[AST] Fix a crash on calling getASTRecordLayout on an invalid RecordDecl.

[hokein]

Summary

The getASTRecordLayout method requires a valid decl.

[hokein]

clang/lib/AST/ASTContext.cpp

2463

another proper fix might be change the getPreferredTypeAlign signature to support error handling (e.g. return a bool), but that would require a large invasive change for its callers, not sure it is worth.

I think the current implementation is OK, as the result doesn't really matter for an invalid RecordDecl, and we do similar thing int getTypeInfoImpl (returning a fake Align/Width for invalid decl).

[sammccall]

clang/lib/AST/ASTContext.cpp

2463

I think this is fine though it could be clearer/more explicit about how the special case is handled.

if (RT->getDecl()->isInvalidDecl())
  return ABIAlign;

[hokein]

Richard fixed this in https://github.com/llvm/llvm-project/commit/269bc3f5df6c3b75de515a48063c6941ef8fbbe6 last night.

clang/lib/AST/ASTContext.cpp

unsigned ASTContext::getPreferredTypeAlign(const Type *T) const {
  
   if (!Target->allowsLargerPreferedTypeAlignment())
     return ABIAlign;
 
   if (const auto *RT = T->getAs<RecordType>()) {
     if (TI.AlignIsRequired)
       return ABIAlign;
 
+    if (!RT->getDecl()->isInvalidDecl()) {
       unsigned PreferredAlign = static_cast<unsigned>(
           toBits(getASTRecordLayout(RT->getDecl()).PreferredAlignment));
       assert(PreferredAlign >= ABIAlign &&
              "PreferredAlign should be at least as large as ABIAlign.");
       return PreferredAlign;
     }
+  }
 
   // Double (and, for targets supporting AIX `power` alignment, long double) and
   // long long should be naturally aligned (despite requiring less alignment) if
   // possible.
   if (const auto *CT = T->getAs<ComplexType>())
     T = CT->getElementType().getTypePtr();
   if (const auto *ET = T->getAs<EnumType>())
     T = ET->getDecl()->getIntegerType().getTypePtr();

clang/test/SemaCXX/alignof.cpp

struct S {
   typedef __attribute__((aligned(N))) int Field[sizeof(N)]; // expected-error {{requested alignment is dependent but declaration is not dependent}}
 };
 }
 
 typedef int __attribute__((aligned(16))) aligned_int;
 template <typename>
 using template_alias = aligned_int;
 static_assert(alignof(template_alias<void>) == 16, "Expected alignment of 16" );
+
+namespace nocrash {
+struct ForwardDecl; // expected-note {{forward declaration of}}
+struct S {
+  ForwardDecl a; // expected-error {{field has incomplete type}}
+};
+static_assert(__alignof__(S), "");
+}