[Clang] Realize generic lambda call operators are dependent sooner

[cor3ntin]

Summary

When parsing a trailing return type / noexcept / constraint
of a generic lambda, we need to know that we are in a dependent
context. We currently don't because we only create a TemplateDecl
for the call operator one its fully parsed.

This patch attach a template decl to the call operator as soon
as the parameter declaration clause is parsed - the point at which
we have collected all template parameters

Fixes #64689

[erichkeane]

This is great! Probably worth the cherry pick to Clang17 as well!

[cor3ntin]

Rebase

[cor3ntin]

Fix tests

clang/docs/ReleaseNotes.rst

 - Fix crash when parsing ill-formed lambda trailing return type. Fixes:
   (`#64962 <https://github.com/llvm/llvm-project/issues/64962>`_) and
   (`#28679 <https://github.com/llvm/llvm-project/issues/28679>`_).
 
 - Fix a crash caused by substitution failure in expression requirements.
   (`#64172 <https://github.com/llvm/llvm-project/issues/64172>`_) and
   (`#64723 <https://github.com/llvm/llvm-project/issues/64723>`_).
 
+ Fix crash when parsing the requires clause of some generic lambdas.
+  (`#64689 <https://github.com/llvm/llvm-project/issues/64689>`_`)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.
   `Issue 64169 <https://github.com/llvm/llvm-project/issues/64169>`_
 - Remove unnecessary RecordLayout computation when importing UnaryOperator. The
   computed RecordLayout is incorrect if fields are not completely imported and
   should not be cached.
   `Issue 64170 <https://github.com/llvm/llvm-project/issues/64170>`_

clang/include/clang/Sema/Sema.h

   void buildLambdaScope(sema::LambdaScopeInfo *LSI, CXXMethodDecl *CallOperator,
                         SourceRange IntroducerRange,
                         LambdaCaptureDefault CaptureDefault,
                         SourceLocation CaptureDefaultLoc, bool ExplicitParams,
                         bool Mutable);
 
   CXXMethodDecl *CreateLambdaCallOperator(SourceRange IntroducerRange,
                                           CXXRecordDecl *Class);
+
+  void AddTemplateParametersToLambdaCallOperator(
+      CXXMethodDecl *CallOperator, CXXRecordDecl *Class,
+      TemplateParameterList *TemplateParams);
+
   void CompleteLambdaCallOperator(
       CXXMethodDecl *Method, SourceLocation LambdaLoc,
       SourceLocation CallOperatorLoc, Expr *TrailingRequiresClause,
       TypeSourceInfo *MethodTyInfo, ConstexprSpecKind ConstexprKind,
       StorageClass SC, ArrayRef<ParmVarDecl *> Params,
       bool HasExplicitResultType);
 
   /// Perform initialization analysis of the init-capture and perform

clang/lib/Sema/SemaLambda.cpp

CXXMethodDecl *Method = CXXMethodDecl::Create(
       QualType(), /*Tinfo=*/nullptr, SC_None,
       getCurFPFeatures().isFPConstrained(),
       /*isInline=*/true, ConstexprSpecKind::Unspecified, SourceLocation(),
       /*TrailingRequiresClause=*/nullptr);
   Method->setAccess(AS_public);
   return Method;
 }
 
+void Sema::AddTemplateParametersToLambdaCallOperator(
+    CXXMethodDecl *CallOperator, CXXRecordDecl *Class,
+    TemplateParameterList *TemplateParams) {
+  assert(TemplateParams && "no template parameters");
+  FunctionTemplateDecl *TemplateMethod = FunctionTemplateDecl::Create(
+      Context, Class, CallOperator->getLocation(), CallOperator->getDeclName(),
+      TemplateParams, CallOperator);
+  TemplateMethod->setAccess(AS_public);
+  CallOperator->setDescribedFunctionTemplate(TemplateMethod);
+}
+
 void Sema::CompleteLambdaCallOperator(
     CXXMethodDecl *Method, SourceLocation LambdaLoc,
     SourceLocation CallOperatorLoc, Expr *TrailingRequiresClause,
     TypeSourceInfo *MethodTyInfo, ConstexprSpecKind ConstexprKind,
     StorageClass SC, ArrayRef<ParmVarDecl *> Params,
     bool HasExplicitResultType) {
 
