[libc++] Make bad_function_call::what() existence a matter of availability instead of ABI

[philnik777]

We're currently adding bad_function_call::what() behind an ABI flag, even though adding it is not an ABI break and can be handled through availability.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

We're currently adding bad_function_call::what() behind an ABI flag, even though adding it is not an ABI break and can be handled through availability.

---

Full diff: https://github.com/llvm/llvm-project/pull/127697.diff

3 Files Affected:

• (modified) libcxx/include/__configuration/availability.h (+10)
• (modified) libcxx/include/__functional/function.h (+1-1)
• (modified) libcxx/src/functional.cpp (-2)

diff --git a/libcxx/include/__configuration/availability.h b/libcxx/include/__configuration/availability.h
index 261cf9c1ae9d8..bdb72d86c2603 100644
--- a/libcxx/include/__configuration/availability.h
+++ b/libcxx/include/__configuration/availability.h
@@ -78,6 +78,9 @@
 // in all versions of the library are available.
 #if !_LIBCPP_HAS_VENDOR_AVAILABILITY_ANNOTATIONS
 
+#  define _LIBCPP_INTRODUCED_IN_LLVM_21 1
+#  define _LIBCPP_INTRODUCED_IN_LLVM_21_ATTRIBUTE /* nothing */
+
 #  define _LIBCPP_INTRODUCED_IN_LLVM_20 1
 #  define _LIBCPP_INTRODUCED_IN_LLVM_20_ATTRIBUTE /* nothing */
 
@@ -114,6 +117,11 @@
 
 // clang-format off
 
+// LLVM 21
+// TODO: Fill this in
+#  define _LIBCPP_INTRODUCED_IN_LLVM_21 0
+#  define _LIBCPP_INTRODUCED_IN_LLVM_21_ATTRIBUTE __attribute__((unavailable))
+
 // LLVM 20
 // TODO: Fill this in
 #  define _LIBCPP_INTRODUCED_IN_LLVM_20 0
@@ -359,6 +367,8 @@
 #define _LIBCPP_AVAILABILITY_HAS_FROM_CHARS_FLOATING_POINT _LIBCPP_INTRODUCED_IN_LLVM_20
 #define _LIBCPP_AVAILABILITY_FROM_CHARS_FLOATING_POINT _LIBCPP_INTRODUCED_IN_LLVM_20_ATTRIBUTE
 
+#define _LIBCPP_AVAILABILITY_HAS_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE _LIBCPP_INTRODUCED_IN_LLVM_21
+
 // Define availability attributes that depend on _LIBCPP_HAS_EXCEPTIONS.
 // Those are defined in terms of the availability attributes above, and
 // should not be vendor-specific.
diff --git a/libcxx/include/__functional/function.h b/libcxx/include/__functional/function.h
index 2a1293cfcc26b..cd77012173512 100644
--- a/libcxx/include/__functional/function.h
+++ b/libcxx/include/__functional/function.h
@@ -71,7 +71,7 @@ class _LIBCPP_EXPORTED_FROM_ABI bad_function_call : public exception {
   _LIBCPP_HIDE_FROM_ABI_VIRTUAL ~bad_function_call() _NOEXCEPT override {}
 #  endif
 
-#  ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
+#  if _LIBCPP_AVAILABILITY_HAS_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
   const char* what() const _NOEXCEPT override;
 #  endif
 };
diff --git a/libcxx/src/functional.cpp b/libcxx/src/functional.cpp
index ef53e3e84da0e..f2c70fc54392d 100644
--- a/libcxx/src/functional.cpp
+++ b/libcxx/src/functional.cpp
@@ -12,8 +12,6 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 bad_function_call::~bad_function_call() noexcept {}
 
-#ifdef _LIBCPP_ABI_BAD_FUNCTION_CALL_GOOD_WHAT_MESSAGE
 const char* bad_function_call::what() const noexcept { return "std::bad_function_call"; }
-#endif
 
 _LIBCPP_END_NAMESPACE_STD

[ldionne]

This LGTM. However, since I do expect we might run into some unforeseen issues with this change, let me take it for a spin internally just to see if anything breaks. But assuming that's not the case, let's ship this. I'll let you know early next week.

Edit for future reference: 148470934&148470973

[ldionne]

As we discussed in Hagenberg with Oliver Hunt, this information is actually untrue and I think we (I?) put it there by mistaking an issue I had seen with another kind of issue, likely related to ABI tags.

[ldionne]

I didn't see any issues with this, I think this is good to merge once my comments have been addressed.