[Clang] Fix dependence handling of nttp for variable templates (#69075)

The dependence of a template argument is not only determined by the
argument itself, but also by the type of the template parameter:

> Furthermore, a non-type
[template-argument](https://eel.is/c++draft/temp.names#nt:template-argument)
is dependent if the corresponding non-type
[template-parameter](https://eel.is/c++draft/temp.param#nt:template-parameter)
is of reference or pointer type and the
[template-argument](https://eel.is/c++draft/temp.names#nt:template-argument)
designates or points to a member of the current instantiation or a
member of a dependent
type[.](https://eel.is/c++draft/temp.dep#temp-3.sentence-1)

For example:

```cpp
struct A{};

template <const A& T>
const A JoinStringViews = T;

template <int V>
class Builder {
public:
    static constexpr A Equal{};
    static constexpr auto Val = JoinStringViews<Equal>;
};
```

The constant expression `Equal` is not dependent, but because the type
of the template parameter is a reference type and `Equal` is a member of
the current instantiation, the template argument of
`JoinStringViews<Equal>` is actually dependent, which makes
`JoinStringViews<Equal>` dependent.

When a template-id of a variable template is dependent,
`CheckVarTemplateId` will return an `UnresolvedLookupExpr`, but
`UnresolvedLookupExpr` calculates dependence by template arguments only
(the `ConstantExpr` `Equal` here), which is not dependent. This causes
type deduction to think that `JoinStringViews<Equal>` is `OverloadTy`
and treat it as a function template, which is clearly wrong.

This PR adds a `KnownDependent` parameter to the constructor of
`UnresolvedLookupExpr`. After canonicalization, if `CanonicalConverted`
contains any dependent argument, `KnownDependent` is set to `true`. This
fixes the dependence calculation of `UnresolvedLookupExpr` for dependent
variable templates.

Fixes #65153 .

clang/docs/ReleaseNotes.rst

@@ -516,6 +516,10 @@ Bug Fixes to C++ Support
   rather than prefer the non-templated constructor as specified in
   [standard.group]p3.
 
+- Fixed a crash caused by incorrect handling of dependence on variable templates
+  with non-type template parameters of reference type. Fixes:
+  (`#65153 <https://github.com/llvm/llvm-project/issues/65153>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.

clang/include/clang/AST/ExprCXX.h

@@ -3191,7 +3191,8 @@ class UnresolvedLookupExpr final
                        const DeclarationNameInfo &NameInfo, bool RequiresADL,
                        bool Overloaded,
                        const TemplateArgumentListInfo *TemplateArgs,
-                       UnresolvedSetIterator Begin, UnresolvedSetIterator End);
+                       UnresolvedSetIterator Begin, UnresolvedSetIterator End,
+                       bool KnownDependent);
 
   UnresolvedLookupExpr(EmptyShell Empty, unsigned NumResults,
                        bool HasTemplateKWAndArgsInfo);
@@ -3211,12 +3212,15 @@ class UnresolvedLookupExpr final
          const DeclarationNameInfo &NameInfo, bool RequiresADL, bool Overloaded,
          UnresolvedSetIterator Begin, UnresolvedSetIterator End);
 
+  // After canonicalization, there may be dependent template arguments in
+  // CanonicalConverted But none of Args is dependent. When any of
+  // CanonicalConverted dependent, KnownDependent is true.
   static UnresolvedLookupExpr *
   Create(const ASTContext &Context, CXXRecordDecl *NamingClass,
          NestedNameSpecifierLoc QualifierLoc, SourceLocation TemplateKWLoc,
          const DeclarationNameInfo &NameInfo, bool RequiresADL,
          const TemplateArgumentListInfo *Args, UnresolvedSetIterator Begin,
-         UnresolvedSetIterator End);
+         UnresolvedSetIterator End, bool KnownDependent);
 
   static UnresolvedLookupExpr *CreateEmpty(const ASTContext &Context,
                                            unsigned NumResults,

clang/lib/AST/ASTImporter.cpp

@@ -8395,10 +8395,13 @@ ASTNodeImporter::VisitUnresolvedLookupExpr(UnresolvedLookupExpr *E) {
     if (!ToTemplateKeywordLocOrErr)
       return ToTemplateKeywordLocOrErr.takeError();
 
+    const bool KnownDependent =
+        (E->getDependence() & ExprDependence::TypeValue) ==
+        ExprDependence::TypeValue;
     return UnresolvedLookupExpr::Create(
         Importer.getToContext(), *ToNamingClassOrErr, *ToQualifierLocOrErr,
         *ToTemplateKeywordLocOrErr, ToNameInfo, E->requiresADL(), &ToTAInfo,
-        ToDecls.begin(), ToDecls.end());
+        ToDecls.begin(), ToDecls.end(), KnownDependent);
   }
 
