Re-analyze functions as top-level

[chrisbazley]

Summary

If a function with a _Nullable parameter has at least
one caller, the checker behaves as though no other
parameter value is possible. This is because the static
analyzer doesn't re-analyse functions as top level
(with the exception of C++ copy and move operators, and
ObjC methods).

Fixed by modifying shouldSkipFunction to return false.

After this change, Clang-tidy even emits different
warnings for the same line of code, if appropriate:
"Nullable pointer is dereferenced" for a (simulated)
top-level call and "Dereference of null pointer"
for a call to the same function with a known null value.

[chrisbazley]

This change is admittedly a hack. It might be better to check the function parameter types, or maybe the list of exceptions in this function is now so long that it no longer serves any purpose.

[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/lib/StaticAnalyzer/Frontend/AnalysisConsumer.cpp

static bool shouldSkipFunction(const Decl *D,
   // - The 'init' methods should be reanalyzed because
   //   ObjCNonNilReturnValueChecker assumes that '[super init]' never returns
   //   'nil' and unless we analyze the 'init' functions as top level, we will
   //   not catch errors within defensive code.
   // - We want to reanalyze all ObjC methods as top level to report Retain
   //   Count naming convention errors more aggressively.
   if (isa<ObjCMethodDecl>(D))
     return false;
+
+  // If a function with a _Nullable parameter has at least one caller, the
+  // checker behaves as though no other parameter value is possible. This is
+  // because the static analyzer doesn't re-analyse functions as top level.
+  return false;
+
   // We also want to reanalyze all C++ copy and move assignment operators to
   // separately check the two cases where 'this' aliases with the parameter and
   // where it may not. (cplusplus.SelfAssignmentChecker)
   if (const auto *MD = dyn_cast<CXXMethodDecl>(D)) {
     if (MD->isCopyAssignmentOperator() || MD->isMoveAssignmentOperator())
       return false;
   }