int>();   // NOTE: Non-dependent name uses are type-checked at template definition time.
+  f<int>();   // expected-note 3{{here}}
   fi<10>();   // expected-note 3{{here}}
 }
 

clang/test/SemaCXX/undefined-internal.cpp

@@ -186,10 +186,15 @@ namespace OverloadUse {
   namespace {
     void f();
     void f(int); // expected-warning {{function 'OverloadUse::(anonymous namespace)::f' has internal linkage but is not defined}}
+    void f(int, int); // expected-warning {{function 'OverloadUse::(anonymous namespace)::f' has internal linkage but is not defined}}
+  }
+  template<void x()> void t() { x(); }
+  template<void x(int)> void t(int*) { x(10); }
+  template<void x(int, int)> void t(int*, int*) {}
+  void g(int n) {
+    t<f>(&n); // expected-note {{used here}}
+    t<f>(&n, &n); // expected-note {{used here}}
   }
-  template<void x()> void t(int*) { x(); }
-  template<void x(int)> void t(long*) { x(10); } // expected-note {{used here}}
-  void g() { long a; t<f>(&a); }
 }
 
 namespace test7 {