[Clang] Fix ICE where C++ Template Instantiation failed to handle attributed lambdas (#76523)

This PR is proposing a fix for
#76521.

Clang used to assume that during template instantiation, Lambda
expressions can only have `FunctionProtoTypeLoc`s. However, this is not
true for certain attributes like `__attribute__((pcs("aapcs-vfp")))`,
whose interpretation happens after template instantiation. This PR
changes the transformation logic for lambdas.

Author: yuxuanchen1997

clang/docs/ReleaseNotes.rst

@@ -856,6 +856,9 @@ Bug Fixes to AST Handling
 - Fixed a bug where RecursiveASTVisitor fails to visit the
   initializer of a bitfield.
   `Issue 64916 <https://github.com/llvm/llvm-project/issues/64916>`_
+- Fixed a bug where Template Instantiation failed to handle Lambda Expressions
+  with certain types of Attributes.
+  (`#76521 <https://github.com/llvm/llvm-project/issues/76521>`_)
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/TreeTransform.h

@@ -674,6 +674,10 @@ class TreeTransform {
                                       Qualifiers ThisTypeQuals,
                                       Fn TransformExceptionSpec);
 
+  template <typename Fn>
+  QualType TransformAttributedType(TypeLocBuilder &TLB, AttributedTypeLoc TL,
+                                   Fn TransformModifiedType);
+
   bool TransformExceptionSpec(SourceLocation Loc,
                               FunctionProtoType::ExceptionSpecInfo &ESI,
                               SmallVectorImpl<QualType> &Exceptions,
@@ -7050,12 +7054,12 @@ TreeTransform<Derived>::TransformElaboratedType(TypeLocBuilder &TLB,
   return Result;
 }
 
-template<typename Derived>
+template <typename Derived>
+template <typename Fn>
 QualType TreeTransform<Derived>::TransformAttributedType(
-                                                TypeLocBuilder &TLB,
-                                                AttributedTypeLoc TL) {
+    TypeLocBuilder &TLB, AttributedTypeLoc TL, Fn TransformModifiedTypeFn) {
   const AttributedType *oldType = TL.getTypePtr();
-  QualType modifiedType = getDerived().TransformType(TLB, TL.getModifiedLoc());
+  QualType modifiedType = TransformModifiedTypeFn(TLB, TL.getModifiedLoc());
   if (modifiedType.isNull())
     return QualType();
 
@@ -7099,6 +7103,15 @@ QualType TreeTransform<Derived>::TransformAttributedType(
   return result;
 }
 
+template <typename Derived>
+QualType TreeTransform<Derived>::TransformAttributedType(TypeLocBuilder &TLB,
+                                                         AttributedTypeLoc TL) {
+  return getDerived().TransformAttributedType(
+      TLB, TL, [&](TypeLocBuilder &TLB, TypeLoc ModifiedLoc) -> QualType {
+        return getDerived().TransformType(TLB, ModifiedLoc);
+      });
+}
+
 template <typename Derived>
 QualType TreeTransform<Derived>::TransformBTFTagAttributedType(
     TypeLocBuilder &TLB, BTFTagAttributedTypeLoc TL) {
@@ -13600,32 +13613,56 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   // transformed parameters.
   TypeSourceInfo *NewCallOpTSI = nullptr;
   {
-    TypeSourceInfo *OldCallOpTSI = E->getCallOperator()->getTypeSourceInfo();
-    auto OldCallOpFPTL =
-        OldCallOpTSI->getTypeLoc().getAs<FunctionProtoTypeLoc>();
+    auto OldCallOpTypeLoc =
+        E->getCallOperator()->getTypeSourceInfo()->getTypeLoc();
+
+    auto TransformFunctionProtoTypeLoc =
+        [this](TypeLocBuilder &TLB, FunctionProtoTypeLoc FPTL) -> QualType {
+      SmallVector<QualType, 4> ExceptionStorage;
+      TreeTransform *This = this; // Work around gcc.gnu.org/PR56135.
+      return this->TransformFunctionProtoType(
+          TLB, FPTL, nullptr, Qualifiers(),
+          [&](FunctionProtoType::ExceptionSpecInfo &ESI, bool &Changed) {
+            return This->TransformExceptionSpec(FPTL.getBeginLoc(), ESI,
+                                                ExceptionStorage, Changed);
+          });
+    };
 
+    QualType NewCallOpType;
     TypeLocBuilder NewCallOpTLBuilder;
-    SmallVector<QualType, 4> ExceptionStorage;
-    TreeTransform *This = this; // Work around gcc.gnu.org/PR56135.
-    QualType NewCallOpType = TransformFunctionProtoType(
-        NewCallOpTLBuilder, OldCallOpFPTL, nullptr, Qualifiers(),
-        [&](FunctionProtoType::ExceptionSpecInfo &ESI, bool &Changed) {
-          return This->TransformExceptionSpec(OldCallOpFPTL.getBeginLoc(), ESI,
-                                              ExceptionStorage, Changed);
-        });
+
+    if (auto ATL = OldCallOpTypeLoc.getAs<AttributedTypeLoc>()) {
+      NewCallOpType = this->TransformAttributedType(
+          NewCallOpTLBuilder, ATL,
+          [&](TypeLocBuilder &TLB, TypeLoc TL) -> QualType {
+            return TransformFunctionProtoTypeLoc(
+                TLB, TL.castAs<FunctionProtoTypeLoc>());
+          });
+    } else {
+      auto FPTL = OldCallOpTypeLoc.castAs<FunctionProtoTypeLoc>();
+      NewCallOpType = TransformFunctionProtoTypeLoc(NewCallOpTLBuilder, FPTL);
+    }
+
     if (NewCallOpType.isNull())
       return ExprError();
     NewCallOpTSI =
         NewCallOpTLBuilder.getTypeSourceInfo(getSema().Context, NewCallOpType);
   }
 
+  ArrayRef<ParmVarDecl *> Params;
+  if (auto ATL = NewCallOpTSI->getTypeLoc().getAs<AttributedTypeLoc>()) {
+    Params = ATL.getModifiedLoc().castAs<FunctionProtoTypeLoc>().getParams();
+  } else {
+    auto FPTL = NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>();
+    Params = FPTL.getParams();
+  }
+
   getSema().CompleteLambdaCallOperator(
       NewCallOperator, E->getCallOperator()->getLocation(),
       E->getCallOperator()->getInnerLocStart(),
       E->getCallOperator()->getTrailingRequiresClause(), NewCallOpTSI,
       E->getCallOperator()->getConstexprKind(),
-      E->getCallOperator()->getStorageClass(),
-      NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams(),
+      E->getCallOperator()->getStorageClass(), Params,
       E->hasExplicitResultType());
 
   getDerived().transformAttrs(E->getCallOperator(), NewCallOperator);

clang/test/SemaCXX/template-instantiation.cpp

@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -verify -fsyntax-only -Wno-ignored-attributes %s
+// expected-no-diagnostics
+
+namespace GH76521 {
+
+template <typename T>
+void foo() {
+  auto l = []() __attribute__((pcs("aapcs-vfp"))) {};
+}
+
+void bar() {
+  foo<int>();
+}
+
+}