Don't emit `-Wnullability-completeness` warnings on `weak` Objective-C properties.

[mwyman]

Summary

Zeroing weak references are by definition nullable, and adding nonnull or _Nonnull yields a mutual-exclusivity error. However, when -Wnullability-completeness is enabled, in non-audited header regions it's necessary to add the nullable property keyword to avoid a warning. It should not be necessary to add code clutter to restate the nullability of a weak property even under -Wnullability-completeness.

Additionally, the fix-it hints are both non-idiomatic Objective-C (adding _Nullable to the property's pointer type rather than in the @property attributes) and suggest the option of adding _Nonnull (which would be an error).

[mwyman]

Created Github issue to track: https://github.com/llvm/llvm-project/issues/56096

[mwyman]

Adding Doug Gregor who appears to have created the original warning.

[dmaclach]

LGTM

clang/lib/Sema/SemaType.cpp

if (IsTypedefName) {
 
     case DeclaratorContext::Member:
       if (state.getDeclarator().isObjCIvar() && !isFunctionOrMethod) {
         complainAboutMissingNullability = CAMN_No;
         break;
       }
 
       // Weak properties are inferred to be nullable.
-      if (state.getDeclarator().isObjCWeakProperty() && inAssumeNonNullRegion) {
+      if (state.getDeclarator().isObjCWeakProperty()) {
+        // Weak properties cannot be nonnull, and should not complain about
+        // missing nullable attributes during completeness checks.
+        complainAboutMissingNullability = CAMN_No;
+        if (inAssumeNonNullRegion) {
           inferNullability = NullabilityKind::Nullable;
+        }
         break;
       }
 
       [[fallthrough]];
 
     case DeclaratorContext::File:
     case DeclaratorContext::KNRTypeList: {
       complainAboutMissingNullability = CAMN_Yes;

clang/test/SemaObjCXX/Inputs/nullability-consistency-2.h

 // expected-note@-2 {{insert '_Nonnull' if the pointer should never be null}}
         ); 
 
 @interface SomeClass
 @property (retain,nonnull) id property1;
 @property (retain,nullable) SomeClass *property2;
 - (nullable SomeClass *)method1;
 - (void)method2:(nonnull SomeClass *)param;
-@property (readonly, weak) SomeClass *property3; // expected-warning{{missing a nullability type specifier}}
-// expected-note@-1 {{insert '_Nullable' if the pointer may be null}}
-// expected-note@-2 {{insert '_Nonnull' if the pointer should never be null}}
+@property (readonly, weak) SomeClass *property3;
 @end
 
 @interface SomeClass ()
-@property (readonly, weak) SomeClass *property4; // expected-warning{{missing a nullability type specifier}}
-// expected-note@-1 {{insert '_Nullable' if the pointer may be null}}
-// expected-note@-2 {{insert '_Nonnull' if the pointer should never be null}}
+@property (readonly, weak) SomeClass *property4;
 @end