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

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)

Author: bpfoley

clang/docs/ReleaseNotes.rst

@@ -113,6 +113,10 @@ Attribute Changes in Clang
 Improvements to Clang's diagnostics
 -----------------------------------
 
+- Improve the diagnostics for deleted default constructor errors for C++ class
+  initializer lists that don't explicitly list a class member and thus attempt
+  to implicitly default construct that member.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6113,6 +6113,8 @@ 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 "

clang/lib/Sema/SemaInit.cpp

@@ -9148,6 +9148,17 @@ 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;
       }

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'}}
+};
+}

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}}

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@-6{{default constructor of 'B' is implicitly deleted because field 's' has a deleted default constructor}}
            // expected-note@-15{{'S' has been explicitly marked deleted here}}