[Sema] Don't disallow placing `__attribute__((alloc_align(param_idx)))` on `std::align_val_t`-typed parameters

[lebedev.ri]

Summary

I kind-of understand why it is restricted to integer-typed arguments,
for general enum's the value passed is not nessesairly the alignment implied,
although one might say that user would know best.

But we clearly should whitelist std::align_val_t,
which is just a thin wrapper over std::size_t,
and is the C++ standard way of specifying alignment.

[lebedev.ri]

NFC, more tests.

[erichkeane]

This makes a lot of sense to me.

[lebedev.ri]

In D73019#1836529, @erichkeane wrote:

This makes a lot of sense to me.

Thank you for the review!

[lebedev.ri]

Rebased, NFC.

clang/lib/Sema/SemaDeclAttr.cpp

void Sema::AddAllocAlignAttr(Decl *D, const AttributeCommonInfo &CI,
 
   ParamIdx Idx;
   const auto *FuncDecl = cast<FunctionDecl>(D);
   if (!checkFunctionOrMethodParameterIndex(*this, FuncDecl, TmpAttr,
                                            /*AttrArgNum=*/1, ParamExpr, Idx))
     return;
 
   QualType Ty = getFunctionOrMethodParamType(D, Idx.getASTIndex());
-  if (!Ty->isDependentType() && !Ty->isIntegralType(Context)) {
+  if (!Ty->isDependentType() && !Ty->isIntegralType(Context) &&
+      !Ty->isAlignValT()) {
     Diag(ParamExpr->getBeginLoc(), diag::err_attribute_integers_only)
         << &TmpAttr
         << FuncDecl->getParamDecl(Idx.getASTIndex())->getSourceRange();
     return;
   }
 
   D->addAttr(::new (Context) AllocAlignAttr(Context, CI, Idx));
 }

clang/test/SemaCXX/std-align-val-t-in-operator-new.cpp

+// RUN: %clang_cc1 -std=c++11                      -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++11 -faligned-allocation -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++17                      -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++17 -faligned-allocation -fsyntax-only -verify %s
+
+namespace std {
+typedef __SIZE_TYPE__ size_t;
+struct nothrow_t {};
+#if __cplusplus >= 201103L
+enum class align_val_t : size_t {};
+#else
+enum align_val_t {
+// We can't force an underlying type when targeting windows.
+#ifndef _WIN32
+  __zero = 0,
+  __max = (size_t)-1
+#endif
+};
+#endif
+} // namespace std
+
+void *operator new(std::size_t count, std::align_val_t al) __attribute__((alloc_align(2)));
+
+#define OVERALIGNED alignas(__STDCPP_DEFAULT_NEW_ALIGNMENT__ * 2)
+
+struct OVERALIGNED A {
+  A();
+  int n[128];
+};
+
+void *ptr_variable(int align) { return new (std::align_val_t(align)) A; }
+void *ptr_align16() { return new (std::align_val_t(16)) A; }
+void *ptr_align15() { return new (std::align_val_t(15)) A; }
+
+std::align_val_t align_variable(int align) { return std::align_val_t(align); }
+std::align_val_t align_align16() { return std::align_val_t(16); }
+std::align_val_t align_align15() { return std::align_val_t(15); }
+
+// expected-no-diagnostics