[clang] [clang] Error on explicit specialization of lambda call operator (PR #84343)

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon created https://github.com/llvm/llvm-project/pull/84343

Fixes https://github.com/llvm/llvm-project/issues/83267

>From 9c2fccb61a058385171415f863eaccb666e7b120 Mon Sep 17 00:00:00 2001
From: "Podchishchaeva, Mariya" <mariya.podchishchaeva at intel.com>
Date: Thu, 7 Mar 2024 08:56:35 -0800
Subject: [PATCH] [clang] Error on explicit specialization of lambda call
 operator

Fixes https://github.com/llvm/llvm-project/issues/83267
---
 clang/docs/ReleaseNotes.rst                   |  4 ++++
 .../clang/Basic/DiagnosticSemaKinds.td        |  2 ++
 clang/lib/Sema/SemaDecl.cpp                   | 22 +++++++++++++------
 clang/test/SemaCXX/lambda-expressions.cpp     | 15 +++++++++++++
 4 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1b901a27fd19d1..aa28ebf2558509 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -241,6 +241,10 @@ Bug Fixes in This Version
   for variables created through copy initialization having side-effects in C++17 and later.
   Fixes (#GH64356) (#GH79518).
 
+- Clang now emits errors for explicit specializations/instatiations of lambda call
+  operator.
+  Fixes (#GH83267).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index c8dfdc08f5ea07..a80c8b75e863ed 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -8170,6 +8170,8 @@ let CategoryName = "Lambda Issue" in {
   def warn_cxx11_compat_generic_lambda : Warning<
     "generic lambdas are incompatible with C++11">,
     InGroup<CXXPre14Compat>, DefaultIgnore;
+  def err_lambda_explicit_spec : Error<
+    "lambda call operator should not be explicitly specialized or instantiated">;
 
   // C++17 '*this' captures.
   def warn_cxx14_compat_star_this_lambda_capture : Warning<
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6b81ee183cc440..31d615189385e5 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -15795,10 +15795,19 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
   // captures during transformation of nested lambdas, it is necessary to
   // have the LSI properly restored.
   if (isGenericLambdaCallOperatorSpecialization(FD)) {
-    assert(inTemplateInstantiation() &&
-           "There should be an active template instantiation on the stack "
-           "when instantiating a generic lambda!");
-    RebuildLambdaScopeInfo(cast<CXXMethodDecl>(D));
+    // C++2c 7.5.5.2p17 A member of a closure type shall not be explicitly
+    // instantiated, explicitly specialized.
+    if (FD->getTemplateSpecializationInfo()
+            ->isExplicitInstantiationOrSpecialization()) {
+      Diag(FD->getLocation(), diag::err_lambda_explicit_spec);
+      FD->setInvalidDecl();
+      PushFunctionScope();
+    } else {
+      assert(inTemplateInstantiation() &&
+             "There should be an active template instantiation on the stack "
+             "when instantiating a generic lambda!");
+      RebuildLambdaScopeInfo(cast<CXXMethodDecl>(D));
+    }
   } else {
     // Enter a new function scope
     PushFunctionScope();
@@ -16317,9 +16326,8 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
         }
       }
 
-      assert(
-          (FD == getCurFunctionDecl() || getCurLambda()->CallOperator == FD) &&
-          "Function parsing confused");
+      assert((FD == getCurFunctionDecl(/*AllowLambdas=*/true)) &&
+             "Function parsing confused");
     } else if (ObjCMethodDecl *MD = dyn_cast_or_null<ObjCMethodDecl>(dcl)) {
       assert(MD == getCurMethodDecl() && "Method parsing confused");
       MD->setBody(Body);
diff --git a/clang/test/SemaCXX/lambda-expressions.cpp b/clang/test/SemaCXX/lambda-expressions.cpp
index 41cf5a46c38922..0516a5da31ae9a 100644
--- a/clang/test/SemaCXX/lambda-expressions.cpp
+++ b/clang/test/SemaCXX/lambda-expressions.cpp
@@ -732,3 +732,18 @@ void GH67492() {
   constexpr auto test = 42;
   auto lambda = (test, []() noexcept(true) {});
 }
+
+namespace GH83267 {
+auto l = [](auto a) { return 1; };
+using type = decltype(l);
+
+template<>
+auto type::operator()(int a) const { // expected-error{{lambda call operator should not be explicitly specialized or instantiated}}
+  return c; // expected-error {{use of undeclared identifier 'c'}}
+}
+
+auto ll = [](auto a) { return 1; }; // expected-error{{lambda call operator should not be explicitly specialized or instantiated}}
+using t = decltype(ll);
+template auto t::operator()<int>(int a) const; // expected-note {{in instantiation}}
+
+}