diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -1256,25 +1256,6 @@
   return OpenCLAccessAttr::Keyword_read_only;
 }
 
-static QualType ConvertConstrainedAutoDeclSpecToType(Sema &S, DeclSpec &DS,
-                                                     AutoTypeKeyword AutoKW) {
-  assert(DS.isConstrainedAuto());
-  TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId();
-  TemplateArgumentListInfo TemplateArgsInfo;
-  TemplateArgsInfo.setLAngleLoc(TemplateId->LAngleLoc);
-  TemplateArgsInfo.setRAngleLoc(TemplateId->RAngleLoc);
-  ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
-                                     TemplateId->NumArgs);
-  S.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
-  llvm::SmallVector<TemplateArgument, 8> TemplateArgs;
-  for (auto &ArgLoc : TemplateArgsInfo.arguments())
-    TemplateArgs.push_back(ArgLoc.getArgument());
-  return S.Context.getAutoType(
-      QualType(), AutoKW, false, /*IsPack=*/false,
-      cast<ConceptDecl>(TemplateId->Template.get().getAsTemplateDecl()),
-      TemplateArgs);
-}
-
 /// Convert the specified declspec to the appropriate type
 /// object.
 /// \param state Specifies the declarator containing the declaration specifier
@@ -1655,29 +1636,39 @@
     break;
 
   case DeclSpec::TST_auto:
+  case DeclSpec::TST_decltype_auto: {
+    auto AutoKW = DS.getTypeSpecType() == DeclSpec::TST_decltype_auto
+                      ? AutoTypeKeyword::DecltypeAuto
+                      : AutoTypeKeyword::Auto;
+
+    ConceptDecl *TypeConstraintConcept = nullptr;
+    llvm::SmallVector<TemplateArgument, 8> TemplateArgs;
     if (DS.isConstrainedAuto()) {
-      Result = ConvertConstrainedAutoDeclSpecToType(S, DS,
-                                                    AutoTypeKeyword::Auto);
-      break;
+      if (TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId()) {
+        TypeConstraintConcept =
+            cast<ConceptDecl>(TemplateId->Template.get().getAsTemplateDecl());
+        TemplateArgumentListInfo TemplateArgsInfo;
+        TemplateArgsInfo.setLAngleLoc(TemplateId->LAngleLoc);
+        TemplateArgsInfo.setRAngleLoc(TemplateId->RAngleLoc);
+        ASTTemplateArgsPtr TemplateArgsPtr(TemplateId->getTemplateArgs(),
+                                           TemplateId->NumArgs);
+        S.translateTemplateArguments(TemplateArgsPtr, TemplateArgsInfo);
+        for (const auto &ArgLoc : TemplateArgsInfo.arguments())
+          TemplateArgs.push_back(ArgLoc.getArgument());
+      } else {
+        declarator.setInvalidType(true);
+      }
     }
-    Result = Context.getAutoType(QualType(), AutoTypeKeyword::Auto, false);
+    Result = S.Context.getAutoType(QualType(), AutoKW,
+                                   /*IsDependent*/ false, /*IsPack=*/false,
+                                   TypeConstraintConcept, TemplateArgs);
     break;
+  }
 
   case DeclSpec::TST_auto_type:
     Result = Context.getAutoType(QualType(), AutoTypeKeyword::GNUAutoType, false);
     break;
 
-  case DeclSpec::TST_decltype_auto:
-    if (DS.isConstrainedAuto()) {
-      Result =
-          ConvertConstrainedAutoDeclSpecToType(S, DS,
-                                               AutoTypeKeyword::DecltypeAuto);
-      break;
-    }
-    Result = Context.getAutoType(QualType(), AutoTypeKeyword::DecltypeAuto,
-                                 /*IsDependent*/ false);
-    break;
-
   case DeclSpec::TST_unknown_anytype:
     Result = Context.UnknownAnyTy;
     break;
@@ -5956,6 +5947,8 @@
       if (!DS.isConstrainedAuto())
         return;
       TemplateIdAnnotation *TemplateId = DS.getRepAsTemplateId();
+      if (!TemplateId)
+        return;
       if (DS.getTypeSpecScope().isNotEmpty())
         TL.setNestedNameSpecifierLoc(
             DS.getTypeSpecScope().getWithLocInContext(Context));
diff --git a/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp b/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp
--- a/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp
+++ b/clang/test/CXX/dcl/dcl.spec/dcl.type/dcl.spec.auto/p6.cpp
@@ -68,3 +68,11 @@
   True decltype(auto) h = (b);
   static_assert(is_same_v<decltype(h), int&>);
 }
+
+namespace PR48593 {
+  template <class T, class U> concept a = true;
+  a<B> auto c = 0; // expected-error{{use of undeclared identifier 'B'}}
+
+  template<class> concept d = true;
+  d<,> auto e = 0; // expected-error{{expected expression}}
+}