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

Summary:
Unfortunately, #95660 has caused a regression in DeduceReturnType(),
where some of the local recursive lambdas are getting incorrectly rejected.

This reverts commit 5548ea3. Also, this
adds an offending case to the test.

Closes #98526

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251570

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -1017,7 +1017,6 @@ Bug Fixes to C++ Support
   (#GH88081), (#GH89496), (#GH90669), (#GH91633) and (#GH97453).
 - Fixed a crash in constraint instantiation under nested lambdas with dependent parameters.
 - 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)

clang/lib/Sema/SemaExpr.cpp

@@ -17958,17 +17958,16 @@ 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(

clang/test/SemaTemplate/instantiate-local-class.cpp

@@ -512,24 +512,26 @@ namespace LambdaInDefaultMemberInitializer {
 }
 
 #if __cplusplus >= 201703L
-namespace GH35052 {
 
-template <typename F> constexpr int func(F f) {
-  if constexpr (f(1UL)) {
-    return 1;
+// 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 {
+  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>;
 
-  static_assert(predicate(1));
-  return func(predicate);
+void foo() {
+  recursive_lambda{[&](auto &self_fn, int) -> int {
+    return self_fn(0);
+  }}(0);
 }
 
-} // namespace GH35052
+} // namespace local_recursive_lambda
 
 #endif