[Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context #80802

jcsxky · 2024-02-06T06:16:29Z

Try to fix issue
When transform a lambda expression which is declared in an unevaluated context, isInstantiationDependentType() and isVariablyModifiedType() both return false and lead to skip transforming the lambda expression. On the other hand, AlreadyTransformed also skip transformation in this case. Add the condition to check whether it's in decltype makes it work.

llvmbot · 2024-02-06T06:17:02Z

@llvm/pr-subscribers-clang

Author: Qizhi Hu (jcsxky)

Changes

Try to fix issue
When transform a lambda expression which is declared in an unevaluated context, isInstantiationDependentType() and isVariablyModifiedType() both return false and lead to skip transforming the lambda expression. On the other hand, AlreadyTransformed also skip transform in this case. Add the condition to check whether it's in decltype makes it work.

Full diff: https://github.com/llvm/llvm-project/pull/80802.diff

3 Files Affected:

(modified) clang/docs/ReleaseNotes.rst (+2)
(modified) clang/lib/Sema/SemaTemplateInstantiate.cpp (+4-3)
(added) clang/test/SemaTemplate/PR76674.cpp (+11)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 4d57ea4fd55b8..7ad575e4f57fa 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -204,6 +204,8 @@ Bug Fixes to C++ Support
   parameter where we did an incorrect specialization of the initialization of
   the default parameter.
   Fixes (`#68490 <https://github.com/llvm/llvm-project/issues/68490>`_)
+- Fix a crash in codegen when lambdas declared in an unevaluated context.
+  Fixes (`#76674 <https://github.com/llvm/llvm-project/issues/76674>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 6d59180bc446d..383163ea969a9 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1613,8 +1613,8 @@ namespace {
 bool TemplateInstantiator::AlreadyTransformed(QualType T) {
   if (T.isNull())
     return true;
-
-  if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
+  if (T->isInstantiationDependentType() || T->isVariablyModifiedType() ||
+      (SemaRef.getLangOpts().CPlusPlus20 && T->isDecltypeType()))
     return false;
 
   getSema().MarkDeclarationsReferencedInType(Loc, T);
@@ -2685,7 +2685,8 @@ QualType Sema::SubstType(QualType T,
 
   // If T is not a dependent type or a variably-modified type, there
   // is nothing to do.
-  if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType())
+  if (!T->isInstantiationDependentType() && !T->isVariablyModifiedType() &&
+      (getLangOpts().CPlusPlus20 && !T->isDecltypeType()))
     return T;
 
   TemplateInstantiator Instantiator(*this, TemplateArgs, Loc, Entity);
diff --git a/clang/test/SemaTemplate/PR76674.cpp b/clang/test/SemaTemplate/PR76674.cpp
new file mode 100644
index 0000000000000..50e9053e41e0f
--- /dev/null
+++ b/clang/test/SemaTemplate/PR76674.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -c -std=c++20 -verify=cxx20 -o /dev/null %s
+// expected-no-diagnostics
+
+template <class>
+struct A {
+    template <class U>
+    using Func = decltype([] {return U{};});
+};
+
+A<int>::Func<int> f{};
+int i{f()};

clang/test/CodeGen/PR76674.cpp

erichkeane · 2024-02-06T14:49:44Z

clang/lib/Sema/SemaTemplateInstantiate.cpp

-
-  if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
+  if (T->isInstantiationDependentType() || T->isVariablyModifiedType() ||
+      (SemaRef.getLangOpts().CPlusPlus20 && T->isDecltypeType()))


Can you please add the relevant standards-quote here, particularly for the C++20 vs pre-C++20 change here?

Note that this would at best retransform all the decltype which does sound wrong?

I'd rather try to understand why isInstantiationDependentType gives a bogus result, although that might be involved.

