[Clang][Sema] Revisit the fix for the lambda within a type alias template decl (#89934)

In the last patch #82310, we used template depths to tell if such alias
decls contain lambdas, which is wrong because the lambda can also appear
as a part of the default argument, and that would make
`getTemplateInstantiationArgs` provide extra template arguments in
undesired contexts. This leads to issue #89853.

Moreover, our approach
for https://github.com/llvm/llvm-project/issues/82104 was sadly wrong.
We tried to teach `DeduceReturnType` to consider alias template
arguments; however, giving these arguments in the context where they
should have been substituted in a `TransformCallExpr` call is never
correct.

This patch addresses such problems by using a `RecursiveASTVisitor` to
check if the lambda is contained by an alias `Decl`, as well as
twiddling the lambda dependencies - we should also build a dependent
lambda expression if the surrounding alias template arguments were
dependent.

Fixes #89853
Fixes #102760
Fixes #105885

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -20,6 +20,7 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/ExprConcepts.h"
 #include "clang/AST/PrettyDeclStackTrace.h"
+#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/AST/TypeVisitor.h"
@@ -88,12 +89,19 @@ struct Response {
 // than lambda classes.
 const FunctionDecl *
 getPrimaryTemplateOfGenericLambda(const FunctionDecl *LambdaCallOperator) {
+  if (!isLambdaCallOperator(LambdaCallOperator))
+    return LambdaCallOperator;
   while (true) {
     if (auto *FTD = dyn_cast_if_present<FunctionTemplateDecl>(
             LambdaCallOperator->getDescribedTemplate());
         FTD && FTD->getInstantiatedFromMemberTemplate()) {
       LambdaCallOperator =
           FTD->getInstantiatedFromMemberTemplate()->getTemplatedDecl();
+    } else if (LambdaCallOperator->getPrimaryTemplate()) {
+      // Cases where the lambda operator is instantiated in
+      // TemplateDeclInstantiator::VisitCXXMethodDecl.
+      LambdaCallOperator =
+          LambdaCallOperator->getPrimaryTemplate()->getTemplatedDecl();
     } else if (auto *Prev = cast<CXXMethodDecl>(LambdaCallOperator)
                                 ->getInstantiatedFromMemberFunction())
       LambdaCallOperator = Prev;
@@ -139,22 +147,28 @@ getEnclosingTypeAliasTemplateDecl(Sema &SemaRef) {
 // Check if we are currently inside of a lambda expression that is
 // surrounded by a using alias declaration. e.g.
 //   template <class> using type = decltype([](auto) { ^ }());
-// By checking if:
-//  1. The lambda expression and the using alias declaration share the
-//  same declaration context.
-//  2. They have the same template depth.
 // We have to do so since a TypeAliasTemplateDecl (or a TypeAliasDecl) is never
 // a DeclContext, nor does it have an associated specialization Decl from which
 // we could collect these template arguments.
 bool isLambdaEnclosedByTypeAliasDecl(
-    const FunctionDecl *PrimaryLambdaCallOperator,
+    const FunctionDecl *LambdaCallOperator,
     const TypeAliasTemplateDecl *PrimaryTypeAliasDecl) {
-  return cast<CXXRecordDecl>(PrimaryLambdaCallOperator->getDeclContext())
-                 ->getTemplateDepth() ==
-             PrimaryTypeAliasDecl->getTemplateDepth() &&
-         getLambdaAwareParentOfDeclContext(
-             const_cast<FunctionDecl *>(PrimaryLambdaCallOperator)) ==
-             PrimaryTypeAliasDecl->getDeclContext();
+  struct Visitor : RecursiveASTVisitor<Visitor> {
+    Visitor(const FunctionDecl *CallOperator) : CallOperator(CallOperator) {}
+    bool VisitLambdaExpr(const LambdaExpr *LE) {
+      // Return true to bail out of the traversal, implying the Decl contains
+      // the lambda.
+      return getPrimaryTemplateOfGenericLambda(LE->getCallOperator()) !=
+             CallOperator;
+    }
+    const FunctionDecl *CallOperator;
+  };
+
+  QualType Underlying =
+      PrimaryTypeAliasDecl->getTemplatedDecl()->getUnderlyingType();
+
+  return !Visitor(getPrimaryTemplateOfGenericLambda(LambdaCallOperator))
+              .TraverseType(Underlying);
 }
 
 // Add template arguments from a variable template instantiation.
@@ -293,23 +307,8 @@ Response HandleFunction(Sema &SemaRef, const FunctionDecl *Function,
 
     // If this function is a generic lambda specialization, we are done.
     if (!ForConstraintInstantiation &&
-        isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function)) {
-      // TypeAliasTemplateDecls should be taken into account, e.g.
-      // when we're deducing the return type of a lambda.
-      //
-      // template <class> int Value = 0;
-      // template <class T>
-      // using T = decltype([]<int U = 0>() { return Value<T>; }());
-      //
-      if (auto TypeAlias = getEnclosingTypeAliasTemplateDecl(SemaRef)) {
-        if (isLambdaEnclosedByTypeAliasDecl(
-                /*PrimaryLambdaCallOperator=*/getPrimaryTemplateOfGenericLambda(
-                    Function),
-                /*PrimaryTypeAliasDecl=*/TypeAlias.PrimaryTypeAliasDecl))
-          return Response::UseNextDecl(Function);
-      }
+        isGenericLambdaCallOperatorOrStaticInvokerSpecialization(Function))
       return Response::Done();
-    }
 
