diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -84,6 +84,8 @@
 - Fix assert that triggers a crash during template name lookup when a type was
   incomplete but was not also a TagType. This fixes
   `Issue 57387 <https://github.com/llvm/llvm-project/issues/57387>`_.
+- Fix a crash when emitting a concept-related diagnostic. This fixes
+  `Issue 57415 <https://github.com/llvm/llvm-project/issues/57415>`_.
 
 
 Improvements to Clang's diagnostics
diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -775,7 +775,8 @@
 def err_template_spec_syntax_non_template : Error<
   "identifier followed by '<' indicates a class template specialization but "
   "%0 %select{does not refer to a template|refers to a function template|"
-  "<unused>|refers to a variable template|<unused>|refers to a concept}1">;
+  "<unused>|refers to a variable template|<unused>|<unused>|"
+  "refers to a concept}1">;
 def err_id_after_template_in_nested_name_spec : Error<
   "expected template name after 'template' keyword in nested name specifier">;
 def err_unexpected_template_in_unqualified_id : Error<
diff --git a/clang/test/Parser/cxx-concept-declaration.cpp b/clang/test/Parser/cxx-concept-declaration.cpp
--- a/clang/test/Parser/cxx-concept-declaration.cpp
+++ b/clang/test/Parser/cxx-concept-declaration.cpp
@@ -1,7 +1,12 @@
 
 // Support parsing of concepts
 // Disabled for now.
-// expected-no-diagnostics
 
-// RUN:  %clang_cc1 -std=c++14 -x c++ -verify %s
-// template<typename T> concept C1 = true;
+// RUN:  %clang_cc1 -std=c++20 -x c++ -verify %s
+template<typename T> concept C1 = true;
+
+template<class T>
+concept C = true;
+
+template<class T>
+class C<int> {}; //expected-error{{identifier followed by '<' indicates a class template specialization but 'C' refers to a concept}}