address feedback

Author: cor3ntin

clang/lib/Sema/SemaConcept.cpp

@@ -277,8 +277,12 @@ class HashParameterMapping : public RecursiveASTVisitor<HashParameterMapping> {
   bool VisitTemplateTypeParmType(TemplateTypeParmType *T) {
     // A lambda expression can introduce template parameters that don't have
     // corresponding template arguments yet.
-    if (T->getDepth() >= TemplateArgs.getNumLevels() ||
-        !TemplateArgs.hasTemplateArgument(T->getDepth(), T->getIndex()))
+    if (T->getDepth() >= TemplateArgs.getNumLevels())
+      return true;
+
+    // There might not be a corresponding template argument before substituting
+    // into the parameter mapping, e.g. a sizeof... expression.
+    if (!TemplateArgs.hasTemplateArgument(T->getDepth(), T->getIndex()))
       return true;
 
     TemplateArgument Arg = TemplateArgs(T->getDepth(), T->getIndex());

clang/test/SemaTemplate/concepts.cpp

@@ -1434,7 +1434,7 @@ template<typename, typename... Ts>
 concept true_types = true_int<sizeof...(Ts), void>;
 
 template<typename, typename... Ts>
-concept true_types2 = true_int<Ts...[0]{1}, void>;
+concept true_types2 = true_int<Ts...[0]{1}, void>; // cxx20-warning {{pack indexing is a C++2c extension}}
 
 template<typename... Ts>
 struct s {