[clangd][AST] Handle uninitialized type constraints

The ASTWriter currently assumes template type constraints to be
initialized ((bool)getTypeConstraint() == hasTypeConstraint()).
The attached test case presents a scenario where that is not the case.

This patch removes the assumption and adds another boolean to the
serialization, to explicitly encode whether the type constraint has been
initialized.

Fixes #99036.
Fixes #109354.

clang-tools-extra/clangd/unittests/ASTTests.cpp

@@ -658,6 +658,26 @@ TEST(ClangdAST, PreferredIncludeDirective) {
             Symbol::IncludeDirective::Import);
 }
 
+TEST(ClangdAST, HandleUninitializedTypeConstraints) {
+  auto TU = TestTU::withHeaderCode(R"cpp(
+    template<typename T>
+    concept C; // error-ok
+    template<C U>
+    void f();
+    )cpp");
+  TU.ExtraArgs.push_back("-std=c++20");
+
+  auto AST = TU.build();
+
+  const auto &F = llvm::cast<FunctionTemplateDecl>(findDecl(AST, "f"));
+  const auto *Params = F.getTemplateParameters();
+  const auto *U = llvm::cast<TemplateTypeParmDecl>(Params->getParam(0));
+  const auto *TC = U->getTypeConstraint();
+
+  EXPECT_TRUE(U->hasTypeConstraint());
+  EXPECT_FALSE(/*TypeConstraintInitialized=*/TC != nullptr);
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2674,7 +2674,7 @@ void ASTDeclReader::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
 
   D->setDeclaredWithTypename(Record.readInt());
 
-  if (D->hasTypeConstraint()) {
+  if (Record.readBool() && D->hasTypeConstraint()) {
     ConceptReference *CR = nullptr;
     if (Record.readBool())
       CR = Record.readConceptReference();

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1899,7 +1899,7 @@ void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   Record.push_back(D->wasDeclaredWithTypename());
 
   const TypeConstraint *TC = D->getTypeConstraint();
-  assert((bool)TC == D->hasTypeConstraint());
+  Record.push_back(/*TypeConstraintInitialized=*/TC != nullptr);
   if (TC) {
     auto *CR = TC->getConceptReference();
     Record.push_back(CR != nullptr);
@@ -1917,7 +1917,7 @@ void ASTDeclWriter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   if (OwnsDefaultArg)
     Record.AddTemplateArgumentLoc(D->getDefaultArgument());
 
-  if (!TC && !OwnsDefaultArg &&
+  if (!D->hasTypeConstraint() && !OwnsDefaultArg &&
       D->getDeclContext() == D->getLexicalDeclContext() &&
       !D->isInvalidDecl() && !D->hasAttrs() &&
       !D->isTopLevelDeclInObjCContainer() && !D->isImplicit() &&
@@ -2580,6 +2580,7 @@ void ASTWriter::WriteDeclAbbrevs() {
   // TemplateTypeParmDecl
   Abv->Add(
       BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 1)); // wasDeclaredWithTypename
+  Abv->Add(BitCodeAbbrevOp(0));                    // TypeConstraintInitialized
   Abv->Add(BitCodeAbbrevOp(0));                    // OwnsDefaultArg
   DeclTemplateTypeParmAbbrev = Stream.EmitAbbrev(std::move(Abv));