diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -8,6 +8,7 @@
 //  This file implements semantic analysis for C++ templates.
 //===----------------------------------------------------------------------===//
 
+#include <iostream> // massberg
 #include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
@@ -1587,6 +1588,17 @@
 
   // Check the well-formedness of the default template argument, if provided.
   if (Default) {
+    // Local variables may not be used as default template arguments.
+    if (auto *DRE = dyn_cast<DeclRefExpr>(Default)) {
+      if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
+        if (VD->isLocalVarDecl()) {
+          Diag(DRE->getLocation(),
+               diag::err_param_default_argument_references_local)
+              << VD->getNameAsString();
+        }
+      }
+    }
+
     // Check for unexpanded parameter packs.
     if (DiagnoseUnexpandedParameterPack(Default, UPPC_DefaultArgument))
       return Param;
diff --git a/clang/test/SemaTemplate/default-template-arguments.cpp b/clang/test/SemaTemplate/default-template-arguments.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaTemplate/default-template-arguments.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+
+namespace GH48755 {
+constexpr auto z = 2;
+
+auto f() {
+  const auto x = 1;
+
+  auto lambda1 = [] <auto y = x> {}; // expected-error {{default argument references local variable x of enclosing function}}
+  auto lambda2 = [] <auto y = 42> {};
+  auto lambda3 = [] <auto y = z> {};
+  auto lambda4 = [] <auto y> {};
+}
+}