Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -183,6 +183,9 @@
   initializer is not allowed this is now diagnosed as an error.
 - Clang now correctly emits symbols for implicitly instantiated constexpr
   template function. Fixes `Issue 55560 <https://github.com/llvm/llvm-project/issues/55560>`_.
+- Clang now checks ODR violations when merging concepts from different modules.
+  Note that this is expected to break existing code, and is done so intentionally.
+  Fixes `Issue 56310 <https://github.com/llvm/llvm-project/issues/56310>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Index: clang/lib/AST/ASTContext.cpp
===================================================================
--- clang/lib/AST/ASTContext.cpp
+++ clang/lib/AST/ASTContext.cpp
@@ -6561,6 +6561,20 @@
   // and patterns match.
   if (const auto *TemplateX = dyn_cast<TemplateDecl>(X)) {
     const auto *TemplateY = cast<TemplateDecl>(Y);
+
+    // ConceptDecl wouldn't be the same if their constraint expression differs.
+    if (const auto *ConceptX = dyn_cast<ConceptDecl>(X)) {
+      const auto *ConceptY = cast<ConceptDecl>(Y);
+      const Expr *XCE = ConceptX->getConstraintExpr();
+      const Expr *YCE = ConceptY->getConstraintExpr();
+      assert(XCE && YCE && "ConceptDecl without constraint expression?");
+      llvm::FoldingSetNodeID XID, YID;
+      XCE->Profile(XID, *this, /*Canonical=*/true);
+      YCE->Profile(YID, *this, /*Canonical=*/true);
+      if (XID != YID)
+        return false;
+    }
+
     return isSameEntity(TemplateX->getTemplatedDecl(),
                         TemplateY->getTemplatedDecl()) &&
            isSameTemplateParameterList(TemplateX->getTemplateParameters(),
Index: clang/test/Modules/concept_differ.cpp
===================================================================
--- /dev/null
+++ clang/test/Modules/concept_differ.cpp
@@ -0,0 +1,35 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -x c++ -std=c++20 -fmodules -fmodules-cache-path=%t -fmodule-map-file=%t/module.map %t/foo.cpp -verify
+
+//--- module.map
+module "foo" {
+  export * 
+  header "foo.h"
+}
+module "bar" {
+  export * 
+  header "bar.h"
+}
+
+//--- foo.h
+template <class T>
+concept A = true;
+
+//--- bar.h
+template <class T>
+concept A = false;
+
+//--- foo.cpp
+#include "bar.h"
+#include "foo.h"
+
+template <class T> void foo() requires A<T> {}  // expected-error 1+{{reference to 'A' is ambiguous}}
+                                                // expected-note@* 1+{{candidate found by name lookup}}
+
+int main() {
+  foo<int>();
+  return 0;
+}
Index: clang/test/Modules/concept_differ.cppm
===================================================================
--- /dev/null
+++ clang/test/Modules/concept_differ.cppm
@@ -0,0 +1,39 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -x c++ -std=c++20 %t/A.cppm -I%t -emit-module-interface -o %t/A.pcm
+// RUN: %clang_cc1 -x c++ -std=c++20 %t/B.cppm -I%t -emit-module-interface -o %t/B.pcm
+// RUN: %clang_cc1 -x c++ -std=c++20 -fprebuilt-module-path=%t %t/foo.cpp -verify
+
+//--- foo.h
+template <class T>
+concept A = true;
+
+//--- bar.h
+template <class T>
+concept A = false;
+
+//--- A.cppm
+module;
+#include "foo.h"
+export module A;
+export using ::A;
+
+//--- B.cppm
+module;
+#include "bar.h"
+export module B;
+export using ::A;
+
+//--- foo.cpp
+import A;
+import B;
+
+template <class T> void foo() requires A<T> {}  // expected-error 1+{{reference to 'A' is ambiguous}}
+                                                // expected-note@* 1+{{candidate found by name lookup}}
+
+int main() {
+  foo<int>();
+  return 0;
+}