[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.

Author: falbrechtskirchinger

clang-tools-extra/clangd/test/GH99036.test

@@ -0,0 +1,29 @@
+# RUN: rm -rf %t
+# RUN: mkdir -p %t
+# RUN: split-file %s %t
+#
+# RUN: not clangd --check=%t/main.cpp 2>&1 | FileCheck -strict-whitespace %s
+#
+# CHECK:      Loaded compilation database
+# CHECK-NEXT: Compile command inferred from main.cpp is:
+# CHECK:      Indexing headers...
+# CHECK-NEXT: [unknown_typename] Line 1: in included file: unknown type name '_Up'
+# CHECK:      All checks completed, 1 errors
+
+#--- header.h
+template<_Up>
+concept __decayed_same_as;
+template<__decayed_same_as>
+partial_ordering operator0
+
+#--- main.cpp
+#include "header.h"
+
+#--- compile_commands.json
+[
+  {
+    "directory": "/",
+    "command": "c++ -std=c++20",
+    "file": "main.cpp"
+  }
+]

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(TC != nullptr); // reflects TypeConstraintInitialized
   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));