[clang][Interp] Allow getFieldPtr ops on null pointers in C

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1224,7 +1224,8 @@ inline bool GetPtrGlobal(InterpState &S, CodePtr OpPC, uint32_t I) {
 inline bool GetPtrField(InterpState &S, CodePtr OpPC, uint32_t Off) {
   const Pointer &Ptr = S.Stk.pop<Pointer>();
 
-  if (S.inConstantContext() && !CheckNull(S, OpPC, Ptr, CSK_Field))
+  if (S.getLangOpts().CPlusPlus && S.inConstantContext() &&
+      !CheckNull(S, OpPC, Ptr, CSK_Field))
     return false;
 
   if (CheckDummy(S, OpPC, Ptr)) {

clang/test/AST/Interp/c.c

@@ -79,6 +79,11 @@ int a2[(intptr_t)&((struct y*)0)->y]; // all-warning {{folded to constant array}
 const struct y *yy = (struct y*)0;
 const intptr_t L = (intptr_t)(&(yy->y)); // all-error {{not a compile-time constant}}
 
+_Static_assert((long)&((struct y*)0)->y > 0, ""); // pedantic-ref-warning {{GNU extension}} \
+                                                  // pedantic-ref-note {{this conversion is not allowed in a constant expression}} \
+                                                  // pedantic-expected-warning {{GNU extension}} \
+                                                  // pedantic-expected-note {{this conversion is not allowed in a constant expression}}
+
 const ptrdiff_t m = &m + 137 - &m;
 _Static_assert(m == 137, ""); // pedantic-ref-warning {{GNU extension}} \
                               // pedantic-expected-warning {{GNU extension}}