[clang] prevent crash for nonnull attribut in constant context (Bug 43601)

[Tyker]

Summary

bug : https://bugs.llvm.org/show_bug.cgi?id=43601

[rnk]

lgtm, thanks.

clang/lib/AST/ExprConstant.cpp

5446

As an unrelated, optional suggestion, given how common I-Args.begin() is, it seems like it would be simpler to do the usual index iteration pattern here.

clang/lib/AST/ExprConstant.cpp

for (const auto *Attr : Callee->specific_attrs<NonNullAttr>()) {
           if (ASTIdx >= Args.size())
             continue;
           ForbiddenNullArgs[ASTIdx] = 1;
         }
     }
   }
   for (ArrayRef<const Expr*>::iterator I = Args.begin(), E = Args.end();
        I != E; ++I) {
     if (!Evaluate(ArgValues[I - Args.begin()], Info, *I)) {
       // If we're checking for a potential constant expression, evaluate all
       // initializers even if some of them fail.
       if (!Info.noteFailure())
         return false;
       Success = false;
     } else if (!ForbiddenNullArgs.empty() &&
                ForbiddenNullArgs[I - Args.begin()] &&
+               ArgValues[I - Args.begin()].isLValue() &&
                ArgValues[I - Args.begin()].isNullPointer()) {
       Info.CCEDiag(*I, diag::note_non_null_attribute_failed);
       if (!Info.noteFailure())
         return false;
       Success = false;
     }
   }
   return Success;

clang/test/SemaCXX/attr-nonnull.cpp

 __attribute__((nonnull(2)))
 constexpr int f3(const int*, const int*) {
   return 0;
 }
 constexpr int i3 = f3(&c, 0); //expected-error {{constant expression}} expected-note {{null passed}}
 constexpr int i32 = f3(0, &c);
 
 __attribute__((nonnull(4))) __attribute__((nonnull)) //expected-error {{out of bounds}}
-constexpr int f4(const int*, const int*) {
+constexpr int f4(const int*, const int*, int) {
   return 0;
 }
-constexpr int i4 = f4(&c, 0); //expected-error {{constant expression}} expected-note {{null passed}}
-constexpr int i42 = f4(0, &c); //expected-error {{constant expression}} expected-note {{null passed}}
+constexpr int i4 = f4(&c, 0, 0); //expected-error {{constant expression}} expected-note {{null passed}}
+constexpr int i42 = f4(0, &c, 1); //expected-error {{constant expression}} expected-note {{null passed}}
+constexpr int i43 = f4(&c, &c, 0);
 
 }
 No newline at end of file