@cor3ntin I use this test case locally:

A<double>::Func<long long> f{};

AST looks like:

Breakpoint 42, clang::DecltypeType::DecltypeType (this=0x39d190, E=0x39cfd0, underlyingType=..., can=...) at /home/huqizhi/Downloads/llvm-project/clang/lib/AST/Type.cpp:3797 3797 E(E), UnderlyingType(underlyingType) {} (gdb-ChatDBG) p E->dumpColor() CXXBindTemporaryExpr 0x39cfd0 'class A<double>::(lambda at /home/huqizhi/K.cpp:18:27)' (CXXTemporary 0x39cfd0) `-LambdaExpr 0x39cfa0 'class A<double>::(lambda at /home/huqizhi/K.cpp:18:27)' |-CXXRecordDecl 0x39ca60 implicit class definition | |-DefinitionData lambda pass_in_registers empty standard_layout trivially_copyable trivial literal has_constexpr_non_copy_move_ctor can_const_default_init | | |-DefaultConstructor exists trivial constexpr needs_implicit defaulted_is_constexpr | | |-CopyConstructor simple trivial has_const_param needs_implicit implicit_has_const_param | | |-MoveConstructor exists simple trivial needs_implicit | | |-CopyAssignment simple trivial has_const_param needs_implicit implicit_has_const_param | | |-MoveAssignment exists simple trivial needs_implicit | | `-Destructor simple irrelevant trivial constexpr | |-CXXMethodDecl 0x39cbc8 constexpr operator() 'auto (void) const -> auto' implicit_instantiation inline instantiated_from 0x37cc18 | | `-CompoundStmt 0x39cd58 | | `-ReturnStmt 0x39cd48 | | `-CXXUnresolvedConstructExpr 0x39cd08 'U' 'U' list | | `-InitListExpr 0x39ccc8 'void' | |-CXXConversionDecl 0x39cdf0 implicit constexpr operator auto (*)() 'auto (*(void) const noexcept)(void) -> auto' inline | |-CXXMethodDecl 0x39ceb0 implicit __invoke 'auto (void) -> auto' static inline | `-CXXDestructorDecl 0x39cff8 implicit referenced constexpr ~(lambda at /home/huqizhi/K.cpp:18:27) 'void (void) noexcept' inline default trivial `-CompoundStmt 0x39cd58 `-ReturnStmt 0x39cd48 `-CXXUnresolvedConstructExpr 0x39cd08 'U' 'U' list `-InitListExpr 0x39ccc8 'void'

CXXBindTemporaryExpr and LambdaExpr both are not InstantiationDependentType

(gdb-ChatDBG) p E->isInstantiationDependent() $306 = false (gdb-ChatDBG) p ((CXXBindTemporaryExpr*)E)->getSubExpr()->isInstantiationDependent() $308 = false

This is because type of the lambda is not InstantiationDependentType:

(gdb-ChatDBG) p ((LambdaExpr*)(((CXXBindTemporaryExpr*)E)->getSubExpr()))->getType().dump() RecordType 0x39cba0 'class A<double>::(lambda at /home/huqizhi/K.cpp:18:27)' `-CXXRecord 0x39ca60 '' $312 = void

I think retransforming all the decltype should be right because retransformation should not change the result. But, this may involve some efficiency problem.
Since CXXUnresolvedConstructExpr in the lambda expression is InstantiationDependentType, Maybe visiting the lambda expression and checking whether it has InstantiationDependentType child is a better approach (I think, but more complex).

Can you please add the relevant standards-quote here, particularly for the C++20 vs pre-C++20 change here?

Sorry, I couldn't get what you mean of relevant standards-quote and I am not very familiar with cpp standards. I add SemaRef.getLangOpts().CPlusPlus20 just because command args contains -std=c++20 and clang diagnose says 'lambda expression in an unevaluated operand' and don't retransform decltype all the time(issue only exists in code of cpp20 and later).

jcsxky · 2024-02-12T14:51:55Z

@cor3ntin @erichkeane Could you please take another look at this PR? I think current version would be a reasonable fix since it transforms type only when it's needed. Or you have any other opinions? Thank you for guidance!

erichkeane

A few suggestions, but I REALLY would like @cor3ntin to take a final pass, it mostly LGTM.

erichkeane · 2024-02-12T15:27:01Z

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1614,7 +1614,19 @@ bool TemplateInstantiator::AlreadyTransformed(QualType T) {
  if (T.isNull())
    return true;

-  if (T->isInstantiationDependentType() || T->isVariablyModifiedType())
+  bool DependentLambdaType = false;
+  QualType DesugaredType = T.getDesugaredType(SemaRef.getASTContext());


Is this step necessary? getAsCXXRecordDecl should at least canonicalize.

