[Clang] Eagerly instantiate used constexpr function upon definition. (#73463)

Despite CWG2497 not being resolved, it is reasonable to expect the
following code to compile (and which is supported by other compilers)

```cpp
  template<typename T> constexpr T f();
  constexpr int g() { return f<int>(); } // #1
  template<typename T> constexpr T f() { return 123; }
  int k[g()];
  // #2
```

To that end, we eagerly instantiate all referenced specializations of
constexpr functions when they are defined.

We maintain a map of (pattern, [instantiations]) independent of
`PendingInstantiations` to avoid having to iterate that list after each
function definition.

We should apply the same logic to constexpr variables, but I wanted to
keep the PR small.

Fixes #73232

6 files changed, 87 insertions, 2 deletions

diff --git a/clang/lib/Sema/MultiplexExternalSemaSource.cpp b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
index 058e22c..d0d6a3a 100644
--- a/clang/lib/Sema/MultiplexExternalSemaSource.cpp
+++ b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
@@ -310,6 +310,12 @@ void MultiplexExternalSemaSource::ReadPendingInstantiations(
     Sources[i]->ReadPendingInstantiations(Pending);
 }
 
+void MultiplexExternalSemaSource::ReadPendingInstantiationsOfConstexprEntity(
+    const NamedDecl *D, llvm::SmallSetVector<NamedDecl *, 4> &Decls) {
+  for (size_t i = 0; i < Sources.size(); ++i)
+    Sources[i]->ReadPendingInstantiationsOfConstexprEntity(D, Decls);
+};
+
 void MultiplexExternalSemaSource::ReadLateParsedTemplates(
     llvm::MapVector<const FunctionDecl *, std::unique_ptr<LateParsedTemplate>>
         &LPTMap) {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 77ff4e1..9591f8b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -16275,6 +16275,9 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
   if (FD && !FD->isDeleted())
     checkTypeSupport(FD->getType(), FD->getLocation(), FD);
 
+  if (FD && FD->isConstexpr() && FD->isTemplated())
+    PerformPendingInstantiationsOfConstexprFunctions(FD);
+
   return dcl;
 }
 
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index d1b2b80..b204cb0 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -19053,12 +19053,17 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
               CodeSynthesisContexts.size())
             PendingLocalImplicitInstantiations.push_back(
                 std::make_pair(Func, PointOfInstantiation));
-          else if (Func->isConstexpr())
+          else if (Func->isConstexpr()) {
             // Do not defer instantiations of constexpr functions, to avoid the
             // expression evaluator needing to call back into Sema if it sees a
             // call to such a function.
             InstantiateFunctionDefinition(PointOfInstantiation, Func);
-          else {
+            if (!Func->isDefined()) {
+              PendingInstantiationsOfConstexprEntities
+                  [Func->getTemplateInstantiationPattern()->getCanonicalDecl()]
+                      .push_back(Func);
+            }
+          } else {
             Func->setInstantiationIsPending(true);
             PendingInstantiations.push_back(
                 std::make_pair(Func, PointOfInstantiation));
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d768bb7..aa367e0 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -6495,6 +6495,34 @@ void Sema::PerformPendingInstantiations(bool LocalOnly) {
     PendingInstantiations.swap(delayedPCHInstantiations);
 }
 
+// Instantiate all referenced specializations of the given function template
+// definition. This make sure that constexpr function templates that are defined
+// after the point of instantiation of their use can be evaluated after they
+// are defined. see CWG2497.
+void Sema::PerformPendingInstantiationsOfConstexprFunctions(FunctionDecl *Tpl) {
+
+  auto InstantiateAll = [&](const auto &Range) {
+    for (NamedDecl *D : Range) {
+      FunctionDecl *Fun = cast<FunctionDecl>(D);
+      InstantiateFunctionDefinition(Fun->getPointOfInstantiation(), Fun);
+    }
+  };
+
+  auto It =
+      PendingInstantiationsOfConstexprEntities.find(Tpl->getCanonicalDecl());
+  if (It != PendingInstantiationsOfConstexprEntities.end()) {
+    auto Decls = std::move(It->second);
+    PendingInstantiationsOfConstexprEntities.erase(It);
+    InstantiateAll(Decls);
+  }
+
+  llvm::SmallSetVector<NamedDecl *, 4> Decls;
+  if (ExternalSource) {
+    ExternalSource->ReadPendingInstantiationsOfConstexprEntity(Tpl, Decls);
+    InstantiateAll(Decls);
+  }
+}
+
 void Sema::PerformDependentDiagnostics(const DeclContext *Pattern,
                        const MultiLevelTemplateArgumentList &TemplateArgs) {
   for (auto *DD : Pattern->ddiags()) {
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index f22da83..ef191c2 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3709,6 +3709,19 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
       }
       break;
 
