[clang] Distinguish unresolved templates in UnresolvedLookupExpr (#89019)

This patch revolves around the misuse of UnresolvedLookupExpr in
BuildTemplateIdExpr.
    
Basically, we build up an UnresolvedLookupExpr not only for function
overloads but for "unresolved" templates wherever we need an expression
for template decls. For example, a dependent VarTemplateDecl can be
wrapped with such an expression before template instantiation. (See

6170072)
    
Also, one important thing is that UnresolvedLookupExpr uses a
"canonical"
QualType to describe the containing unresolved decls: a DependentTy is
for dependent expressions and an OverloadTy otherwise. Therefore, this
modeling for non-dependent templates leaves a problem in that the
expression
is marked and perceived as if describing overload functions. The
consumer then
expects functions for every such expression, although the fact is the
reverse.
Hence, we run into crashes.
    
As to the patch, I added a new canonical type "UnresolvedTemplateTy" to
model these cases. Given that we have been using this model
(intentionally or
accidentally) and it is pretty baked in throughout the code, I think
extending the role of UnresolvedLookupExpr is reasonable. Further, I
added
some diagnostics for the direct occurrence of these expressions, which
are supposed to be ill-formed.

As a bonus, this patch also fixes some typos in the diagnostics and
creates
RecoveryExprs rather than nothing in the hope of a better error-recovery
for clangd.
    
Fixes #88832
Fixes #63243
Fixes #48673

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -681,6 +681,8 @@ Bug Fixes to C++ Support
   whose type is `decltype(auto)`. Fixes (#GH68885).
 - Clang now correctly treats the noexcept-specifier of a friend function to be a complete-class context.
 - Fix an assertion failure when parsing an invalid members of an anonymous class. (#GH85447)
+- Fixed a misuse of ``UnresolvedLookupExpr`` for ill-formed templated expressions. Fixes (#GH48673), (#GH63243)
+  and (#GH88832).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/ASTContext.h

@@ -1116,7 +1116,8 @@ class ASTContext : public RefCountedBase<ASTContext> {
   CanQualType BFloat16Ty;
   CanQualType Float16Ty; // C11 extension ISO/IEC TS 18661-3
   CanQualType VoidPtrTy, NullPtrTy;
-  CanQualType DependentTy, OverloadTy, BoundMemberTy, UnknownAnyTy;
+  CanQualType DependentTy, OverloadTy, BoundMemberTy, UnresolvedTemplateTy,
+      UnknownAnyTy;
   CanQualType BuiltinFnTy;
   CanQualType PseudoObjectTy, ARCUnbridgedCastTy;
   CanQualType ObjCBuiltinIdTy, ObjCBuiltinClassTy, ObjCBuiltinSelTy;

clang/include/clang/AST/BuiltinTypes.def

@@ -285,6 +285,9 @@ PLACEHOLDER_TYPE(Overload, OverloadTy)
 //   x->foo       # if only contains non-static members
 PLACEHOLDER_TYPE(BoundMember, BoundMemberTy)
 
+// The type of an unresolved template. Used in UnresolvedLookupExpr.
+PLACEHOLDER_TYPE(UnresolvedTemplate, UnresolvedTemplateTy)
+
 // The type of an expression which refers to a pseudo-object,
 // such as those introduced by Objective C's @property or
 // VS.NET's __property declarations.  A placeholder type.  The

clang/include/clang/AST/ExprCXX.h

@@ -3163,8 +3163,30 @@ class OverloadExpr : public Expr {
 /// This arises in several ways:
 ///   * we might be waiting for argument-dependent lookup;
 ///   * the name might resolve to an overloaded function;
+///   * the name might resolve to a non-function template; for example, in the
+///   following snippet, the return expression of the member function
+///   'foo()' might remain unresolved until instantiation:
+///
+/// \code
+/// struct P {
+///   template <class T> using I = T;
+/// };
+///
+/// struct Q {
+///   template <class T> int foo() {
+///     return T::template I<int>;
+///   }
+/// };
+/// \endcode
+///
+/// ...which is distinct from modeling function overloads, and therefore we use
+/// a different builtin type 'UnresolvedTemplate' to avoid confusion. This is
+/// done in Sema::BuildTemplateIdExpr.
+///
 /// and eventually:
 ///   * the lookup might have included a function template.
+///   * the unresolved template gets transformed in an instantiation or gets
+///   diagnosed for its direct use.
 ///
 /// These never include UnresolvedUsingValueDecls, which are always class
 /// members and therefore appear only in UnresolvedMemberLookupExprs.