   LambdaScopeInfo *LSI = getCurrentLambdaScopeUnsafe(*this);
 
   if (TrailingRequiresClause)
     Method->setTrailingRequiresClause(TrailingRequiresClause);
 
   TemplateParameterList *TemplateParams =
       getGenericLambdaTemplateParameterList(LSI, *this);
 
   DeclContext *DC = Method->getLexicalDeclContext();
   Method->setLexicalDeclContext(LSI->Lambda);
   if (TemplateParams) {
-    FunctionTemplateDecl *TemplateMethod = FunctionTemplateDecl::Create(
-        Context, LSI->Lambda, Method->getLocation(), Method->getDeclName(),
-        TemplateParams, Method);
-    TemplateMethod->setAccess(AS_public);
-    Method->setDescribedFunctionTemplate(TemplateMethod);
+    FunctionTemplateDecl *TemplateMethod =
+        Method->getDescribedFunctionTemplate();
+    assert(TemplateMethod &&
+           "AddTemplateParametersToLambdaCallOperator should have been called");
+
     LSI->Lambda->addDecl(TemplateMethod);
     TemplateMethod->setLexicalDeclContext(DC);
   } else {
     LSI->Lambda->addDecl(Method);
   }
   LSI->Lambda->setLambdaIsGeneric(TemplateParams);
   LSI->Lambda->setLambdaTypeInfo(MethodTyInfo);
 
void Sema::ActOnLambdaClosureParameters(
 
   for (const DeclaratorChunk::ParamInfo &P : Params) {
     auto *Param = cast<ParmVarDecl>(P.Param);
     Param->setOwningFunction(LSI->CallOperator);
     if (Param->getIdentifier())
       PushOnScopeChains(Param, LambdaScope, false);
   }
 
+  // After the parameter list, we may parse a noexcept/requires/trailing return
+  // type which need to know whether the call operator constiture a dependent
+  // context, so we need to setup the FunctionTemplateDecl of generic lambdas
+  // now.
+  TemplateParameterList *TemplateParams =
+      getGenericLambdaTemplateParameterList(LSI, *this);
+  if (TemplateParams) {
+    AddTemplateParametersToLambdaCallOperator(LSI->CallOperator, LSI->Lambda,
+                                              TemplateParams);
+    LSI->Lambda->setLambdaIsGeneric(true);
+  }
   LSI->AfterParameterList = true;
 }
 
 void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
                                         Declarator &ParamInfo,
                                         const DeclSpec &DS) {
 
   LambdaScopeInfo *LSI = getCurrentLambdaScopeUnsafe(*this);

clang/lib/Sema/TreeTransform.h

TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   }
   getSema().finishLambdaExplicitCaptures(LSI);
 
   // Transform the template parameters, and add them to the current
   // instantiation scope. The null case is handled correctly.
   auto TPL = getDerived().TransformTemplateParameterList(
       E->getTemplateParameterList());
   LSI->GLTemplateParameterList = TPL;
+  if (TPL)
+    getSema().AddTemplateParametersToLambdaCallOperator(NewCallOperator, Class,
+                                                        TPL);
 
   // Transform the type of the original lambda's call operator.
   // The transformation MUST be done in the CurrentInstantiationScope since
   // it introduces a mapping of the original to the newly created
   // transformed parameters.
   TypeSourceInfo *NewCallOpTSI = nullptr;
   {
     TypeSourceInfo *OldCallOpTSI = E->getCallOperator()->getTypeSourceInfo();

clang/test/SemaCXX/cxx2a-template-lambdas.cpp

 namespace TypeParam
 {
 template <typename T = decltype([](auto x) {return x;})>
 auto B = T{};
 static_assert(B<>(42) == 42);
 }
 
 }
+
+namespace GH64689 {
+void f();
+void foo() {
+  []<typename T>(int)
+    noexcept(requires(int t) { f(); })
+    -> decltype(requires(int t) { f(); })
+    requires requires(int t) { f(); }
+  {return {};}.operator()<int>(0);
+  [](auto)
+    noexcept(requires(int t) { f(); })
+    -> decltype(requires(int t) { f(); })
+    requires requires(int t) { f(); }
+  {return {};}(1);
+}
+
+}