[clang] Revert "[Clang] Instantiate local constexpr functions eagerly (#95660)" (PR #98991)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 updated https://github.com/llvm/llvm-project/pull/98991

>From 727d92f0651cec3110d321582a884f1c939d8a6d Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Tue, 16 Jul 2024 12:43:33 +0800
Subject: [PATCH 1/2] Revert "[Clang] Instantiate local constexpr functions
 eagerly (#95660)"

Unfortunately, this has caused a regression in DeduceReturnType(), which
causes a problem in that some of the local recursive lambdas are incorrectly
rejected.

This reverts commit 5548ea34341e9d0ae645719c34b466ca3b9eaa5a. Also,
this adds an offending case to the test.
---
 clang/docs/ReleaseNotes.rst                   |  1 -
 clang/lib/Sema/SemaExpr.cpp                   | 13 ++++----
 .../SemaTemplate/instantiate-local-class.cpp  | 33 +++++++++++--------
 3 files changed, 26 insertions(+), 21 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 838cb69f647ee..077063a828cbb 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -960,7 +960,6 @@ Bug Fixes to C++ Support
 - Fixed several bugs in capturing variables within unevaluated contexts. (#GH63845), (#GH67260), (#GH69307),
   (#GH88081), (#GH89496), (#GH90669) and (#GH91633).
 - Fixed handling of brace ellison when building deduction guides. (#GH64625), (#GH83368).
-- Clang now instantiates local constexpr functions eagerly for constant evaluators. (#GH35052), (#GH94849)
 - Fixed a failed assertion when attempting to convert an integer representing the difference
   between the addresses of two labels (a GNU extension) to a pointer within a constant expression. (#GH95366).
 - Fix immediate escalation bugs in the presence of dependent call arguments. (#GH94935)
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 852344d895ffd..9723de6f46fc7 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -17938,17 +17938,16 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
 
         if (FirstInstantiation || TSK != TSK_ImplicitInstantiation ||
             Func->isConstexpr()) {
-          if (Func->isConstexpr())
+          if (isa<CXXRecordDecl>(Func->getDeclContext()) &&
+              cast<CXXRecordDecl>(Func->getDeclContext())->isLocalClass() &&
+              CodeSynthesisContexts.size())
+            PendingLocalImplicitInstantiations.push_back(
+                std::make_pair(Func, PointOfInstantiation));
+          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 (isa<CXXRecordDecl>(Func->getDeclContext()) &&
-                   cast<CXXRecordDecl>(Func->getDeclContext())
-                       ->isLocalClass() &&
-                   CodeSynthesisContexts.size())
-            PendingLocalImplicitInstantiations.push_back(
-                std::make_pair(Func, PointOfInstantiation));
           else {
             Func->setInstantiationIsPending(true);
             PendingInstantiations.push_back(
diff --git a/clang/test/SemaTemplate/instantiate-local-class.cpp b/clang/test/SemaTemplate/instantiate-local-class.cpp
index 7eee131e28d60..a313bdd9d2ea5 100644
--- a/clang/test/SemaTemplate/instantiate-local-class.cpp
+++ b/clang/test/SemaTemplate/instantiate-local-class.cpp
@@ -512,24 +512,31 @@ namespace LambdaInDefaultMemberInitializer {
 }
 
 #if __cplusplus >= 201703L
-namespace GH35052 {
 
-template <typename F> constexpr int func(F f) {
-  if constexpr (f(1UL)) {
-    return 1;
+namespace local_recursive_lambda {
+
+template <typename F> struct recursive_lambda {
+  template <typename... Args> auto operator()(Args &&...args) const {
+    return fn(*this, args...);
   }
-  return 0;
-}
+  F fn;
+};
 
-int main() {
-  auto predicate = [](auto v) /*implicit constexpr*/ -> bool {
-    return v == 1;
-  };
+template <typename F> recursive_lambda(F) -> recursive_lambda<F>;
+
+struct Tree {
+  Tree *left, *right;
+};
+
+int sumSize(Tree *tree) {
+  auto accumulate =
+      recursive_lambda{[&](auto &self_fn, Tree *element_node) -> int {
+        return 1 + self_fn(tree->left) + self_fn(tree->right);
+      }};
 
-  static_assert(predicate(1));
-  return func(predicate);
+  return accumulate(tree);
 }
 
-} // namespace GH35052
+} // namespace local_recursive_lambda
 
 #endif

>From 44752766678a796b294c0cd8073a2e11c18988d8 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Tue, 16 Jul 2024 18:52:10 +0800
Subject: [PATCH 2/2] Add an issue reference

---
 .../SemaTemplate/instantiate-local-class.cpp    | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/clang/test/SemaTemplate/instantiate-local-class.cpp b/clang/test/SemaTemplate/instantiate-local-class.cpp
index a313bdd9d2ea5..298233739900f 100644
--- a/clang/test/SemaTemplate/instantiate-local-class.cpp
+++ b/clang/test/SemaTemplate/instantiate-local-class.cpp
@@ -513,6 +513,8 @@ namespace LambdaInDefaultMemberInitializer {
 
 #if __cplusplus >= 201703L
 
+// Reduced from https://github.com/llvm/llvm-project/issues/98526
+// This relies on the deferral instantiation of the local lambda, otherwise we would fail in DeduceReturnType().
 namespace local_recursive_lambda {
 
 template <typename F> struct recursive_lambda {
@@ -524,17 +526,10 @@ template <typename F> struct recursive_lambda {
 
 template <typename F> recursive_lambda(F) -> recursive_lambda<F>;
 
-struct Tree {
-  Tree *left, *right;
-};
-
-int sumSize(Tree *tree) {
-  auto accumulate =
-      recursive_lambda{[&](auto &self_fn, Tree *element_node) -> int {
-        return 1 + self_fn(tree->left) + self_fn(tree->right);
-      }};
-
-  return accumulate(tree);
+void foo() {
+  recursive_lambda{[&](auto &self_fn, int) -> int {
+    return self_fn(0);
+  }}(0);
 }
 
 } // namespace local_recursive_lambda