   return UnresolvedLookupExpr::Create(

clang/lib/AST/ExprCXX.cpp

@@ -354,10 +354,10 @@ UnresolvedLookupExpr::UnresolvedLookupExpr(
     NestedNameSpecifierLoc QualifierLoc, SourceLocation TemplateKWLoc,
     const DeclarationNameInfo &NameInfo, bool RequiresADL, bool Overloaded,
     const TemplateArgumentListInfo *TemplateArgs, UnresolvedSetIterator Begin,
-    UnresolvedSetIterator End)
+    UnresolvedSetIterator End, bool KnownDependent)
     : OverloadExpr(UnresolvedLookupExprClass, Context, QualifierLoc,
-                   TemplateKWLoc, NameInfo, TemplateArgs, Begin, End, false,
-                   false, false),
+                   TemplateKWLoc, NameInfo, TemplateArgs, Begin, End,
+                   KnownDependent, false, false),
       NamingClass(NamingClass) {
   UnresolvedLookupExprBits.RequiresADL = RequiresADL;
   UnresolvedLookupExprBits.Overloaded = Overloaded;
@@ -380,25 +380,25 @@ UnresolvedLookupExpr *UnresolvedLookupExpr::Create(
   void *Mem = Context.Allocate(Size, alignof(UnresolvedLookupExpr));
   return new (Mem) UnresolvedLookupExpr(Context, NamingClass, QualifierLoc,
                                         SourceLocation(), NameInfo, RequiresADL,
-                                        Overloaded, nullptr, Begin, End);
+                                        Overloaded, nullptr, Begin, End, false);
 }
 
 UnresolvedLookupExpr *UnresolvedLookupExpr::Create(
     const ASTContext &Context, CXXRecordDecl *NamingClass,
     NestedNameSpecifierLoc QualifierLoc, SourceLocation TemplateKWLoc,
     const DeclarationNameInfo &NameInfo, bool RequiresADL,
     const TemplateArgumentListInfo *Args, UnresolvedSetIterator Begin,
-    UnresolvedSetIterator End) {
+    UnresolvedSetIterator End, bool KnownDependent) {
   assert(Args || TemplateKWLoc.isValid());
   unsigned NumResults = End - Begin;
   unsigned NumTemplateArgs = Args ? Args->size() : 0;
   unsigned Size =
       totalSizeToAlloc<DeclAccessPair, ASTTemplateKWAndArgsInfo,
                        TemplateArgumentLoc>(NumResults, 1, NumTemplateArgs);
   void *Mem = Context.Allocate(Size, alignof(UnresolvedLookupExpr));
-  return new (Mem) UnresolvedLookupExpr(Context, NamingClass, QualifierLoc,
-                                        TemplateKWLoc, NameInfo, RequiresADL,
-                                        /*Overloaded*/ true, Args, Begin, End);
+  return new (Mem) UnresolvedLookupExpr(
+      Context, NamingClass, QualifierLoc, TemplateKWLoc, NameInfo, RequiresADL,
+      /*Overloaded=*/true, Args, Begin, End, KnownDependent);
 }
 
 UnresolvedLookupExpr *UnresolvedLookupExpr::CreateEmpty(

clang/lib/Sema/SemaDeclCXX.cpp

@@ -1299,8 +1299,9 @@ static bool checkTupleLikeDecomposition(Sema &S,
       //   in the associated namespaces.
       Expr *Get = UnresolvedLookupExpr::Create(
           S.Context, nullptr, NestedNameSpecifierLoc(), SourceLocation(),
-          DeclarationNameInfo(GetDN, Loc), /*RequiresADL*/true, &Args,
-          UnresolvedSetIterator(), UnresolvedSetIterator());
+          DeclarationNameInfo(GetDN, Loc), /*RequiresADL*/ true, &Args,
+          UnresolvedSetIterator(), UnresolvedSetIterator(),
+          /*KnownDependent=*/false);
 
       Expr *Arg = E.get();
       E = S.BuildCallExpr(nullptr, Get, Loc, Arg, Loc);

clang/lib/Sema/SemaTemplate.cpp

@@ -4982,7 +4982,7 @@ ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
       return ExprError();
     }
   }
-
+  bool KnownDependent = false;
   // In C++1y, check variable template ids.
   if (R.getAsSingle<VarTemplateDecl>()) {
     ExprResult Res = CheckVarTemplateId(SS, R.getLookupNameInfo(),
@@ -4991,6 +4991,7 @@ ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
     if (Res.isInvalid() || Res.isUsable())
       return Res;
     // Result is dependent. Carry on to build an UnresolvedLookupEpxr.
+    KnownDependent = true;
   }
 
   if (R.getAsSingle<ConceptDecl>()) {
@@ -5002,13 +5003,10 @@ ExprResult Sema::BuildTemplateIdExpr(const CXXScopeSpec &SS,
   // We don't want lookup warnings at this point.
   R.suppressDiagnostics();
 
-  UnresolvedLookupExpr *ULE
-    = UnresolvedLookupExpr::Create(Context, R.getNamingClass(),
-                                   SS.getWithLocInContext(Context),
-                                   TemplateKWLoc,
-                                   R.getLookupNameInfo(),
-                                   RequiresADL, TemplateArgs,
-                                   R.begin(), R.end());
+  UnresolvedLookupExpr *ULE = UnresolvedLookupExpr::Create(
+      Context, R.getNamingClass(), SS.getWithLocInContext(Context),
+      TemplateKWLoc, R.getLookupNameInfo(), RequiresADL, TemplateArgs,
+      R.begin(), R.end(), KnownDependent);
 
   return ULE;
 }

clang/test/SemaTemplate/dependent-expr.cpp

@@ -165,3 +165,18 @@ namespace BindingInStmtExpr {
   using U = decltype(num_bindings<T>()); // expected-note {{previous}}
   using U = N<3>; // expected-error-re {{type alias redefinition with different types ('N<3>' vs {{.*}}N<2>}}
 }
+
+namespace PR65153 {
+struct A{};
+
+template <const A& T>
+const A JoinStringViews = T;
+
+template <int V>
+class Builder {
+public:
+    static constexpr A Equal{};
+    // no crash here
+    static constexpr auto Val = JoinStringViews<Equal>;
+};
+} // namespace PR65153