Indeed, code has been fixed.

erichkeane · 2024-02-12T15:27:33Z

clang/lib/Sema/SemaTemplateInstantiate.cpp

+  bool DependentLambdaType = false;
+  QualType DesugaredType = T.getDesugaredType(SemaRef.getASTContext());
+  CXXRecordDecl *RD = DesugaredType->getAsCXXRecordDecl();
+  if (RD && RD->isLambda()) {


if (auto *RD = T->getAsCXXRecordDecl(); RD && RD->isLambda())

??

…clared in an unevaluated context

zyn0217 · 2024-02-20T08:48:38Z

@jcsxky Do you have more test cases other than the one from #76674? I wonder what'll happen if constraint checking is involved.

jcsxky · 2024-02-21T05:21:56Z

@jcsxky Do you have more test cases other than the one from #76674? I wonder what'll happen if constraint checking is involved.

Do you mean concept? If yes, I will have a try.

cor3ntin · 2024-05-03T08:18:17Z

This was fixed by #82310

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Feb 6, 2024

jcsxky force-pushed the fix_lack_transform_crash_of_unevaluated_lambda branch 3 times, most recently from d767344 to 741793c Compare February 6, 2024 06:46

jcsxky requested review from cor3ntin, AaronBallman and erichkeane February 6, 2024 07:10

Fznamznon reviewed Feb 6, 2024

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clang/test/CodeGen/PR76674.cpp Outdated Show resolved Hide resolved

erichkeane reviewed Feb 6, 2024

View reviewed changes

jcsxky force-pushed the fix_lack_transform_crash_of_unevaluated_lambda branch 4 times, most recently from 1706840 to 376d5b0 Compare February 8, 2024 11:31

jcsxky requested review from erichkeane and Fznamznon February 8, 2024 12:46

jcsxky force-pushed the fix_lack_transform_crash_of_unevaluated_lambda branch 4 times, most recently from 59fb684 to 0063efb Compare February 11, 2024 07:50

erichkeane reviewed Feb 12, 2024

View reviewed changes

jcsxky force-pushed the fix_lack_transform_crash_of_unevaluated_lambda branch 2 times, most recently from 2f09be5 to 5430d07 Compare February 13, 2024 10:26

erichkeane approved these changes Feb 13, 2024

View reviewed changes

huqizhi added 2 commits February 17, 2024 09:34

[Clang][Sema] fix crash in codegen stage when an lambda expression de…

c19a66e

…clared in an unevaluated context

resolve conflict of ReleaseNote

842f848

jcsxky force-pushed the fix_lack_transform_crash_of_unevaluated_lambda branch from 5430d07 to 842f848 Compare February 17, 2024 01:37

cor3ntin closed this May 3, 2024

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[Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context #80802

[Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context #80802

jcsxky commented Feb 6, 2024 •

edited

llvmbot commented Feb 6, 2024

erichkeane Feb 6, 2024

cor3ntin Feb 6, 2024

jcsxky Feb 7, 2024 •

edited

jcsxky Feb 7, 2024

jcsxky commented Feb 12, 2024

erichkeane left a comment

erichkeane Feb 12, 2024

jcsxky Feb 13, 2024

erichkeane Feb 12, 2024

jcsxky Feb 13, 2024

zyn0217 commented Feb 20, 2024

jcsxky commented Feb 21, 2024

cor3ntin commented May 3, 2024

[Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context #80802

[Clang][Sema] fix crash in codegen stage when an lambda expression declared in an unevaluated context #80802

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jcsxky commented Feb 6, 2024 • edited

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