1 files changed, 40 insertions, 0 deletions
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index 26dc5d92f23..dff336f2ff2 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1057,6 +1057,8 @@ namespace {
                                             NonTypeTemplateParmDecl *D);
     ExprResult TransformSubstNonTypeTemplateParmPackExpr(
                                            SubstNonTypeTemplateParmPackExpr *E);
+    ExprResult TransformSubstNonTypeTemplateParmExpr(
+                                           SubstNonTypeTemplateParmExpr *E);
 
     /// Rebuild a DeclRefExpr for a VarDecl reference.
     ExprResult RebuildVarDeclRefExpr(VarDecl *PD, SourceLocation Loc);
@@ -1535,6 +1537,44 @@ TemplateInstantiator::TransformSubstNonTypeTemplateParmPackExpr(
                                          Arg);
 }
 
+ExprResult
+TemplateInstantiator::TransformSubstNonTypeTemplateParmExpr(
+                                          SubstNonTypeTemplateParmExpr *E) {
+  ExprResult SubstReplacement = TransformExpr(E->getReplacement());
+  if (SubstReplacement.isInvalid())
+    return true;
+  QualType SubstType = TransformType(E->getType());
+  if (SubstType.isNull())
+    return true;
+  // The type may have been previously dependent and not now, which means we
+  // might have to implicit cast the argument to the new type, for example:
+  // template<auto T, decltype(T) U>
+  // concept C = sizeof(U) == 4;
+  // void foo() requires C<2, 'a'> { }
+  // When normalizing foo(), we first form the normalized constraints of C:
+  // AtomicExpr(sizeof(U) == 4,
+  //            U=SubstNonTypeTemplateParmExpr(Param=U,
+  //                                           Expr=DeclRef(U),
+  //                                           Type=decltype(T)))
+  // Then we substitute T = 2, U = 'a' into the parameter mapping, and need to
+  // produce:
+  // AtomicExpr(sizeof(U) == 4,
+  //            U=SubstNonTypeTemplateParmExpr(Param=U,
+  //                                           Expr=ImpCast(
+  //                                               decltype(2),
+  //                                               SubstNTTPE(Param=U, Expr='a',
+  //                                                          Type=char)),
+  //                                           Type=decltype(2)))
+  // The call to CheckTemplateArgument here produces the ImpCast.
+  TemplateArgument Converted;
+  if (SemaRef.CheckTemplateArgument(E->getParameter(), SubstType,
+                                    SubstReplacement.get(),
+                                    Converted).isInvalid())
+    return true;
+  return transformNonTypeTemplateParmRef(E->getParameter(),
+                                         E->getExprLoc(), Converted);
+}
+
 ExprResult TemplateInstantiator::RebuildVarDeclRefExpr(VarDecl *PD,
                                                        SourceLocation Loc) {
   DeclarationNameInfo NameInfo(PD->getDeclName(), Loc);