[Sema] Don't permit dependent alignments on non-dependent typedef-names

[majnemer]

Summary

A dependent alignment attribute (like attribute((aligned(...))) or
__declspec(align(...))) on a non-dependent typedef or using declaration
poses a considerable challenge: the type is _not_ dependent, the size
_may_ be dependent if the type is used as an array type, the alignment
_is_ dependent.

It is reasonable for a compiler to be able to query the size and
alignment of a complete type. Let's help that become an invariant.

This fixes PR22042.

[majnemer]

ping.

[rsmith]

lib/Sema/SemaDeclAttr.cpp

2866

The 'type dependent' check here is redundant.

2869

I don't think you should check instantiation-dependence here. I think you should reject this, for instance:

template<int N> struct X {
  typedef __attribute__((aligned(N))) int X[sizeof(N)];
};

[majnemer]

• Address review comments

[rsmith]

LGTM. Can you also update the documentation to list this divergence from GCC behavior, with a brief explanation of why we think it's unreasonable to support?

include/clang/Basic/DiagnosticSemaKinds.td

def err_property_is_variably_modified : Error<
   "property %0 has a variably modified type">;
 def err_no_accessor_for_property : Error<
   "no %select{getter|setter}0 defined for property %1">;
 def error_cannot_find_suitable_accessor : Error<
   "cannot find suitable %select{getter|setter}0 for property %1">;
 
 def err_alignment_not_power_of_two : Error<
   "requested alignment is not a power of 2">;
+def err_alignment_dependent_typedef_name : Error<
+  "requested alignment is dependent but declaration is not dependent">;
 
 def err_attribute_aligned_too_great : Error<
   "requested alignment must be %0 bytes or smaller">;
 def warn_redeclaration_without_attribute_prev_attribute_ignored : Warning<
   "%q0 redeclared without %1 attribute: previous %1 ignored">,
   InGroup<DiagGroup<"inconsistent-dllimport">>;
 def warn_dllimport_dropped_from_inline_function : Warning<
   "%q0 redeclared inline; %1 attribute ignored">,

lib/Sema/SemaDeclAttr.cpp

if (Attr.isPackExpansion() && !E->containsUnexpandedParameterPack()) {
     S.Diag(Attr.getEllipsisLoc(),
            diag::err_pack_expansion_without_parameter_packs);
     return;
   }
 
   if (!Attr.isPackExpansion() && S.DiagnoseUnexpandedParameterPack(E))
     return;
 
+  if (E->isValueDependent()) {
+    if (const auto *TND = dyn_cast<TypedefNameDecl>(D)) {
+      if (!TND->getUnderlyingType()->isDependentType()) {
+        S.Diag(Attr.getLoc(), diag::err_alignment_dependent_typedef_name)
+            << E->getSourceRange();
+        return;
+      }
+    }
+  }
+
   S.AddAlignedAttr(Attr.getRange(), D, E, Attr.getAttributeSpellingListIndex(),
                    Attr.isPackExpansion());
 }
 
 void Sema::AddAlignedAttr(SourceRange AttrRange, Decl *D, Expr *E,
                           unsigned SpellingListIndex, bool IsPackExpansion) {
   AlignedAttr TmpAttr(AttrRange, Context, true, E, SpellingListIndex);
   SourceLocation AttrLoc = AttrRange.getBegin();

test/SemaCXX/alignof.cpp

 }
 
 template <typename T> void n(T) {
   alignas(T) int T1;
   char k[__alignof__(T1)];
   static_assert(sizeof(k) == alignof(long long), "");
 }
 template void n(long long);
+
+namespace PR22042 {
+template <typename T>
+void Fun(T A) {
+  typedef int __attribute__((__aligned__(A))) T1; // expected-error {{requested alignment is dependent but declaration is not dependent}}
+  int k1[__alignof__(T1)];
+}
+
+template <int N>
+struct S {
+  typedef __attribute__((aligned(N))) int Field[sizeof(N)]; // expected-error {{requested alignment is dependent but declaration is not dependent}}
+};
+}