[Clang][Parser] Don't always destroy template annotations at the end of a declaration (#89494)

Since
[6163aa9](6163aa9#diff-3a7ef0bff7d2b73b4100de636f09ea68b72eda191b39c8091a6a1765d917c1a2),
we have introduced an optimization that almost always destroys
TemplateIdAnnotations at the end of a function declaration. This doesn't
always work properly: a lambda within a default template argument could
also result in such deallocation and hence a use-after-free bug while
building a type constraint on the template parameter.

This patch adds another flag to the parser to tell apart cases when we
shouldn't do such cleanups eagerly. A bit complicated as it is, this retains
the optimization on a highly templated function with lots of generic lambdas.

Note the test doesn't always trigger a conspicuous bug/crash even with a
debug build. But a sanitizer build can detect them, I believe.

Fixes #67235
Fixes #89127

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -560,6 +560,7 @@ Bug Fixes to C++ Support
 - Fix the Itanium mangling of lambdas defined in a member of a local class (#GH88906)
 - Fixed a crash when trying to evaluate a user-defined ``static_assert`` message whose ``size()``
   function returns a large or negative value. Fixes (#GH89407).
+- Fixed a use-after-free bug in parsing of type constraints with default arguments that involve lambdas. (#GH67235)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Parse/Parser.h

@@ -313,7 +313,15 @@ class Parser : public CodeCompletionHandler {
   /// top-level declaration is finished.
   SmallVector<TemplateIdAnnotation *, 16> TemplateIds;
 
+  /// Don't destroy template annotations in MaybeDestroyTemplateIds even if
+  /// we're at the end of a declaration. Instead, we defer the destruction until
+  /// after a top-level declaration.
+  /// Use DelayTemplateIdDestructionRAII rather than setting it directly.
+  bool DelayTemplateIdDestruction = false;
+
   void MaybeDestroyTemplateIds() {
+    if (DelayTemplateIdDestruction)
+      return;
     if (!TemplateIds.empty() &&
         (Tok.is(tok::eof) || !PP.mightHavePendingAnnotationTokens()))
       DestroyTemplateIds();
@@ -329,6 +337,22 @@ class Parser : public CodeCompletionHandler {
     ~DestroyTemplateIdAnnotationsRAIIObj() { Self.MaybeDestroyTemplateIds(); }
   };
 
+  struct DelayTemplateIdDestructionRAII {
+    Parser &Self;
+    bool PrevDelayTemplateIdDestruction;
+
+    DelayTemplateIdDestructionRAII(Parser &Self,
+                                   bool DelayTemplateIdDestruction) noexcept
+        : Self(Self),
+          PrevDelayTemplateIdDestruction(Self.DelayTemplateIdDestruction) {
+      Self.DelayTemplateIdDestruction = DelayTemplateIdDestruction;
+    }
+
+    ~DelayTemplateIdDestructionRAII() noexcept {
+      Self.DelayTemplateIdDestruction = PrevDelayTemplateIdDestruction;
+    }
+  };
+
   /// Identifiers which have been declared within a tentative parse.
   SmallVector<const IdentifierInfo *, 8> TentativelyDeclaredIdentifiers;
 

clang/lib/Parse/ParseTemplate.cpp

@@ -733,7 +733,12 @@ NamedDecl *Parser::ParseTypeParameter(unsigned Depth, unsigned Position) {
   // we introduce the type parameter into the local scope.
   SourceLocation EqualLoc;
   ParsedType DefaultArg;
+  std::optional<DelayTemplateIdDestructionRAII> DontDestructTemplateIds;
   if (TryConsumeToken(tok::equal, EqualLoc)) {
+    // The default argument might contain a lambda declaration; avoid destroying
+    // parsed template ids at the end of that declaration because they can be
+    // used in a type constraint later.
+    DontDestructTemplateIds.emplace(*this, /*DelayTemplateIdDestruction=*/true);
     // The default argument may declare template parameters, notably
     // if it contains a generic lambda, so we need to increase
     // the template depth as these parameters would not be instantiated

clang/test/Parser/cxx2a-constrained-template-param.cpp

@@ -49,4 +49,22 @@ namespace temp
   template<C1 TT, C1 UU = test1> // expected-error{{use of class template 'test1' requires template arguments}}
   // expected-error@-1 2{{concept named in type constraint is not a type concept}}
   using A = TT<int>; // expected-error{{expected ';' after alias declaration}}
-}
+}
+
+namespace PR67235 {
+
+template <class T>
+concept C = true;
+
+template <auto D>
+struct S {};
+
+// Don't destroy annotation 'C' at the end of the lambda; else we'll run into a
+// use-after-free bug while constructing the type constraint 'C' on 'Default'.
+template <typename Ret, C Default = decltype([] { return Ret(); })>
+void func() {}
+
+template <typename Ret, C Default = S<[] { return Ret(); }>>
+void func2() {}
+
+}