Skip NonNull sema checks in unevaluated contexts.

[EricWF]

Summary

Currently when a function annotated with attribute((nonnull)) is called in an unevaluated context with a null argument a -Wnonnull warning is emitted.
This warning seems like a false positive unless the call expression is potentially evaluated. Change this behavior so that the non-null warnings use DiagRuntimeBehavior so they wont emit when they won't be evaluated.

[rsmith]

This seems reasonable.

I wonder if it might be better to also suppress warnings for dynamically-unreachable code (you can do this by changing the Diag calls inside CheckNonNullArguments to DiagRuntimeBehavior instead of the current patch). That would help for a case like:

#define SAFE_FOO(x) if (x) foo(x)
SAFE_FOO(0);

... but admittedly that's pretty contrived.

Anyway, this looks fine as far as it goes.

[EricWF]

DiagRuntimeBehavior seems like a better way to do this. I'll commit that change.

lib/Sema/SemaChecking.cpp

                                  const Expr *ArgExpr,
                                  SourceLocation CallSiteLoc) {
   if (CheckNonNullExpr(S, ArgExpr))
-    S.Diag(CallSiteLoc, diag::warn_null_arg) << ArgExpr->getSourceRange();
+    S.DiagRuntimeBehavior(CallSiteLoc, ArgExpr,
+           S.PDiag(diag::warn_null_arg) << ArgExpr->getSourceRange());
 }
 
 bool Sema::GetFormatNSStringIdx(const FormatAttr *Format, unsigned &Idx) {

test/Sema/non-null-warning.c

   return 0; // expected-warning {{null returned from function that requires a non-null return value}}
 }
 
+#define SAFE_CALL(X) if (X) foo(X)
 int main () {
   foo(0); // expected-warning {{null passed to a callee that requires a non-null argument}}
+  (void)sizeof(foo(0)); // expect no diagnostic in unevaluated context.
+  SAFE_CALL(0); // expect no diagnostic for unreachable code.
 }