[Clang][Sema] Do not try to analyze dependent alignment during -Wcast-align

[serge-sans-paille]

Summary

Fix #63007

[Fznamznon]

The change looks reasonable for me, though I'm not experienced in related code. Have you checked that it fixes the non-reduced original example?

[shafik]

clang/lib/Sema/SemaChecking.cpp

16163

Can't we just use hasDependentAlignment() which basically does this.

I am curious that we don't have to do this check in every location we use getDeclAlign

[serge-sans-paille]

Take review into account

[serge-sans-paille]

In D151753#4383602, @Fznamznon wrote:

Have you checked that it fixes the non-reduced original example?

Just did and it passes.

[serge-sans-paille]

clang/lib/Sema/SemaChecking.cpp

16163

I missed that method, thanks for point it out. I think we should be doing that check in the other reference in SemaChecking.cpp, but I failed to find a reproducer...

[serge-sans-paille]

Gentle ping :-)

[erichkeane]

Needs a release note, else LGTM.

clang/lib/Sema/SemaChecking.cpp

     auto *ASE = cast<ArraySubscriptExpr>(E);
     return getAlignmentAndOffsetFromBinAddOrSub(ASE->getBase(), ASE->getIdx(),
                                                 false, Ctx);
   }
   case Stmt::DeclRefExprClass: {
     if (auto *VD = dyn_cast<VarDecl>(cast<DeclRefExpr>(E)->getDecl())) {
       // FIXME: If VD is captured by copy or is an escaping __block variable,
       // use the alignment of VD's type.
-      if (!VD->getType()->isReferenceType())
+      if (!VD->getType()->isReferenceType()) {
+        // Dependent alignment cannot be resolved -> bail out.
+        if (VD->hasDependentAlignment())
+          break;
         return std::make_pair(Ctx.getDeclAlign(VD), CharUnits::Zero());
+      }
       if (VD->hasInit())
         return getBaseAlignmentAndOffsetFromLValue(VD->getInit(), Ctx);
     }
     break;
   }
   case Stmt::MemberExprClass: {
     auto *ME = cast<MemberExpr>(E);
     auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());

clang/test/SemaCXX/warn-cast-align.cpp

void test1(void *P) {
 
   c = (int*) P;
   c = static_cast<int*>(P);
   c = reinterpret_cast<int*>(P);
   typedef int *IntPtr;
   c = IntPtr(P);
 }
 
+template <class A> void DependentAlign() {
+  alignas(A) int lut[]{};
+  (long *)lut; // expected-warning {{cast from 'int *' to 'long *'}}
+}
+
 struct __attribute__((aligned(16))) AlignedS {
   char m[16];
 };
 
 struct __attribute__((aligned(16))) A {
   char m0[16];
   char m1[16];
   AlignedS *getAlignedS() {