[clang] name mangling for lambdas in local classes broken since clang-13

[daantimmer]

Minimal Reproducible Example:

#include <iostream>
#include <functional>

int main(){
  class Test{
    public:
    std::function<void()> a{[]{ std::cout << "a\n"; }};
    std::function<void()> b{[]{ std::cout << "b\n"; }};
    std::function<void()> c{[]{ std::cout << "c\n"; }};
  };
  
  Test test{};
  test.a();
  test.b();
  test.c();
}

// output:
c
c
c

// expected:
a
b
c

godbolt setup with clang-trunk, clang-12 and gcc-trunk: https://godbolt.org/z/EM36r4Ybo

So, if you have a local class with lambda's the name mangling seems to be missing/going wrong. Resulting in only one actual lambda being declared.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Daan Timmer (daantimmer)

Minimal Reproducible Example:

#include <iostream>
#include <functional>

int main(){
  class Test{
    public:
    std::function<void()> a{[]{ std::cout << "a\n"; }};
    std::function<void()> b{[]{ std::cout << "b\n"; }};
    std::function<void()> c{[]{ std::cout << "c\n"; }};
  };
  
  Test test{};
  test.a();
  test.b();
  test.c();
}

// output:
c
c
c

// expected:
a
b
c

godbolt setup with clang-trunk, clang-12 and gcc-trunk: https://godbolt.org/z/EM36r4Ybo

So, if you have a local class with lambda's the name mangling seems to be missing/going wrong. Resulting in only one actual lambda being declared.

[cor3ntin]

Here

llvm-project/clang/lib/AST/ItaniumMangle.cpp

Lines 2086 to 2102 in b632476

	void CXXNameMangler::mangleLambda(const CXXRecordDecl *Lambda) {
	// When trying to be ABI-compatibility with clang 12 and before, mangle a
	// <data-member-prefix> now, with no substitutions.
	if (Decl *Context = Lambda->getLambdaContextDecl()) {
	if (isCompatibleWith(LangOptions::ClangABI::Ver12) &&
	(isa<VarDecl>(Context) || isa<FieldDecl>(Context)) &&
	!isa<ParmVarDecl>(Context)) {
	if (const IdentifierInfo *Name
	= cast<NamedDecl>(Context)->getIdentifier()) {
	mangleSourceName(Name);
	const TemplateArgumentList *TemplateArgs = nullptr;
	if (GlobalDecl TD = isTemplate(cast<NamedDecl>(Context), TemplateArgs))
	mangleTemplateArgs(asTemplateName(TD), *TemplateArgs);
	Out << 'M';
	}
	}
	}

We seem to do the right thing for the 12ABI, but not for newer ABIs, which is odd.

Itanium specification: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#closure-types

If the context of a closure type is an initializer for a class member (static or nonstatic), inline variable, or variable template, it is encoded in a qualified name with a of the form:

    <closure-prefix> ::= [ <prefix> ] <variable or member unqualified-name> M
                     ::= <variable template template-prefix> <template-args> M

[cor3ntin]

This might have been changed by 5bb7e81
Any idea @zygoloid ?

[zygoloid]

We're supposed to call mangleClosurePrefix at a higher level in mangling to mangle the prefix, I think. We should figure out why that's not happening in this case.