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

We had a code path in `Sema::MarkFunctionReferenced()` that deferred
local lambda instantiation even for constexprs. This resulted in any
calls to them referring to function decls that lack bodies and hence
failures at constant evaluation.

The issue doesn't occur when the lambda has no explicit return type
because the return type deduction requires instantiation.

Fixes #35052
Fixes #94849

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -858,6 +858,7 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExpr.cpp

@@ -18112,16 +18112,17 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
 
         if (FirstInstantiation || TSK != TSK_ImplicitInstantiation ||
             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())
+          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

@@ -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