[AArch64] Fix FMV crash on unspecified number of parameters function (llvm#65671)

Fix Function Multi Versioning crash reported in
llvm#65669

Author: ilinpv

clang/lib/Sema/SemaDecl.cpp

@@ -11627,20 +11627,22 @@ static bool CheckMultiVersionFunction(Sema &S, FunctionDecl *NewFD,
   FunctionDecl *OldFD = OldDecl->getAsFunction();
 
   if (!OldFD->isMultiVersion() && MVKind == MultiVersionKind::None) {
-    // No target_version attributes mean default
-    if (!NewTVA) {
-      const auto *OldTVA = OldFD->getAttr<TargetVersionAttr>();
-      if (OldTVA) {
-        NewFD->addAttr(TargetVersionAttr::CreateImplicit(
-            S.Context, "default", NewFD->getSourceRange()));
-        NewFD->setIsMultiVersion();
-        OldFD->setIsMultiVersion();
-        OldDecl = OldFD;
-        Redeclaration = true;
-        return true;
-      }
+    if (NewTVA || !OldFD->getAttr<TargetVersionAttr>())
+      return false;
+    if (!NewFD->getType()->getAs<FunctionProtoType>()) {
+      // Multiversion declaration doesn't have prototype.
+      S.Diag(NewFD->getLocation(), diag::err_multiversion_noproto);
+      NewFD->setInvalidDecl();
+    } else {
+      // No "target_version" attribute is equivalent to "default" attribute.
+      NewFD->addAttr(TargetVersionAttr::CreateImplicit(
+          S.Context, "default", NewFD->getSourceRange()));
+      NewFD->setIsMultiVersion();
+      OldFD->setIsMultiVersion();
+      OldDecl = OldFD;
+      Redeclaration = true;
     }
-    return false;
+    return true;
   }
 
   // Multiversioned redeclarations aren't allowed to omit the attribute, except

clang/test/Sema/attr-target-version.c

@@ -89,3 +89,8 @@ float __attribute__((target_version("rdm"))) rtype(int);
 int __attribute__((target_version("sha2"))) combine(void) { return 1; }
 // expected-error@+1 {{multiversioned function declaration has a different calling convention}}
 int __attribute__((aarch64_vector_pcs, target_version("sha3"))) combine(void) { return 2; }
+
+int __attribute__((target_version("fp+aes+pmull+rcpc"))) unspec_args() { return -1; }
+// expected-error@+1 {{multiversioned function must have a prototype}}
+int __attribute__((target_version("default"))) unspec_args() { return 0; }
+int cargs() { return unspec_args(); }