Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -129,6 +129,9 @@
   of a base class is not called in the constructor of its derived class.
 - Clang no longer emits ``-Wmissing-variable-declarations`` for variables declared
   with the ``register`` storage class.
+- 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-cxx2a.cpp
===================================================================
--- clang/test/SemaCXX/constant-expression-cxx2a.cpp
+++ clang/test/SemaCXX/constant-expression-cxx2a.cpp
@@ -1492,3 +1492,33 @@
 class D : B{}; // expected-error {{deleted function '~D' cannot override a non-deleted function}}
 // expected-note@-1 {{destructor of 'D' is implicitly deleted because base class 'B' has an inaccessible destructor}}
 }
+
+namespace UninitCompoundAssign {
+constexpr int scalar(int a) {
+  int sum;
+  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];
+  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;
+  constexpr Foo() {}
+};
+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)'}}
+}