   } else if (Function->getDescribedFunctionTemplate()) {
     assert(
@@ -421,10 +420,9 @@ Response HandleRecordDecl(Sema &SemaRef, const CXXRecordDecl *Rec,
     // Retrieve the template arguments for a using alias declaration.
     // This is necessary for constraint checking, since we always keep
     // constraints relative to the primary template.
-    if (auto TypeAlias = getEnclosingTypeAliasTemplateDecl(SemaRef)) {
-      const FunctionDecl *PrimaryLambdaCallOperator =
-          getPrimaryTemplateOfGenericLambda(Rec->getLambdaCallOperator());
-      if (isLambdaEnclosedByTypeAliasDecl(PrimaryLambdaCallOperator,
+    if (auto TypeAlias = getEnclosingTypeAliasTemplateDecl(SemaRef);
+        ForConstraintInstantiation && TypeAlias) {
+      if (isLambdaEnclosedByTypeAliasDecl(Rec->getLambdaCallOperator(),
                                           TypeAlias.PrimaryTypeAliasDecl)) {
         Result.addOuterTemplateArguments(TypeAlias.Template,
                                          TypeAlias.AssociatedTemplateArguments,
@@ -1647,12 +1645,17 @@ namespace {
 
     CXXRecordDecl::LambdaDependencyKind
     ComputeLambdaDependency(LambdaScopeInfo *LSI) {
-      auto &CCS = SemaRef.CodeSynthesisContexts.back();
-      if (CCS.Kind ==
-          Sema::CodeSynthesisContext::TypeAliasTemplateInstantiation) {
-        unsigned TypeAliasDeclDepth = CCS.Entity->getTemplateDepth();
+      if (auto TypeAlias =
+              TemplateInstArgsHelpers::getEnclosingTypeAliasTemplateDecl(
+                  getSema());
+          TypeAlias && TemplateInstArgsHelpers::isLambdaEnclosedByTypeAliasDecl(
+                           LSI->CallOperator, TypeAlias.PrimaryTypeAliasDecl)) {
+        unsigned TypeAliasDeclDepth = TypeAlias.Template->getTemplateDepth();
         if (TypeAliasDeclDepth >= TemplateArgs.getNumSubstitutedLevels())
           return CXXRecordDecl::LambdaDependencyKind::LDK_AlwaysDependent;
+        for (const TemplateArgument &TA : TypeAlias.AssociatedTemplateArguments)
+          if (TA.isDependent())
+            return CXXRecordDecl::LambdaDependencyKind::LDK_AlwaysDependent;
       }
       return inherited::ComputeLambdaDependency(LSI);
     }

clang/test/SemaTemplate/alias-template-with-lambdas.cpp

@@ -91,15 +91,84 @@ void bar() {
 
 namespace GH82104 {
 
-template <typename, typename...> int Zero = 0;
+template <typename, typename... D> constexpr int Value = sizeof...(D);
 
-template <typename T, typename...U>
-using T14 = decltype([]<int V = 0>() { return Zero<T, U...>; }());
+template <typename T, typename... U>
+using T14 = decltype([]<int V = 0>(auto Param) {
+  return Value<T, U...> + V + (int)sizeof(Param);
+}("hello"));
 
 template <typename T> using T15 = T14<T, T>;
 
 static_assert(__is_same(T15<char>, int));
 
+// FIXME: This still crashes because we can't extract template arguments T and U
+// outside of the instantiation context of T16.
+#if 0
+template <typename T, typename... U>
+using T16 = decltype([](auto Param) requires (sizeof(Param) != 1 && sizeof...(U) > 0) {
+  return Value<T, U...> + sizeof(Param);
+});
+static_assert(T16<int, char, float>()(42) == 2 + sizeof(42));
+#endif
 } // namespace GH82104
 
+namespace GH89853 {
+
+template <typename = void>
+static constexpr auto innocuous = []<int m> { return m; };
+
+template <auto Pred = innocuous<>>
+using broken = decltype(Pred.template operator()<42>());
+
+broken<> *boom;
+
+template <auto Pred =
+              []<char c> {
+                (void)static_cast<char>(c);
+              }>
+using broken2 = decltype(Pred.template operator()<42>());
+
+broken2<> *boom2;
+
+template <auto Pred = []<char m> { return m; }>
+using broken3 = decltype(Pred.template operator()<42>());
+
+broken3<> *boom3;
+
+static constexpr auto non_default = []<char c>(True auto) {
+    (void) static_cast<char>(c);
+};
+
+template<True auto Pred>
+using broken4 = decltype(Pred.template operator()<42>(Pred));
+
+broken4<non_default>* boom4;
+
+} // namespace GH89853
+
+namespace GH105885 {
+
+template<int>
+using test = decltype([](auto...) {
+}());
+
+static_assert(__is_same(test<0>, void));
+
+} // namespace GH105885
+
+namespace GH102760 {
+
+auto make_tuple = []< class Tag, class... Captures>(Tag, Captures...) {
+  return []< class _Fun >( _Fun) -> void requires requires { 0; }
+  {};
+};
+
+template < class, class... _As >
+using Result = decltype(make_tuple(0)(_As{}...));
+
+using T = Result<int, int>;
+
+} // namespace GH102760
+
 } // namespace lambda_calls