Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -132,6 +132,9 @@
 - Clang now warns on unused variables declared and initialized in condition
   expressions.
   (`#61681: <https://github.com/llvm/llvm-project/issues/61681>`_)
+- Clang constexpr evaluator now diagnoses compound assignment operators against
+  uninitialized variables as a read of uninitialized object.
+  (`#51536 <https://github.com/llvm/llvm-project/issues/51536>_`)
 
 Bug Fixes in This Version
 -------------------------
Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -4442,6 +4442,10 @@
       return foundPointer(Subobj, SubobjType);
     case APValue::Vector:
       return foundVector(Subobj, SubobjType);
+    case APValue::Indeterminate:
+      Info.FFDiag(E, diag::note_constexpr_access_uninit)
+          << /*read of=*/0 << /*uninitialized object=*/1;
+      return false;
     default:
       // FIXME: can this happen?
       Info.FFDiag(E);
Index: clang/test/SemaCXX/constant-expression-cxx14.cpp
===================================================================
--- clang/test/SemaCXX/constant-expression-cxx14.cpp
+++ clang/test/SemaCXX/constant-expression-cxx14.cpp
@@ -1275,3 +1275,33 @@
     (dbt2.wp = nullptr, 0)
   };
 }
+
+namespace UninitCompoundAssign {
+constexpr int scalar(int a) {
+  int sum; // cxx14-warning {{uninitialized variable in a constexpr function is a C++20 extension}}
+  sum += a; // expected-note {{read of uninitialized object}};
+  return 0;
+}
+static_assert(scalar(3), ""); // expected-error {{constant expression}} \
+                              // expected-note {{in call to 'scalar(3)'}}
+
+constexpr int array(int a) {
+  int arr[3]; // cxx14-warning {{uninitialized variable in a constexpr function is a C++20 extension}}
+  arr[1] += a; // expected-note {{read of uninitialized object}};
+  return 0;
+}
+static_assert(array(3), ""); // expected-error {{constant expression}} \
+                             // expected-note {{in call to 'array(3)'}}
+
+struct Foo {
+  int val; // cxx14-note{{member not initialized by constructor}}
+  constexpr Foo() {} // cxx14-warning {{constexpr constructor that does not initialize all members is a C++20 extension}}
+};
+constexpr int field(int a) {
+  Foo f;
+  f.val += a; // expected-note {{read of uninitialized object}};
+  return 0;
+}
+static_assert(field(3), ""); // expected-error {{constant expression}} \
+                             // expected-note {{in call to 'field(3)'}}
+}