diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3781,9 +3781,13 @@
 
   // If the declaration declares a template, it may inherit default arguments
   // from the previous declaration.
-  if (auto *TD = dyn_cast<TemplateDecl>(D))
-    inheritDefaultTemplateArguments(Reader.getContext(),
-                                    cast<TemplateDecl>(Previous), TD);
+  if (auto *TD = dyn_cast<TemplateDecl>(D)) {
+    // CXXDeductionGuideDecls reference the class template parameters so we need
+    // to make sure not to call this twice on the same template parameters.
+    if (!isa<CXXDeductionGuideDecl>(TD->getTemplatedDecl()))
+      inheritDefaultTemplateArguments(Reader.getContext(),
+                                      cast<TemplateDecl>(Previous), TD);
+  }
 
   // If any of the declaration in the chain contains an Inheritable attribute,
   // it needs to be added to all the declarations in the redeclarable chain.
diff --git a/clang/test/Modules/Inputs/ctad/a.h b/clang/test/Modules/Inputs/ctad/a.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/ctad/a.h
@@ -0,0 +1,7 @@
+#pragma once
+
+template<typename T=int> struct A {
+  A(T) {}
+};
+
+A _a(2);
diff --git a/clang/test/Modules/Inputs/ctad/b.h b/clang/test/Modules/Inputs/ctad/b.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/ctad/b.h
@@ -0,0 +1,7 @@
+#pragma once
+
+template<typename T=int> struct A {
+  A(T) {}
+};
+
+A _b(5);
diff --git a/clang/test/Modules/Inputs/ctad/module.modulemap b/clang/test/Modules/Inputs/ctad/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/ctad/module.modulemap
@@ -0,0 +1,2 @@
+module a { header "a.h" export * }
+module b { header "b.h" export * }
diff --git a/clang/test/Modules/ctad.cpp b/clang/test/Modules/ctad.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/ctad.cpp
@@ -0,0 +1,12 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -x c++ -emit-module %S/Inputs/ctad/module.modulemap -fmodule-name=a -o %t/a.pcm -std=c++17
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -x c++ -emit-module %S/Inputs/ctad/module.modulemap -fmodule-name=b -o %t/b.pcm -std=c++17
+// RUN: %clang_cc1 -fmodules -fno-implicit-modules -x c++ -emit-llvm -I%S/Inputs/ctad -o - %s -fmodule-file=%t/a.pcm -fmodule-file=%t/b.pcm -std=c++17 | FileCheck %s
+
+// CHECK: @_a = global
+// CHECK: @_b = global
+// CHECK: call void @__cxx_global_var_init()
+// CHECK: call void @__cxx_global_var_init.1()
+
+#include "a.h"
+#include "b.h"