[clang][-Wunsafe-buffer-usage] Ignore consteval functions

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -4458,6 +4458,10 @@ void clang::checkUnsafeBufferUsage(const Decl *D,
   SmallVector<Stmt *> Stmts;
 
   if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
+    // Consteval functions are free of UB by the spec, so we don't need to
+    // visit them or produce diagnostics.
+    if (FD->isConsteval())
+      return;
     // We do not want to visit a Lambda expression defined inside a method
     // independently. Instead, it should be visited along with the outer method.
     // FIXME: do we want to do the same thing for `BlockDecl`s?

clang/test/SemaCXX/warn-unsafe-buffer-usage.cpp

@@ -406,4 +406,11 @@ void testMultiLineDeclStmt(int * p) {
   foo(ap1[1]);  // expected-note{{used in buffer access here}}
 }
 
+#if __cplusplus >= 202002L
+consteval void testConstevalPtrArithmetic(int idx) {
+  int y[3] = {0, 1, 2};
+  foo(y[idx]);
+}
+#endif
+
 #endif