[clang] The ms-extension __noop should return zero in a constexpr context. (#106849)

Fixes #106713.

Author: c8ef

clang/lib/AST/ExprConstant.cpp

@@ -12720,8 +12720,8 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
   }
 
   case Builtin::BI__noop:
-  // __noop always evaluates successfully
-    return true;
+    // __noop always evaluates successfully and returns 0.
+    return Success(0, E);
 
   case Builtin::BI__builtin_is_constant_evaluated: {
     const auto *Callee = Info.CurrentCall->getCallee();

clang/test/SemaCXX/builtins.cpp

@@ -177,5 +177,21 @@ static void __builtin_cpu_init(); // expected-error {{static declaration of '__b
 #endif
 
 #ifdef _MSC_VER
-constexpr int x = []{ __noop; return 0; }(); // expected-no-diagnostics
+constexpr int x = [] {
+  __noop;
+  return 0;
+}(); // expected-no-diagnostics
+static_assert([] { return __noop; }() == 0);
+static_assert([] { return __noop(4); }() == 0);
+extern int not_accessed;
+void not_called();
+static_assert([] { return __noop(not_accessed *= 6); }() == 0);
+static_assert([] { return __noop(not_called()); }() == 0);
+static_assert([] { return __noop(throw ""); }() == 0);
+static_assert([] { return __noop(throw "", throw ""); }() == 0);
+static_assert([] {
+  int a = 5;
+  __noop(++a);
+  return a;
+}() == 5);
 #endif