diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 232003f256ee7..be1d8f924e680 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1540,22 +1540,15 @@ void Sema::DiagnoseUnsatisfiedConstraint(
 const NormalizedConstraint *Sema::getNormalizedAssociatedConstraints(
     ConstrainedDeclOrNestedRequirement ConstrainedDeclOrNestedReq,
     ArrayRef<AssociatedConstraint> AssociatedConstraints) {
-  // In case the ConstrainedDecl comes from modules, it is necessary to use
-  // the canonical decl to avoid different atomic constraints with the 'same'
-  // declarations.
-
   if (!ConstrainedDeclOrNestedReq)
     return NormalizedConstraint::fromAssociatedConstraints(
         *this, nullptr, AssociatedConstraints);
 
   const NamedDecl *ND =
       ConstrainedDeclOrNestedReq.dyn_cast<const NamedDecl *>();
-  if (ND)
-    ND = cast<NamedDecl>(ND->getCanonicalDecl());
-
   auto CacheEntry = NormalizationCache.find(ConstrainedDeclOrNestedReq);
   if (CacheEntry == NormalizationCache.end()) {
-    auto Normalized = NormalizedConstraint::fromAssociatedConstraints(
+    auto *Normalized = NormalizedConstraint::fromAssociatedConstraints(
         *this, ND, AssociatedConstraints);
     CacheEntry =
         NormalizationCache.try_emplace(ConstrainedDeclOrNestedReq, Normalized)
@@ -1795,10 +1788,12 @@ NormalizedConstraint *NormalizedConstraint::fromConstraintExpr(
       // constraint. If any such substitution results in an invalid type or
       // expression, the program is ill-formed; no diagnostic is required.
       // [...]
-      ConceptDecl *CD = CSE->getNamedConcept();
+
+      // Use canonical declarations to merge ConceptDecls across
+      // different modules.
+      ConceptDecl *CD = CSE->getNamedConcept()->getCanonicalDecl();
       SubNF = NormalizedConstraint::fromAssociatedConstraints(
-          S, CSE->getNamedConcept(),
-          AssociatedConstraint(CD->getConstraintExpr(), SubstIndex));
+          S, CD, AssociatedConstraint(CD->getConstraintExpr(), SubstIndex));
 
       if (!SubNF)
         return nullptr;
diff --git a/clang/lib/Serialization/ASTReaderStmt.cpp b/clang/lib/Serialization/ASTReaderStmt.cpp
index 01c838b955755..126d3246e38b7 100644
--- a/clang/lib/Serialization/ASTReaderStmt.cpp
+++ b/clang/lib/Serialization/ASTReaderStmt.cpp
@@ -802,15 +802,20 @@ readConstraintSatisfaction(ASTRecordReader &Record) {
   if (!Satisfaction.IsSatisfied) {
     unsigned NumDetailRecords = Record.readInt();
     for (unsigned i = 0; i != NumDetailRecords; ++i) {
-      if (/* IsDiagnostic */Record.readInt()) {
+      auto Kind = Record.readInt();
+      if (Kind == 0) {
         SourceLocation DiagLocation = Record.readSourceLocation();
         StringRef DiagMessage = C.backupStr(Record.readString());
 
         Satisfaction.Details.emplace_back(
             new (C) ConstraintSatisfaction::SubstitutionDiagnostic(
                 DiagLocation, DiagMessage));
-      } else
+      } else if (Kind == 1) {
         Satisfaction.Details.emplace_back(Record.readExpr());
+      } else {
+        assert(Kind == 2);
+        Satisfaction.Details.emplace_back(Record.readConceptReference());
+      }
     }
   }
   return Satisfaction;
diff --git a/clang/lib/Serialization/ASTWriterStmt.cpp b/clang/lib/Serialization/ASTWriterStmt.cpp
index bf19cbab65b8f..e0f14fecc6638 100644
--- a/clang/lib/Serialization/ASTWriterStmt.cpp
+++ b/clang/lib/Serialization/ASTWriterStmt.cpp
@@ -475,14 +475,20 @@ addConstraintSatisfaction(ASTRecordWriter &Record,
   if (!Satisfaction.IsSatisfied) {
     Record.push_back(Satisfaction.NumRecords);
     for (const auto &DetailRecord : Satisfaction) {
-      auto *E = dyn_cast<const Expr *>(DetailRecord);
-      Record.push_back(/* IsDiagnostic */ E == nullptr);
-      if (E)
-        Record.AddStmt(const_cast<Expr *>(E));
-      else {
-        auto *Diag = cast<std::pair<SourceLocation, StringRef> *>(DetailRecord);
+      if (auto *Diag =
+              dyn_cast<std::pair<SourceLocation, StringRef> *>(DetailRecord)) {
+        Record.push_back(/*Kind=*/0);
         Record.AddSourceLocation(Diag->first);
         Record.AddString(Diag->second);
+        continue;
+      }
+      if (auto *E = dyn_cast<const Expr *>(DetailRecord)) {
+        Record.push_back(/*Kind=*/1);
+        Record.AddStmt(const_cast<Expr *>(E));
+      } else {
+        Record.push_back(/*Kind=*/2);
+        auto *CR = cast<const ConceptReference *>(DetailRecord);
+        Record.AddConceptReference(CR);
       }
     }
   }