[clang][Interp] Handle __extension__ unary operators

[tbaeder]

Summary

AFAIK this is just used to suppress warnings, so ignore it altogether.

[tbaeder]

clang/test/AST/Interp/literals.cpp

864–881

It's weird that the above two statements warn about using __FUNCTION__ and the return statement doesn't.

[erichkeane]

clang/test/AST/Interp/literals.cpp

864–881

Aren't they warning about it being 'unused'? If so, the 'return' uses them...

[tbaeder]

clang/test/AST/Interp/literals.cpp

864–881

Right, my message wasn't clear but I meant the c++14 extension diagnostic: https://godbolt.org/z/Kso8qv7jh

[tbaeder]

Ping

[aaron.ballman]

LGTM

clang/test/AST/Interp/literals.cpp

864–881

That is pretty strange, tbh. Might be worth investigating in a follow-up.

[tbaeder]

clang/test/AST/Interp/literals.cpp

864–881

Yeah, in CheckConstexprFunctionStmt in SemaDeclCXX.cpp, the return expression is not checked.

clang/lib/AST/Interp/ByteCodeExprGen.cpp

return dereference(
         });
   case UO_Not:    // ~x
     if (!this->visit(SubExpr))
       return false;
     return DiscardResult ? this->emitPop(*T, E) : this->emitComp(*T, E);
   case UO_Real:   // __real x
   case UO_Imag:   // __imag x
   case UO_Extension:
+    if (DiscardResult)
+      return this->discard(SubExpr);
+    return this->visit(SubExpr);
   case UO_Coawait:
     assert(false && "Unhandled opcode");
   }
 
   return false;
 }
 
 template <class Emitter>

clang/test/AST/Interp/literals.cpp

(a); // expected-warning {{unused}} \
        // ref-warning {{unused}}
 
   return 0;
 }
 
 #endif
 
 namespace PredefinedExprs {
 #if __cplusplus >= 201402L
   template<typename CharT>
   constexpr bool strings_match(const CharT *str1, const CharT *str2) {
     while (*str1 && *str2) {
       if (*str1++ != *str2++)
         return false;
     };
 
     return *str1 == *str2;
   }
 
   void foo() {
     static_assert(strings_match(__FUNCSIG__, "void __cdecl PredefinedExprs::foo(void)"), "");
     static_assert(strings_match(L__FUNCSIG__, L"void __cdecl PredefinedExprs::foo(void)"), "");
     static_assert(strings_match(L__FUNCTION__, L"foo"), "");
     static_assert(strings_match(__FUNCTION__, "foo"), "");
     static_assert(strings_match(__func__, "foo"), "");
     static_assert(strings_match(__PRETTY_FUNCTION__, "void PredefinedExprs::foo()"), "");
   }
+
+  constexpr char heh(unsigned index) {
+    __FUNCTION__;               // ref-warning {{result unused}} \
+                                // expected-warning {{result unused}}
+    __extension__ __FUNCTION__; // ref-warning {{result unused}} \
+                                // expected-warning {{result unused}}
+    return __FUNCTION__[index];
+  }
+  static_assert(heh(0) == 'h', "");
+  static_assert(heh(1) == 'e', "");
+  static_assert(heh(2) == 'h', "");
 #endif
 }