diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -688,6 +688,29 @@
                                 Not(WithFix(_)))));
 }
 
+TEST(DiagnosticsTest, Issue890) {
+  Annotations Header(R"cpp(
+template<char = 'x'> struct S;
+
+template<char> struct S {
+    template<typename U> requires(false) S(U);
+    template<typename U> requires(true) S(U);
+};
+
+template<typename T> S(T) -> S<>;
+  )cpp");
+  Annotations Main(R"cpp(
+    #include "a.h"
+    void foo() {
+     S(0); // Simply ::x; is enough to crash!
+    }
+  )cpp");
+  TestTU TU = TestTU::withCode(Main.code());
+  TU.ExtraArgs.push_back("-std=c++20");
+  TU.AdditionalFiles = {{"a.h", std::string(Header.code())}};
+  EXPECT_THAT(*TU.build().getDiagnostics(), IsEmpty());
+}
+
 TEST(ClangdTest, MSAsm) {
   // Parsing MS assembly tries to use the target MCAsmInfo, which we don't link.
   // We used to crash here. Now clang emits a diagnostic, which we filter out.
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
@@ -3740,6 +3740,9 @@
     NamedDecl *FromParam = FromTP->getParam(I);
     NamedDecl *ToParam = ToTP->getParam(I);
 
+    if (FromParam == ToParam)
+      continue;
+
     if (auto *FTTP = dyn_cast<TemplateTypeParmDecl>(FromParam))
       inheritDefaultTemplateArgument(Context, FTTP, ToParam);
     else if (auto *FNTTP = dyn_cast<NonTypeTemplateParmDecl>(FromParam))