clang/include/clang/Serialization/ASTBitCodes.h

@@ -1091,6 +1091,9 @@ enum PredefinedTypeIDs {
 // \brief WebAssembly reference types with auto numeration
 #define WASM_TYPE(Name, Id, SingletonId) PREDEF_TYPE_##Id##_ID,
 #include "clang/Basic/WebAssemblyReferenceTypes.def"
+
+  /// The placeholder type for unresolved templates.
+  PREDEF_TYPE_UNRESOLVED_TEMPLATE,
   // Sentinel value. Considered a predefined type but not useable as one.
   PREDEF_TYPE_LAST_ID
 };
@@ -1100,7 +1103,7 @@ enum PredefinedTypeIDs {
 ///
 /// Type IDs for non-predefined types will start at
 /// NUM_PREDEF_TYPE_IDs.
-const unsigned NUM_PREDEF_TYPE_IDS = 502;
+const unsigned NUM_PREDEF_TYPE_IDS = 503;
 
 // Ensure we do not overrun the predefined types we reserved
 // in the enum PredefinedTypeIDs above.

clang/lib/AST/ASTContext.cpp

@@ -1307,6 +1307,9 @@ void ASTContext::InitBuiltinTypes(const TargetInfo &Target,
   // Placeholder type for bound members.
   InitBuiltinType(BoundMemberTy,       BuiltinType::BoundMember);
 
+  // Placeholder type for unresolved templates.
+  InitBuiltinType(UnresolvedTemplateTy, BuiltinType::UnresolvedTemplate);
+
   // Placeholder type for pseudo-objects.
   InitBuiltinType(PseudoObjectTy,      BuiltinType::PseudoObject);
 

clang/lib/AST/NSAPI.cpp

@@ -454,6 +454,7 @@ NSAPI::getNSNumberFactoryMethodKind(QualType T) const {
 #define WASM_TYPE(Name, Id, SingletonId) case BuiltinType::Id:
 #include "clang/Basic/WebAssemblyReferenceTypes.def"
   case BuiltinType::BoundMember:
+  case BuiltinType::UnresolvedTemplate:
   case BuiltinType::Dependent:
   case BuiltinType::Overload:
   case BuiltinType::UnknownAny:

clang/lib/AST/Type.cpp

@@ -3393,6 +3393,8 @@ StringRef BuiltinType::getName(const PrintingPolicy &Policy) const {
     return "<overloaded function type>";
   case BoundMember:
     return "<bound member function type>";
+  case UnresolvedTemplate:
+    return "<unresolved template type>";
   case PseudoObject:
     return "<pseudo-object type>";
   case Dependent:
@@ -4685,6 +4687,7 @@ bool Type::canHaveNullability(bool ResultIfUnknown) const {
 #include "clang/AST/BuiltinTypes.def"
       return false;
 
+    case BuiltinType::UnresolvedTemplate:
     // Dependent types that could instantiate to a pointer type.
     case BuiltinType::Dependent:
     case BuiltinType::Overload:

clang/lib/AST/TypeLoc.cpp

@@ -399,6 +399,7 @@ TypeSpecifierType BuiltinTypeLoc::getWrittenTypeSpec() const {
   case BuiltinType::NullPtr:
   case BuiltinType::Overload:
   case BuiltinType::Dependent:
+  case BuiltinType::UnresolvedTemplate:
   case BuiltinType::BoundMember:
   case BuiltinType::UnknownAny:
   case BuiltinType::ARCUnbridgedCast:

clang/lib/Parse/ParseExprCXX.cpp

@@ -407,6 +407,20 @@ bool Parser::ParseOptionalCXXScopeSpecifier(
       continue;
     }
 
+    switch (Tok.getKind()) {
+#define TRANSFORM_TYPE_TRAIT_DEF(_, Trait) case tok::kw___##Trait:
+#include "clang/Basic/TransformTypeTraits.def"
+      if (!NextToken().is(tok::l_paren)) {
+        Tok.setKind(tok::identifier);
+        Diag(Tok, diag::ext_keyword_as_ident)
+            << Tok.getIdentifierInfo()->getName() << 0;
+        continue;
+      }
+      [[fallthrough]];
+    default:
+      break;
+    }
+
     // The rest of the nested-name-specifier possibilities start with
     // tok::identifier.
     if (Tok.isNot(tok::identifier))