diff --git a/clang/include/clang/AST/ExprConcepts.h b/clang/include/clang/AST/ExprConcepts.h
--- a/clang/include/clang/AST/ExprConcepts.h
+++ b/clang/include/clang/AST/ExprConcepts.h
@@ -14,20 +14,21 @@
 #ifndef LLVM_CLANG_AST_EXPRCONCEPTS_H
 #define LLVM_CLANG_AST_EXPRCONCEPTS_H
 
-#include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTConcept.h"
+#include "clang/AST/ASTContext.h"
 #include "clang/AST/Decl.h"
-#include "clang/AST/DeclarationName.h"
 #include "clang/AST/DeclTemplate.h"
+#include "clang/AST/DeclarationName.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/SourceLocation.h"
+#include "llvm/ADT/STLFunctionalExtras.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/TrailingObjects.h"
-#include <utility>
 #include <string>
+#include <utility>
 
 namespace clang {
 class ASTStmtReader;
@@ -467,6 +468,13 @@
   }
 };
 
+using EntityPrinter = llvm::function_ref<void(llvm::raw_ostream &)>;
+
+/// \brief create a Requirement::SubstitutionDiagnostic with only a
+/// SubstitutedEntity and DiagLoc using Sema's allocator.
+Requirement::SubstitutionDiagnostic *
+createSubstDiagAt(Sema &S, SourceLocation Location, EntityPrinter Printer);
+
 } // namespace concepts
 
 /// C++2a [expr.prim.req]:
diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -19,6 +19,7 @@
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/ExprConcepts.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/Type.h"
@@ -9074,16 +9075,23 @@
     MLTAL.addOuterRetainedLevels(TPL->getDepth());
     const TypeConstraint *TC = Param->getTypeConstraint();
     assert(TC && "Type Constraint cannot be null here");
-    ExprResult Constraint =
-        SubstExpr(TC->getImmediatelyDeclaredConstraint(), MLTAL);
+    auto *IDC = TC->getImmediatelyDeclaredConstraint();
+    assert(IDC && "ImmediatelyDeclaredConstraint can't be null here.");
+    ExprResult Constraint = SubstExpr(IDC, MLTAL);
     if (Constraint.isInvalid()) {
       Status = concepts::ExprRequirement::SS_ExprSubstitutionFailure;
-    } else {
-      SubstitutedConstraintExpr =
-          cast<ConceptSpecializationExpr>(Constraint.get());
-      if (!SubstitutedConstraintExpr->isSatisfied())
-        Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
-    }
+      return new (Context) concepts::ExprRequirement(
+          concepts::createSubstDiagAt(*this, IDC->getExprLoc(),
+                                      [&](llvm::raw_ostream &OS) {
+                                        IDC->printPretty(OS, /*Helper=*/nullptr,
+                                                         getPrintingPolicy());
+                                      }),
+          IsSimple, NoexceptLoc, ReturnTypeRequirement);
+    }
+    SubstitutedConstraintExpr =
+        cast<ConceptSpecializationExpr>(Constraint.get());
+    if (!SubstitutedConstraintExpr->isSatisfied())
+      Status = concepts::ExprRequirement::SS_ConstraintsNotSatisfied;
   }
   return new (Context) concepts::ExprRequirement(E, IsSimple, NoexceptLoc,
                                                  ReturnTypeRequirement, Status,
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2271,9 +2271,9 @@
       getPackIndex(Pack), Arg, TL.getNameLoc());
 }
 
-template<typename EntityPrinter>
 static concepts::Requirement::SubstitutionDiagnostic *
-createSubstDiag(Sema &S, TemplateDeductionInfo &Info, EntityPrinter Printer) {
+createSubstDiag(Sema &S, TemplateDeductionInfo &Info,
+                concepts::EntityPrinter Printer) {
   SmallString<128> Message;
   SourceLocation ErrorLoc;
   if (Info.hasSFINAEDiagnostic()) {
@@ -2297,6 +2297,19 @@
       StringRef(MessageBuf, Message.size())};
 }
 
+concepts::Requirement::SubstitutionDiagnostic *
+concepts::createSubstDiagAt(Sema &S, SourceLocation Location,
+                            EntityPrinter Printer) {
+  SmallString<128> Entity;
+  llvm::raw_svector_ostream OS(Entity);
+  Printer(OS);
+  char *EntityBuf = new (S.Context) char[Entity.size()];
+  llvm::copy(Entity, EntityBuf);
+  return new (S.Context) concepts::Requirement::SubstitutionDiagnostic{
+      /*SubstitutedEntity=*/StringRef(EntityBuf, Entity.size()),
+      /*DiagLoc=*/Location, /*DiagMessage=*/StringRef()};
+}
+
 ExprResult TemplateInstantiator::TransformRequiresTypeParams(
     SourceLocation KWLoc, SourceLocation RBraceLoc, const RequiresExpr *RE,
     RequiresExprBodyDecl *Body, ArrayRef<ParmVarDecl *> Params,
diff --git a/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp b/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/concept-crash-on-diagnostic.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+
+template <typename _Iterator> class normal_iterator {};
+
+template <typename From, typename To> struct is_convertible {};
+
+template <typename From, typename To>
+inline constexpr bool is_convertible_v = is_convertible<From, To>::value; // #1
+
+// expected-error@#1 {{no member named 'value' in 'is_convertible<bool, bool>'}}
+
+template <typename From, typename To>
+concept convertible_to = is_convertible_v<From, To>; // expected-note 0+{{}}
+template <typename IteratorL, typename IteratorR>
+  requires requires(IteratorL lhs, IteratorR rhs) { // expected-note 0+{{}}
+    { lhs == rhs } -> convertible_to<bool>; // #2
+  }
+constexpr bool compare(normal_iterator<IteratorL> lhs, normal_iterator<IteratorR> rhs) {
+  return false;
+}
+
+// We don't know exactly the substituted type for `lhs == rhs`, thus a placeholder 'expr-type' is emitted.
+// expected-note@#2 {{'convertible_to<expr-type, bool>'}}
+
+// Consume remaining notes/errors.
+// expected-note@* 0+{{}}
+// expected-error@* 0+{{}}
+class Object;
+
+void function() {
+  normal_iterator<Object *> begin, end;
+  compare(begin, end);
+}
diff --git a/clang/test/SemaCXX/concept-fatal-error.cpp b/clang/test/SemaCXX/concept-fatal-error.cpp
--- a/clang/test/SemaCXX/concept-fatal-error.cpp
+++ b/clang/test/SemaCXX/concept-fatal-error.cpp
@@ -1,4 +1,4 @@
-// RUN: not %clang_cc1 -fsyntax-only -std=c++20 -ferror-limit 1 -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -ferror-limit 1 -verify %s
 
 template <class>
 concept f = requires { 42; };
@@ -6,5 +6,5 @@
   // The missing semicolon will trigger an error and -ferror-limit=1 will make it fatal
   // We test that we do not crash in such cases (#55401)
   int i = requires { { i } f } // expected-error {{expected ';' at end of declaration list}}
-                               // expected-error@* {{too many errros emitted}}
+                               // expected-error@* {{too many errors emitted}}
 };