+    case PENDING_INSTANTIATIONS_OF_CONSTEXPR_ENTITIES:
+      if (Record.size() % 2 != 0)
+        return llvm::createStringError(
+            std::errc::illegal_byte_sequence,
+            "Invalid PENDING_INSTANTIATIONS_OF_CONSTEXPR_ENTITIES block");
+
+      for (unsigned I = 0, N = Record.size(); I != N; /* in loop */) {
+        DeclID Key = getGlobalDeclID(F, Record[I++]);
+        DeclID Value = getGlobalDeclID(F, Record[I++]);
+        PendingInstantiationsOfConstexprEntities[Key].insert(Value);
+      }
+      break;
+
     case SEMA_DECL_REFS:
       if (Record.size() != 3)
         return llvm::createStringError(std::errc::illegal_byte_sequence,
@@ -8718,6 +8731,20 @@ void ASTReader::ReadPendingInstantiations(
   PendingInstantiations.clear();
 }
 
+void ASTReader::ReadPendingInstantiationsOfConstexprEntity(
+    const NamedDecl *D, llvm::SmallSetVector<NamedDecl *, 4> &Decls) {
+  for (auto *Redecl : D->redecls()) {
+    if (!Redecl->isFromASTFile())
+      continue;
+    DeclID Id = Redecl->getGlobalID();
+    auto It = PendingInstantiationsOfConstexprEntities.find(Id);
+    if (It == PendingInstantiationsOfConstexprEntities.end())
+      continue;
+    for (DeclID InstantiationId : It->second)
+      Decls.insert(cast<NamedDecl>(GetDecl(InstantiationId)));
+  }
+}
+
 void ASTReader::ReadLateParsedTemplates(
     llvm::MapVector<const FunctionDecl *, std::unique_ptr<LateParsedTemplate>>
         &LPTMap) {
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 6df8152..8c8048f 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -849,6 +849,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(SEMA_DECL_REFS);
   RECORD(WEAK_UNDECLARED_IDENTIFIERS);
   RECORD(PENDING_IMPLICIT_INSTANTIATIONS);
+  RECORD(PENDING_INSTANTIATIONS_OF_CONSTEXPR_ENTITIES);
   RECORD(UPDATE_VISIBLE);
   RECORD(DECL_UPDATE_OFFSETS);
   RECORD(DECL_UPDATES);
@@ -4836,6 +4837,16 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   assert(SemaRef.PendingLocalImplicitInstantiations.empty() &&
          "There are local ones at end of translation unit!");
 
+  // Build a record containing all pending instantiations of constexpr
+  // entities.
+  RecordData PendingInstantiationsOfConstexprEntities;
+  for (const auto &I : SemaRef.PendingInstantiationsOfConstexprEntities) {
+    for (const auto &Elem : I.second) {
+      AddDeclRef(I.first, PendingInstantiationsOfConstexprEntities);
+      AddDeclRef(Elem, PendingInstantiationsOfConstexprEntities);
+    }
+  }
+
   // Build a record containing some declaration references.
   RecordData SemaDeclRefs;
   if (SemaRef.StdNamespace || SemaRef.StdBadAlloc || SemaRef.StdAlignValT) {
@@ -5153,6 +5164,11 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   if (!PendingInstantiations.empty())
     Stream.EmitRecord(PENDING_IMPLICIT_INSTANTIATIONS, PendingInstantiations);
 
+  // Write the record containing pending instantiations of constexpr entities.
+  if (!PendingInstantiationsOfConstexprEntities.empty())
+    Stream.EmitRecord(PENDING_INSTANTIATIONS_OF_CONSTEXPR_ENTITIES,
+                      PendingInstantiationsOfConstexprEntities);
+
   // Write the record containing declaration references of Sema.
   if (!SemaDeclRefs.empty())
     Stream.EmitRecord(SEMA_DECL_REFS, SemaDeclRefs);