[Clang] Correctly recognize unexpanded packs in lambda template params

[cor3ntin]

Fixes #48937
Fixes #49099

[llvmbot]

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

Changes

Fixes #48937

---

Full diff: https://github.com/llvm/llvm-project/pull/98496.diff

3 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+3-1)
• (modified) clang/lib/Sema/SemaLambda.cpp (+1)
• (modified) clang/test/SemaCXX/lambda-pack-expansion.cpp (+21)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 6adf57da42e65..c6a2237113ace 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -103,7 +103,7 @@ ABI Changes in This Version
   ifuncs. Its purpose was to preserve backwards compatibility when the ".ifunc"
   suffix got removed from the name mangling. The alias interacts badly with
   GlobalOpt (see the issue #96197).
-  
+
 - Fixed Microsoft name mangling for auto non-type template arguments of pointer
   type for MSVC 1920+. This change resolves incompatibilities with code compiled
   by MSVC 1920+ but will introduce incompatibilities with code compiled by
@@ -1024,6 +1024,8 @@ Bug Fixes to C++ Support
 - Fixed a bug where references to lambda capture inside a ``noexcept`` specifier were not correctly
   instantiated. (#GH95735).
 - Fixed a CTAD substitution bug involving type aliases that reference outer template parameters. (#GH94614).
+- Clang now correctly handles unexpanded packs in the template parameter list of a generic lambda expression
+  (#GH48937)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 4b19296e8a23a..7b93e5df25b36 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1380,6 +1380,7 @@ void Sema::ActOnLambdaClosureParameters(
     AddTemplateParametersToLambdaCallOperator(LSI->CallOperator, LSI->Lambda,
                                               TemplateParams);
     LSI->Lambda->setLambdaIsGeneric(true);
+    LSI->ContainsUnexpandedParameterPack |= TemplateParams->containsUnexpandedParameterPack();
   }
   LSI->AfterParameterList = true;
 }
diff --git a/clang/test/SemaCXX/lambda-pack-expansion.cpp b/clang/test/SemaCXX/lambda-pack-expansion.cpp
index e3e968e2704ed..221d1d01a06ae 100644
--- a/clang/test/SemaCXX/lambda-pack-expansion.cpp
+++ b/clang/test/SemaCXX/lambda-pack-expansion.cpp
@@ -20,3 +20,24 @@ void foo() {
   take_by_ref(x);
 }
 }
+
+namespace GH48937 {
+
+template <typename... Ts>
+consteval int f(Ts... ts) {
+  return ([]<Ts a = 42>(){ return a;}, ...)();
+}
+
+static_assert(f(0, 42) == 42);
+
+template <typename Ts>
+int g(Ts ts) {
+  return ([]<Ts a = 42>(){ return a;}, ...)();  // expected-error {{pack expansion does not contain any unexpanded parameter packs}}
+}
+
+template <typename... Ts>
+int h(Ts... ts) {
+  return ([]<Ts a = 42>(){ return a;})();  // expected-error {{expression contains unexpanded parameter pack 'Ts'}}
+}
+
+}

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[zyn0217]

Thanks. Actually we're missing this flag many times elsewhere in TransformLambdaExpr().
(See my patch #86265, let me try to revive that these days before 19 is cut)

[cor3ntin]

Thanks. Actually we're missing this flag many times elsewhere in TransformLambdaExpr(). (See my patch #86265, let me try to revive that these days before 19 is cut)

I was not aware of that PR, thanks (The general direction of it looks correct to me.)
Ah, lambdas, the gift that keeps on giving!