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

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 updated https://github.com/llvm/llvm-project/pull/95660

>From c288190c313d6990580048368c60a846919f98ae Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 15 Jun 2024 20:29:24 +0800
Subject: [PATCH 1/2] [Clang] Instantiate local constexpr functions eagerly

---
 clang/docs/ReleaseNotes.rst                   |  1 +
 clang/lib/Sema/SemaExpr.cpp                   |  3 ++-
 .../SemaTemplate/instantiate-local-class.cpp  | 24 +++++++++++++++++++
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8c2f737836a9d..a424554ed821d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -847,6 +847,7 @@ 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)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 99a8704298314..88e9b2b00f84d 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -18112,7 +18112,8 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
 
         if (FirstInstantiation || TSK != TSK_ImplicitInstantiation ||
             Func->isConstexpr()) {
-          if (isa<CXXRecordDecl>(Func->getDeclContext()) &&
+          if (!Func->isConstexpr() &&
+              isa<CXXRecordDecl>(Func->getDeclContext()) &&
               cast<CXXRecordDecl>(Func->getDeclContext())->isLocalClass() &&
               CodeSynthesisContexts.size())
             PendingLocalImplicitInstantiations.push_back(
diff --git a/clang/test/SemaTemplate/instantiate-local-class.cpp b/clang/test/SemaTemplate/instantiate-local-class.cpp
index 47591045fd26e..7eee131e28d60 100644
--- a/clang/test/SemaTemplate/instantiate-local-class.cpp
+++ b/clang/test/SemaTemplate/instantiate-local-class.cpp
@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -verify -std=c++11 %s
 // RUN: %clang_cc1 -verify -std=c++11 -fdelayed-template-parsing %s
+// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
 
 template<typename T>
 void f0() {
@@ -509,3 +510,26 @@ namespace LambdaInDefaultMemberInitializer {
   }
   template void f<int>();
 }
+
+#if __cplusplus >= 201703L
+namespace GH35052 {
+
+template <typename F> constexpr int func(F f) {
+  if constexpr (f(1UL)) {
+    return 1;
+  }
+  return 0;
+}
+
+int main() {
+  auto predicate = [](auto v) /*implicit constexpr*/ -> bool {
+    return v == 1;
+  };
+
+  static_assert(predicate(1));
+  return func(predicate);
+}
+
+} // namespace GH35052
+
+#endif

>From cd7ef4f9680f2d43576a6710f448955cca6f800c Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sun, 16 Jun 2024 20:56:54 +0800
Subject: [PATCH 2/2] Address feedback

---
 clang/lib/Sema/SemaExpr.cpp | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 88e9b2b00f84d..4db8b4130c3c7 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -18112,17 +18112,17 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
 
         if (FirstInstantiation || TSK != TSK_ImplicitInstantiation ||
             Func->isConstexpr()) {
-          if (!Func->isConstexpr() &&
-              isa<CXXRecordDecl>(Func->getDeclContext()) &&
-              cast<CXXRecordDecl>(Func->getDeclContext())->isLocalClass() &&
-              CodeSynthesisContexts.size())
-            PendingLocalImplicitInstantiations.push_back(
-                std::make_pair(Func, PointOfInstantiation));
-          else if (Func->isConstexpr())
+          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(