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

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 llvm#136432
Fixes llvm#137014
Fixes llvm#138018

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -621,6 +621,8 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^

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!");
 
@@ -1750,6 +1754,7 @@ CXXMethodDecl *CXXRecordDecl::getLambdaCallOperator() const {
 
 CXXMethodDecl* CXXRecordDecl::getLambdaStaticInvoker() const {
   CXXMethodDecl *CallOp = getLambdaCallOperator();
+  assert(CallOp && "null call operator");
   CallingConv CC = CallOp->getType()->castAs<FunctionType>()->getCallConv();
   return getLambdaStaticInvoker(CC);
 }

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