[Concepts] Fix MarkUsedTemplateParameters for exprs

D41910 introduced a recursive visitor to MarkUsedTemplateParameters, but
disregarded the 'Depth' parameter, and had incorrect assertions. This fixes
the visitor and removes the assertions.

Author: saarraz

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -5384,46 +5384,40 @@ bool Sema::isTemplateTemplateParameterAtLeastAsSpecializedAs(
   return isAtLeastAsSpecializedAs(*this, PType, AType, AArg, Info);
 }
 
-struct OccurringTemplateParameterFinder :
-    RecursiveASTVisitor<OccurringTemplateParameterFinder> {
-  llvm::SmallBitVector &OccurringIndices;
+namespace {
+struct MarkUsedTemplateParameterVisitor :
+    RecursiveASTVisitor<MarkUsedTemplateParameterVisitor> {
+  llvm::SmallBitVector &Used;
+  unsigned Depth;
 
-  OccurringTemplateParameterFinder(llvm::SmallBitVector &OccurringIndices)
-      : OccurringIndices(OccurringIndices) { }
+  MarkUsedTemplateParameterVisitor(llvm::SmallBitVector &Used,
+                                   unsigned Depth)
+      : Used(Used), Depth(Depth) { }
 
   bool VisitTemplateTypeParmType(TemplateTypeParmType *T) {
-    assert(T->getDepth() == 0 && "This assumes that we allow concepts at "
-                                 "namespace scope only");
-    noteParameter(T->getIndex());
+    if (T->getDepth() == Depth)
+      Used[T->getIndex()] = true;
     return true;
   }
 
   bool TraverseTemplateName(TemplateName Template) {
     if (auto *TTP =
-            dyn_cast<TemplateTemplateParmDecl>(Template.getAsTemplateDecl())) {
-      assert(TTP->getDepth() == 0 && "This assumes that we allow concepts at "
-                                     "namespace scope only");
-      noteParameter(TTP->getIndex());
-    }
-    RecursiveASTVisitor<OccurringTemplateParameterFinder>::
+            dyn_cast<TemplateTemplateParmDecl>(Template.getAsTemplateDecl()))
+      if (TTP->getDepth() == Depth)
+        Used[TTP->getIndex()] = true;
+    RecursiveASTVisitor<MarkUsedTemplateParameterVisitor>::
         TraverseTemplateName(Template);
     return true;
   }
 
   bool VisitDeclRefExpr(DeclRefExpr *E) {
-    if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(E->getDecl())) {
-      assert(NTTP->getDepth() == 0 && "This assumes that we allow concepts at "
-                                      "namespace scope only");
-      noteParameter(NTTP->getIndex());
-    }
+    if (auto *NTTP = dyn_cast<NonTypeTemplateParmDecl>(E->getDecl()))
+      if (NTTP->getDepth() == Depth)
+        Used[NTTP->getIndex()] = true;
     return true;
   }
-
-protected:
-  void noteParameter(unsigned Index) {
-    OccurringIndices.set(Index);
-  }
 };
+}
 
 /// Mark the template parameters that are used by the given
 /// expression.
@@ -5434,7 +5428,8 @@ MarkUsedTemplateParameters(ASTContext &Ctx,
                            unsigned Depth,
                            llvm::SmallBitVector &Used) {
   if (!OnlyDeduced) {
-    OccurringTemplateParameterFinder(Used).TraverseStmt(const_cast<Expr *>(E));
+    MarkUsedTemplateParameterVisitor(Used, Depth)
+        .TraverseStmt(const_cast<Expr *>(E));
     return;
   }