[clang] Error on explicit specialization of lambda call operator

[Fznamznon]

Fixes #83267

[llvmbot]

@llvm/pr-subscribers-clang

Author: Mariya Podchishchaeva (Fznamznon)

Changes

Fixes #83267

---

Full diff: https://github.com/llvm/llvm-project/pull/84343.diff

4 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+4)
• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
• (modified) clang/lib/Sema/SemaDecl.cpp (+15-7)
• (modified) clang/test/SemaCXX/lambda-expressions.cpp (+15)

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}}
+
+}

[cor3ntin]

LGTM, thanks!

[shafik]

Can we please get more descriptive summaries in the future. We should at a minimum state 1) what the underlying problem is 2) what the approach for fixing the problem is.

In this case the title describes the problem but it would be nice to have a summary of the fix as well. So folks reading the git log do not have to do an extra step to understand what the fix was at a high level.

[Fznamznon]

Can we please get more descriptive summaries in the future.

Sure, sorry about that.