[clang] [Sema] Note member decl when initializer list default constructs member (PR #121854)

[Brian Foley via cfe-commits]

https://github.com/bpfoley updated https://github.com/llvm/llvm-project/pull/121854

>From 72ffbe4ef6b4f8fe6fa31fef7fd3f5c961495c9f Mon Sep 17 00:00:00 2001
From: Brian Foley <bpfoley at gmail.com>
Date: Thu, 5 Dec 2024 14:35:25 -0800
Subject: [PATCH] [Sema] Note member decl when initializer list default
 constructs member

Recently I had a scenario where I had:
1. A class C with many members m_1...m_n of the same type T
2. T's default constructor was deleted
3. I accidentally omitted an explicitly constructed member in the
   initializer list C() : m_1(foo), m_2(bar), ... { }

Clang told me that T's default constructor was deleted, and told
me that the call to T() was in C() (which it implicitly was), but
didn't tell me which member was being default constructed.

It was difficult to fix this problem because I had no easy way to
list all the members of type T in C and C's superclasses which
would have let me find which member was missing,

clang/test/CXX/class/class.init/p1.cpp is a simplified version
of this problem (a2 is missing from the initializer list of B)
---
 clang/include/clang/Basic/DiagnosticSemaKinds.td |  2 ++
 clang/lib/Sema/SemaInit.cpp                      | 11 +++++++++++
 clang/test/CXX/class/class.init/p1.cpp           | 14 ++++++++++++++
 clang/test/CXX/dcl.decl/dcl.init/p14-0x.cpp      |  2 +-
 clang/test/SemaCUDA/inherited-ctor.cu            |  2 +-
 5 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/CXX/class/class.init/p1.cpp

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 03fb7ca9bc3c3b..e98ad631b6d6ab 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6069,6 +6069,8 @@ def note_deleted_special_member_class_subobject : Note<
   "destructor}5"
   "%select{||s||}4"
   "|is an ObjC pointer}6">;
+def note_default_constructed_field
+    : Note<"default constructed field %0 declared here">;
 def note_deleted_default_ctor_uninit_field : Note<
   "%select{default constructor of|constructor inherited by}0 "
   "%1 is implicitly deleted because field %2 of "
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 0dd5f468cf60bf..184c88cceb6fa8 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -9088,6 +9088,17 @@ bool InitializationSequence::Diagnose(Sema &S,
               << (Msg ? Msg->getString() : StringRef()) << ArgsRange;
         }
 
+        // If it's a default constructed member, but it's not in the
+        // constructor's initializer list, explicitly note where the member is
+        // declared so the user can see which member is erroneously initialized
+        // with a deleted default constructor.
+        if (Kind.getKind() == InitializationKind::IK_Default &&
+            (Entity.getKind() == InitializedEntity::EK_Member ||
+             Entity.getKind() == InitializedEntity::EK_ParenAggInitMember)) {
+          S.Diag(Entity.getDecl()->getLocation(),
+                 diag::note_default_constructed_field)
+              << Entity.getDecl();
+        }
         S.NoteDeletedFunction(Best->Function);
         break;
       }
diff --git a/clang/test/CXX/class/class.init/p1.cpp b/clang/test/CXX/class/class.init/p1.cpp
new file mode 100644
index 00000000000000..717dfba89763a7
--- /dev/null
+++ b/clang/test/CXX/class/class.init/p1.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+namespace test_deleted_ctor_note {
+struct A {
+  int a;
+  A() = delete; // expected-note {{'A' has been explicitly marked deleted here}}
+  A(int a_) : a(a_) { }
+};
+
+struct B {
+  A a1, a2, a3; // expected-note {{default constructed field 'a2' declared here}}
+  B(int a_) : a1(a_), a3(a_) { } // expected-error{{call to deleted constructor of 'A'}}
+};
+}
diff --git a/clang/test/CXX/dcl.decl/dcl.init/p14-0x.cpp b/clang/test/CXX/dcl.decl/dcl.init/p14-0x.cpp
index e7f501352168ce..d548f9c8c2fdf1 100644
--- a/clang/test/CXX/dcl.decl/dcl.init/p14-0x.cpp
+++ b/clang/test/CXX/dcl.decl/dcl.init/p14-0x.cpp
@@ -27,7 +27,7 @@ class Friend {
 
 
 class S {
-  NoDefault nd1;
+  NoDefault nd1; // expected-note {{default constructed field 'nd1' declared here}}
   NoDefault nd2 = 42;
   Explicit e1; // expected-note {{here}}
   Explicit e2 = 42; // expected-error {{no viable conversion}}
diff --git a/clang/test/SemaCUDA/inherited-ctor.cu b/clang/test/SemaCUDA/inherited-ctor.cu
index 8ac59e7b539f37..ef3938555b9834 100644
--- a/clang/test/SemaCUDA/inherited-ctor.cu
+++ b/clang/test/SemaCUDA/inherited-ctor.cu
@@ -81,7 +81,7 @@ namespace DefaultCtorInvalid {
   };
 
   struct C {
-    struct B b;
+    struct B b; // expected-note{{default constructed field 'b' declared here}}
     C() {} // expected-error{{call to implicitly-deleted default constructor of 'struct B'}}
            // expected-note at -6{{default constructor of 'B' is implicitly deleted because field 's' has a deleted default constructor}}
            // expected-note at -15{{'S' has been explicitly marked deleted here}}