Warn if _Optional used at top-level of decl

[chrisbazley]

Summary

Parameter declarations using [] syntax can be
written more naturally using an _Optional
qualifier than using Clang's _Nullable
annotation, e.g.

void myfunc(_Optional const char s[]);
// s may be a null pointer

With the above exception, it isn't useful to
declare a non-pointed-to object as _Optional
(although so-qualified types exist during
expression evaluation). Such declarations are
therefore disallowed, like equivalent abuse of
the restrict qualifier, to avoid confusion.

[chrisbazley]

Please refer to https://discourse.llvm.org/t/rfc-optional-a-type-qualifier-to-indicate-pointer-nullability/ and https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3089.pdf for the wider context of this commit.

clang/include/clang/Basic/DiagnosticSemaKinds.td

def warn_typecheck_reference_qualifiers : Warning<
   "'%0' qualifier on reference type %1 has no effect">,
   InGroup<IgnoredReferenceQualifiers>;
 def err_typecheck_invalid_restrict_not_pointer : Error<
   "restrict requires a pointer or reference (%0 is invalid)">;
 def err_typecheck_invalid_restrict_not_pointer_noarg : Error<
   "restrict requires a pointer or reference">;
 def err_typecheck_invalid_restrict_invalid_pointee : Error<
   "pointer to function type %0 may not be 'restrict' qualified">;
+def err_typecheck_invalid_optional_not_pointee : Error<
+  "_Optional is only valid on a pointed-to object or incomplete type (%0 is invalid)">;
+def err_typecheck_invalid_optional_not_pointee_noarg : Error<
+  "_Optional is only valid on a pointed-to object or incomplete type">;
 def ext_typecheck_zero_array_size : Extension<
   "zero size arrays are an extension">, InGroup<ZeroLengthArray>;
 def err_typecheck_zero_array_size : Error<
   "zero-length arrays are not permitted in %select{C++|SYCL device code}0">;
 def err_array_size_non_int : Error<"size of array has non-integer type %0">;
 def err_init_element_not_constant : Error<
   "initializer element is not a compile-time constant">;
 def ext_aggregate_init_not_constant : Extension<

clang/lib/Sema/SemaDecl.cpp

Decl *Sema::ParsedFreeStandingDeclSpec(Scope *S, AccessSpecifier AS,
 
   if (unsigned TypeQuals = DS.getTypeQualifiers()) {
     // Enforce C99 6.7.3p2: "Types other than pointer types derived from object
     // or incomplete types shall not be restrict-qualified."
     if (TypeQuals & DeclSpec::TQ_restrict)
       Diag(DS.getRestrictSpecLoc(),
            diag::err_typecheck_invalid_restrict_not_pointer_noarg)
            << DS.getSourceRange();
+
+    // Types other than those of a pointed-to object or pointed-to incomplete
+    // type shall not be _Optional-qualified in a declaration.
+    if (TypeQuals & DeclSpec::TQ_optional)
+      Diag(DS.getOptionalSpecLoc(),
+           diag::err_typecheck_invalid_optional_not_pointee_noarg)
+          << DS.getSourceRange();
   }
 
   if (DS.isInlineSpecified())
     Diag(DS.getInlineSpecLoc(), diag::err_inline_non_function)
         << getLangOpts().CPlusPlus17;
 
   if (DS.hasConstexprSpecifier()) {
     // C++0x [dcl.constexpr]p1: constexpr can only be applied to declarations

clang/lib/Sema/SemaType.cpp

if (IsQualifiedFunction &&
       for (unsigned i = 0, e = D.getNumTypeObjects(); i != e; ++i) {
         if (D.getTypeObject(i).Kind != DeclaratorChunk::Paren)
           break;
         T = S.BuildParenType(T);
       }
     }
   }
 
+  // Types other than those of a pointed-to object or pointed-to incomplete type
+  // shall not be _Optional-qualified in a declaration.
+  if (T.isOptionalQualified() && !IsTypedefName &&
+      (!T->isArrayType() || !D.isPrototypeContext())) {
+    S.Diag(D.getDeclSpec().getOptionalSpecLoc(),
+           diag::err_typecheck_invalid_optional_not_pointee)
+        << T;
+    Context.getNonOptionalType(T);
+  }
+
   // Apply any undistributed attributes from the declaration or declarator.
   ParsedAttributesView NonSlidingAttrs;
   for (ParsedAttr &AL : D.getDeclarationAttributes()) {
     if (!AL.slidesFromDeclToDeclSpecLegacyBehavior()) {
       NonSlidingAttrs.addAtEnd(&AL);
     }
   }
   processTypeAttrs(state, T, TAL_DeclName, NonSlidingAttrs);