[clang] [Clang][Sema] Account for types coming from template parameter when diagnosing mem initializer order issues (PR #201586)

[Gábor Spaits via cfe-commits]

https://github.com/spaits created https://github.com/llvm/llvm-project/pull/201586

https://godbolt.org/z/xPz5r7ad3

>From 10efd54ff2f30e3b46fa94ccfa50eb0743ff9685 Mon Sep 17 00:00:00 2001
From: Gabor Spaits <gaborspaits1 at gmail.com>
Date: Thu, 4 Jun 2026 15:34:19 +0200
Subject: [PATCH] [clang][sema] Account for tempate parameter types when
 diagnosing mem initializer order issues

---
 clang/lib/Sema/SemaDeclCXX.cpp                        |  5 +++++
 .../test/SemaCXX/warn-reorder-ctor-initialization.cpp | 11 +++++++++++
 2 files changed, 16 insertions(+)

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index de837ff0608d0..d523f9d80c157 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -5766,6 +5766,11 @@ static void DiagnoseBaseOrMemInitializerOrder(
         if (InitKey == IdealInitKeys[IdealIndex])
           break;
 
+      // There was a template parameter in the initializers, that isn't a base
+      // of the class to be instantiated.
+      if (IdealIndex >= NumIdealInits)
+        break;
+
       assert(IdealIndex < NumIdealInits &&
              "initializer not found in initializer list");
     }
diff --git a/clang/test/SemaCXX/warn-reorder-ctor-initialization.cpp b/clang/test/SemaCXX/warn-reorder-ctor-initialization.cpp
index 0613338945978..7181305dc5850 100644
--- a/clang/test/SemaCXX/warn-reorder-ctor-initialization.cpp
+++ b/clang/test/SemaCXX/warn-reorder-ctor-initialization.cpp
@@ -84,6 +84,17 @@ class Anon3 {
   Anon3() : b(1) {}
 };
 
+struct S0 {
+    S0(int) {}
+    template <class T>
+    S0(T) : Member(42), T(42) {}; // expected-warning {{field 'Member' will be initialized after base 'T'}}
+    int Member{21};
+};
+
+void InstantiateS0() {
+    S0 S0Instance(67);
+}
+
 namespace T1 {
 
 struct S1 { };