`<type_traits>`: `__vectorcall` _should_ be rejected on ARM64EC but isn't

[CaseyCarter]

Per https://learn.microsoft.com/en-us/cpp/cpp/vectorcall?view=msvc-170:

On ARM64EC, __vectorcall is unsupported and rejected by the compiler.

but we use __vectorcall on ARM64EC:

STL/stl/inc/type_traits

Lines 398 to 402 in 9aca224

	#if ((defined(_M_IX86) && _M_IX86_FP >= 2) || defined(_M_X64)) && !defined(_M_CEE)
	#define _EMIT_VECTORCALL(FUNC, OPT1, OPT2, OPT3) FUNC(__vectorcall, OPT1, OPT2, OPT3)
	#else // ^^^ __vectorcall supported / __vectorcall not supported vvv
	#define _EMIT_VECTORCALL(FUNC, OPT1, OPT2, OPT3)
	#endif // ^^^ __vectorcall not supported ^^^

I'm not sure what effect this has. For all I know the compiler specifically doesn't reject our usage of __vectorcall just to support the STL. Removal is simple enough - see the patch below - but before simply yanking it I think we should investigate with the compiler team and ensure nothing actually useful is happening here. Thanks to llvm/llvm-project#87725 (comment) for bringing this to my attention.

diff --git a/stl/inc/type_traits b/stl/inc/type_traits
index 43e2ca6a..3adf3078 100644
--- a/stl/inc/type_traits
+++ b/stl/inc/type_traits
@@ -395,7 +395,7 @@ constexpr bool is_compound_v = !is_fundamental_v<_Ty>;
 #define _EMIT_THISCALL(FUNC, OPT1, OPT2, OPT3)
 #endif // ^^^ __stdcall and __thiscall not supported ^^^
 
-#if ((defined(_M_IX86) && _M_IX86_FP >= 2) || defined(_M_X64)) && !defined(_M_CEE)
+#if ((defined(_M_IX86) && _M_IX86_FP >= 2) || (defined(_M_X64) && !defined(_M_ARM64EC))) && !defined(_M_CEE)
 #define _EMIT_VECTORCALL(FUNC, OPT1, OPT2, OPT3) FUNC(__vectorcall, OPT1, OPT2, OPT3)
 #else // ^^^ __vectorcall supported / __vectorcall not supported vvv
 #define _EMIT_VECTORCALL(FUNC, OPT1, OPT2, OPT3)
diff --git a/tests/std/tests/Dev09_158457_tr1_mem_fn_calling_conventions/test.cpp b/tests/std/tests/Dev09_158457_tr1_mem_fn_calling_conventions/test.cpp
index 6f3576cc..0187f23f 100644
--- a/tests/std/tests/Dev09_158457_tr1_mem_fn_calling_conventions/test.cpp
+++ b/tests/std/tests/Dev09_158457_tr1_mem_fn_calling_conventions/test.cpp
@@ -64,7 +64,7 @@ struct Cat {
     }
 #endif
 
-#if (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE)
+#if (defined(_M_IX86) || (defined(_M_X64) && !defined(_M_ARM64EC))) && !defined(_M_CEE)
     int __vectorcall g() {
         return 2;
     }
@@ -115,7 +115,7 @@ int y(int i) {
 }
 #endif
 
-#if (defined(_M_IX86) || defined(_M_X64)) && !defined(_M_CEE)
+#if (defined(_M_IX86) || (defined(_M_X64) && !defined(_M_ARM64EC))) && !defined(_M_CEE)
 int __vectorcall z(int i) {
     return -4 * i;
 }
diff --git a/tests/std/tests/P0898R3_concepts/test.cpp b/tests/std/tests/P0898R3_concepts/test.cpp
index 25d325f8..6cbfd189 100644
--- a/tests/std/tests/P0898R3_concepts/test.cpp
+++ b/tests/std/tests/P0898R3_concepts/test.cpp
@@ -2879,12 +2879,12 @@ namespace test_invocable_concepts {
 #include "invocable_cc.hpp"
 
 #ifndef _M_CEE // avoid warning C4575: '__vectorcall' incompatible with the '/clr' option: converting to '__stdcall'
-#if !defined(_M_ARM) && !defined(_M_ARM64)
+#if !defined(_M_ARM) && !defined(_M_ARM64) && !defined(_M_ARM64EC)
 #define NAME      test_vector_vector
 #define CALLCONV  __vectorcall
 #define MCALLCONV __vectorcall
 #include "invocable_cc.hpp"
-#endif // ^^^ !ARM && !ARM64 ^^^
+#endif // ^^^ !ARM && !ARM64 && !ARM64EC ^^^
 #endif // _M_CEE
 
 } // namespace test_invocable_concepts

[StephanTLavavej]

We got confirmation from the compiler devs: __vectorcall is currently not supported for ARM64EC, and our usage should be guarded. Support for it is expected at some point in the future but it's low priority and there's no ETA.