[clang] Fix -Warray-bound interaction with -fstrict-flex-arrays=1

[serge-sans-paille]

Summary

The test to check if an array was a FAM in the context of array bound checking and strict-flex-arrays=1 was inverted.
As a by product, improve test coverage.

[aaron.ballman]

Can you add some more details to the patch summary about what this is fixing? Also, should this have a release note? Documentation changes?

[serge-sans-paille]

Description updated, it's just a bug in the previous implementation (by myself!) I don't think it's worth an entry in the release notes.

[aaron.ballman]

Thank you for the updated description! LGTM

clang/lib/Sema/SemaChecking.cpp

static bool IsTailPaddedMemberArray(Sema &S, const llvm::APInt &Size,
                                     const NamedDecl *ND,
                                     unsigned StrictFlexArraysLevel) {
   if (!ND)
     return false;
 
   if (StrictFlexArraysLevel >= 2 && Size != 0)
     return false;
 
-  if (StrictFlexArraysLevel == 1 && Size.ule(1))
+  if (StrictFlexArraysLevel == 1 && Size.uge(2))
     return false;
 
   // FIXME: While the default -fstrict-flex-arrays=0 permits Size>1 trailing
   // arrays to be treated as flexible-array-members, we still emit diagnostics
   // as if they are not. Pending further discussion...
   if (StrictFlexArraysLevel == 0 && Size != 1)
     return false;
 

clang/test/SemaCXX/array-bounds-strict-flex-arrays.cpp

-// RUN: %clang_cc1 -verify -fstrict-flex-arrays=2 %s
+// RUN: %clang_cc1 -verify=relaxed -fstrict-flex-arrays=1 %s
+// RUN: %clang_cc1 -verify=relaxed,strict -fstrict-flex-arrays=2 %s
 
 // We cannot know for sure the size of a flexible array.
 struct t {
   int f;
   int a[];
 };
 void test(t *s2) {
   s2->a[2] = 0; // no-warning
 }
 
-// Under -fstrict-flex-arrays `a` is not a flexible array.
+// Under -fstrict-flex-arrays={1,2} `a` is not a flexible array
+struct t0 {
+  int f;
+  int a[10]; // relaxed-note {{array 'a' declared here}}
+};
+void test0(t0 *s2) {
+  s2->a[12] = 0; // relaxed-warning {{array index 12 is past the end of the array (which contains 10 elements)}}
+}
+
+
+// Under -fstrict-flex-arrays=2 `a` is not a flexible array, but it is under -fstrict-flex-arrays=1
 struct t1 {
   int f;
-  int a[1]; // expected-note {{array 'a' declared here}}
+  int a[1]; // strict-note {{array 'a' declared here}}
 };
 void test1(t1 *s2) {
-  s2->a[2] = 0; // expected-warning {{array index 2 is past the end of the array (which contains 1 element)}}
+  s2->a[2] = 0; // strict-warning {{array index 2 is past the end of the array (which contains 1 element)}}
 }
 
-// Under -fstrict-flex-arrays `a` is a flexible array.
+// Under -fstrict-flex-arrays={1,2} `a` is a flexible array.
 struct t2 {
   int f;
   int a[0];
 };
 void test1(t2 *s2) {
   s2->a[2] = 0; // no-warning
 }