Index: clang/include/clang/Basic/DiagnosticParseKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticParseKinds.td
+++ clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -709,7 +709,7 @@
 def err_ms_property_expected_comma_or_rparen : Error<
   "expected ',' or ')' at end of property accessor list">;
 def err_ms_property_initializer : Error<
-  "property declaration cannot have an in-class initializer">;
+  "property declaration cannot have a default member initializer">;
 
 def warn_cxx20_compat_explicit_bool : Warning<
   "this expression will be parsed as explicit(bool) in C++20">,
@@ -859,13 +859,13 @@
   "%select{defaulted|deleted}0 function definitions are incompatible with C++98">,
   InGroup<CXX98Compat>, DefaultIgnore;
 
-// C++11 in-class member initialization
+// C++11 default member initialization
 def ext_nonstatic_member_init : ExtWarn<
-  "in-class initialization of non-static data member is a C++11 extension">,
-  InGroup<CXX11>;
+  "default member initializer for non-static data member is a C++11 "
+  "extension">, InGroup<CXX11>;
 def warn_cxx98_compat_nonstatic_member_init : Warning<
-  "in-class initialization of non-static data members is incompatible with C++98">,
-  InGroup<CXX98Compat>, DefaultIgnore;
+  "default member initializer for non-static data members is incompatible with "
+  "C++98">, InGroup<CXX98Compat>, DefaultIgnore;
 def ext_bitfield_member_init: ExtWarn<
   "default member initializer for bit-field is a C++20 extension">,
   InGroup<CXX20>;
@@ -873,7 +873,7 @@
   "default member initializer for bit-field is incompatible with "
   "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
 def err_incomplete_array_member_init: Error<
-  "array bound cannot be deduced from an in-class initializer">;
+  "array bound cannot be deduced from a default member initializer">;
 
 // C++11 alias-declaration
 def ext_alias_declaration : ExtWarn<
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -1842,7 +1842,7 @@
 def note_nontrivial_user_provided : Note<
   "because %select{base class of |field of |}0type %1 has a user-provided "
   "%sub{select_special_member_kind}2">;
-def note_nontrivial_in_class_init : Note<
+def note_nontrivial_default_member_init : Note<
   "because field %0 has an initializer">;
 def note_nontrivial_param_type : Note<
   "because its parameter is %diff{of type $, not $|of the wrong type}2,3">;
@@ -8521,12 +8521,12 @@
   "'constexpr' specifier">;
 def err_in_class_initializer_non_constant : Error<
   "in-class initializer for static data member is not a constant expression">;
-def err_in_class_initializer_not_yet_parsed : Error<
+def err_default_member_initializer_not_yet_parsed : Error<
   "default member initializer for %1 needed within definition of enclosing "
   "class %0 outside of member functions">;
-def note_in_class_initializer_not_yet_parsed : Note<
+def note_default_member_initializer_not_yet_parsed : Note<
   "default member initializer declared here">;
-def err_in_class_initializer_cycle
+def err_default_member_initializer_cycle
     : Error<"default member initializer for %0 uses itself">;
 
 def ext_in_class_initializer_non_constant : Extension<
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -9403,7 +9403,8 @@
     //       brace-or-equal-initializer
     if (CSM == Sema::CXXDefaultConstructor && FI->hasInClassInitializer()) {
       if (Diagnose)
-        S.Diag(FI->getLocation(), diag::note_nontrivial_in_class_init) << FI;
+        S.Diag(FI->getLocation(), diag::note_nontrivial_default_member_init)
+            << FI;
       return false;
     }
 
@@ -15080,9 +15081,10 @@
   // constructor before the initializer is lexically complete will ultimately
   // come here at which point we can diagnose it.
   RecordDecl *OutermostClass = ParentRD->getOuterLexicalRecordContext();
-  Diag(Loc, diag::err_in_class_initializer_not_yet_parsed)
+  Diag(Loc, diag::err_default_member_initializer_not_yet_parsed)
       << OutermostClass << Field;
-  Diag(Field->getEndLoc(), diag::note_in_class_initializer_not_yet_parsed);
+  Diag(Field->getEndLoc(),
+       diag::note_default_member_initializer_not_yet_parsed);
   // Recover by marking the field invalid, unless we're in a SFINAE context.
   if (!isSFINAEContext())
     Field->setInvalidDecl();
Index: clang/lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2972,9 +2972,10 @@
     RecordDecl *PatternRD = Pattern->getParent();
     RecordDecl *OutermostClass = PatternRD->getOuterLexicalRecordContext();
     Diag(PointOfInstantiation,
-         diag::err_in_class_initializer_not_yet_parsed)
+         diag::err_default_member_initializer_not_yet_parsed)
         << OutermostClass << Pattern;
-    Diag(Pattern->getEndLoc(), diag::note_in_class_initializer_not_yet_parsed);
+    Diag(Pattern->getEndLoc(),
+         diag::note_default_member_initializer_not_yet_parsed);
     Instantiation->setInvalidDecl();
     return true;
   }
@@ -2984,7 +2985,7 @@
     return true;
   if (Inst.isAlreadyInstantiating()) {
     // Error out if we hit an instantiation cycle for this initializer.
-    Diag(PointOfInstantiation, diag::err_in_class_initializer_cycle)
+    Diag(PointOfInstantiation, diag::err_default_member_initializer_cycle)
       << Instantiation;
     return true;
   }
Index: clang/test/Parser/MicrosoftExtensions.cpp
===================================================================
--- clang/test/Parser/MicrosoftExtensions.cpp
+++ clang/test/Parser/MicrosoftExtensions.cpp
@@ -349,7 +349,7 @@
   __declspec(property(get=GetV,)) int V10; // expected-error {{expected 'get' or 'put' in property declaration}}
   __declspec(property(get=GetV,put=SetV)) int V11; // no-warning
   __declspec(property(get=GetV,put=SetV,get=GetV)) int V12; // expected-error {{property declaration specifies 'get' accessor twice}}
-  __declspec(property(get=GetV)) int V13 = 3; // expected-error {{property declaration cannot have an in-class initializer}}
+  __declspec(property(get=GetV)) int V13 = 3; // expected-error {{property declaration cannot have a default member initializer}}
 
