[clang] add fallback to expr in the template differ when comparing ValueDecl (#93266)

clang/docs/ReleaseNotes.rst

@@ -803,6 +803,7 @@ Miscellaneous Bug Fixes
 - Fixed an infinite recursion in ASTImporter, on return type declared inside
   body of C++11 lambda without trailing return (#GH68775).
 - Fixed declaration name source location of instantiated function definitions (GH71161).
+- Improve diagnostic output to print an expression instead of 'no argument` when comparing Values as template arguments.
 
 Miscellaneous Clang Crashes Fixed
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ASTDiagnostic.cpp

@@ -1936,6 +1936,11 @@ class TemplateDiff {
       return;
     }
 
+    if (E) {
+      PrintExpr(E);
+      return;
+    }
+
     OS << "(no argument)";
   }
 

clang/test/Misc/diag-template-diffing-cxx26.cpp

@@ -19,10 +19,10 @@ namespace GH93068 {
     // expected-note@#A {{no known conversion from 'A<0>' to 'const A<&n[1]> &' for 1st argument}}
     // expected-note@#A {{no known conversion from 'A<0>' to 'A<&n[1]> &&' for 1st argument}}
 
-    // notree-error@#2 {{no viable conversion from 'A<n>' to 'A<(no argument)>'}}
+    // notree-error@#2 {{no viable conversion from 'A<n>' to 'A<n + 1>'}}
     /* tree-error@#2 {{no viable conversion
   A<
-    [n != (no argument)]>}}*/
+    [n != n + 1]>}}*/
 
     A<n + 1> v2 = A<n>(); // #2
     // expected-note@#A {{no known conversion from 'A<n>' to 'const A<&n[1]> &' for 1st argument}}