[clang] [Clang] incorrect assertion when checking template template parameter of a lambda (PR #138121)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: cor3ntin (cor3ntin)

<details>
<summary>Changes</summary>

When a lambda is used in an alias declaration, we were trying to refer to its call operator. However, that could happen before (or during) the call operator is defined.

So we should not assume a lambda always has a call operator.

Fixes #<!-- -->136432
Fixes #<!-- -->137014
Fixes #<!-- -->138018

---
Full diff: https://github.com/llvm/llvm-project/pull/138121.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+2) 
- (modified) clang/lib/AST/DeclCXX.cpp (+5-1) 
- (modified) clang/test/SemaCXX/lambda-unevaluated.cpp (+16) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f5cd1fbeabcfe..8a463f9aca0c9 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -529,6 +529,8 @@ Bug Fixes to C++ Support
 - Clang now issues an error when placement new is used to modify a const-qualified variable
   in a ``constexpr`` function. (#GH131432)
 - Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
+- Fix a crash when checking the template template parameters of a dependent lambda appearing in an alias declaration.
+  (#GH136432), (#GH137014), (#GH138018)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index 4d07efd58f518..9095575ee1175 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -1696,7 +1696,11 @@ static NamedDecl* getLambdaCallOperatorHelper(const CXXRecordDecl &RD) {
       RD.getASTContext().DeclarationNames.getCXXOperatorName(OO_Call);
 
   DeclContext::lookup_result Calls = RD.lookup(Name);
-  assert(!Calls.empty() && "Missing lambda call operator!");
+
+  // This can happen while building the lambda.
+  if (Calls.empty())
+    return nullptr;
+
   assert(allLookupResultsAreTheSame(Calls) &&
          "More than one lambda call operator!");
 
diff --git a/clang/test/SemaCXX/lambda-unevaluated.cpp b/clang/test/SemaCXX/lambda-unevaluated.cpp
index a9bcab58464e2..40f8a729fd912 100644
--- a/clang/test/SemaCXX/lambda-unevaluated.cpp
+++ b/clang/test/SemaCXX/lambda-unevaluated.cpp
@@ -266,3 +266,19 @@ void func() {
 }
 
 } // namespace GH88081
+
+namespace GH138018 {
+
+template <typename T> struct vec {};
+
+auto structure_to_typelist(auto)  {
+    return []<template <typename> typename T>(T<int>) {
+        return 0;
+    }(vec<int>{});
+}
+
+template <typename T> using helper = decltype(structure_to_typelist(T{}));
+static_assert(__is_same_as(int, helper<int>));
+
+
+} // namespace GH138018

``````````

</details>


https://github.com/llvm/llvm-project/pull/138121