   int GetV() { return 123; }
   void SetV(int v) {}
Index: clang/test/Parser/cxx-class.cpp
===================================================================
--- clang/test/Parser/cxx-class.cpp
+++ clang/test/Parser/cxx-class.cpp
@@ -229,34 +229,34 @@
 class PR20760_a {
   int a = ); // expected-error {{expected expression}}
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
   int b = }; // expected-error {{expected expression}}
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
   int c = ]; // expected-error {{expected expression}}
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
 };
 class PR20760_b {
   int d = d); // expected-error {{expected ';'}}
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
   int e = d]; // expected-error {{expected ';'}}
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
   int f = d // expected-error {{expected ';'}}
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
 };
Index: clang/test/SemaCXX/PR9572.cpp
===================================================================
--- clang/test/SemaCXX/PR9572.cpp
+++ clang/test/SemaCXX/PR9572.cpp
@@ -21,7 +21,7 @@
 
   const int kBlah = 3;
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
 
   Foo();
Index: clang/test/SemaCXX/class.cpp
===================================================================
--- clang/test/SemaCXX/class.cpp
+++ clang/test/SemaCXX/class.cpp
@@ -44,7 +44,7 @@
 
   int i = 0;
 #if __cplusplus <= 199711L
-  // expected-warning@-2 {{in-class initialization of non-static data member is a C++11 extension}}
+  // expected-warning@-2 {{default member initializer for non-static data member is a C++11 extension}}
 #endif
   static int si = 0; // expected-error {{non-const static data member must be initialized out of line}}
   static const NestedC ci = 0; // expected-error {{static data member of type 'const C::NestedC' must be initialized out of line}}
@@ -188,7 +188,7 @@
   struct A {
 #if __cplusplus <= 199711L
     static const float x = 5.0f; // expected-warning {{in-class initializer for static data member of type 'const float' is a GNU extension}}
-    static const float y = foo(); // expected-warning {{in-class initializer for static data member of type 'const float' is a GNU extension}} expected-error {{in-class initializer for static data member is not a constant expression}}
+    static const float y = foo(); // expected-warning {{in-class initializer for static data member of type 'const float' is a GNU extension}} expected-error {{default member initializer for static data member is not a constant expression}}
 #else
     static constexpr float x = 5.0f;
     static constexpr float y = foo(); // expected-error {{constexpr variable 'y' must be initialized by a constant expression}} expected-note {{non-constexpr function 'foo' cannot be used in a constant expression}}
Index: clang/test/SemaCXX/cxx98-compat.cpp
===================================================================
--- clang/test/SemaCXX/cxx98-compat.cpp
+++ clang/test/SemaCXX/cxx98-compat.cpp
@@ -122,7 +122,7 @@
 }
 
 struct InClassInit {
-  int n = 0; // expected-warning {{in-class initialization of non-static data members is incompatible with C++98}}
+  int n = 0; // expected-warning {{default member initializer for non-static data members is incompatible with C++98}}
 };
 
 struct OverrideControlBase {
Index: clang/test/SemaCXX/member-init.cpp
===================================================================
--- clang/test/SemaCXX/member-init.cpp
+++ clang/test/SemaCXX/member-init.cpp
@@ -21,20 +21,20 @@
 };
 
 struct UnknownBound {
-  int as[] = { 1, 2, 3 }; // expected-error {{array bound cannot be deduced from an in-class initializer}}
+  int as[] = { 1, 2, 3 }; // expected-error {{array bound cannot be deduced from a default member initializer}}
   int bs[4] = { 4, 5, 6, 7 };
-  int cs[] = { 8, 9, 10 }; // expected-error {{array bound cannot be deduced from an in-class initializer}}
+  int cs[] = { 8, 9, 10 }; // expected-error {{array bound cannot be deduced from a default member initializer}}
 };
 
 template<int n> struct T { static const int B; };
 template<> struct T<2> { template<int C, int D> using B = int; };
 const int C = 0, D = 0;
 struct S {
-  int as[] = { decltype(x)::B<C, D>(0) }; // expected-error {{array bound cannot be deduced from an in-class initializer}}
+  int as[] = { decltype(x)::B<C, D>(0) }; // expected-error {{array bound cannot be deduced from a default member initializer}}
   T<sizeof(as) / sizeof(int)> x;
   // test that we handle invalid array bound deductions without crashing when the declarator name is itself invalid
   operator int[](){}; // expected-error {{'operator int' cannot be the name of a variable or data member}} \
-                      // expected-error {{array bound cannot be deduced from an in-class initializer}}
+                      // expected-error {{array bound cannot be deduced from a default member initializer}}
 };
 
 struct ThrowCtor { ThrowCtor(int) noexcept(false); };