diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -267,17 +267,17 @@
   `GH55216 <https://github.com/llvm/llvm-project/issues/55216>`_.
 - Correctly set expression evaluation context as 'immediate function context' in
   consteval functions.
-  This fixes `GH51182 <https://github.com/llvm/llvm-project/issues/51182>`_.
-
+  This fixes `GH51182 <https://github.com/llvm/llvm-project/issues/51182>`
 - Fixes an assert crash caused by looking up missing vtable information on ``consteval``
   virtual functions. Fixes `GH55065 <https://github.com/llvm/llvm-project/issues/55065>`_.
-
 - Skip rebuilding lambda expressions in arguments of immediate invocations.
   This fixes `GH56183 <https://github.com/llvm/llvm-project/issues/56183>`_,
   `GH51695 <https://github.com/llvm/llvm-project/issues/51695>`_,
   `GH50455 <https://github.com/llvm/llvm-project/issues/50455>`_,
   `GH54872 <https://github.com/llvm/llvm-project/issues/54872>`_,
   `GH54587 <https://github.com/llvm/llvm-project/issues/54587>`_.
+- Fixed a crash during template instantiation on a conversion operator during constraint
+  evaulation. Fixes `GH50891 <https://github.com/llvm/llvm-project/issues/50891>`_.
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -4020,6 +4020,9 @@
   //
   // Note: SecondStepOfCopyInit is only ever true in this case when
   // evaluating whether to produce a C++98 compatibility warning.
+  //
+  // The last check avoids an infinite template expansion loop in
+  // requirements checking by skipping the conversion functions check.
   if (S.getLangOpts().CPlusPlus17 && Args.size() == 1 &&
       !SecondStepOfCopyInit) {
     Expr *Initializer = Args[0];
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -7565,6 +7565,24 @@
     return;
   }
 
+  // We won't go through a user-defined type conversion function to convert a
+  // derived to base as such conversions are given Conversion Rank. They only
+  // go through a copy constructor. 13.3.3.1.2-p4 [over.ics.user]
+  // Peforming this check here avoids possible infinite template expansion
+  // in the following call to DeduceTemplateArguments().
+  QualType FromCanon =
+      Context.getCanonicalType(From->getType().getUnqualifiedType());
+  QualType ToCanon = Context.getCanonicalType(ToType).getUnqualifiedType();
+  if (FromCanon == ToCanon ||
+      IsDerivedFrom(CandidateSet.getLocation(), FromCanon, ToCanon)) {
+    OverloadCandidate &Candidate = CandidateSet.addCandidate();
+    Candidate.FoundDecl = FoundDecl;
+    Candidate.Function = FunctionTemplate->getTemplatedDecl();
+    Candidate.Viable = false;
+    Candidate.FailureKind = ovl_fail_trivial_conversion;
+    return;
+  }
+
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   CXXConversionDecl *Specialization = nullptr;
   if (TemplateDeductionResult Result
diff --git a/clang/test/SemaTemplate/concepts.cpp b/clang/test/SemaTemplate/concepts.cpp
--- a/clang/test/SemaTemplate/concepts.cpp
+++ b/clang/test/SemaTemplate/concepts.cpp
@@ -262,3 +262,18 @@
 template <class> struct S {};
 void f(C<GH55567::S> auto);
 } // namespace GH55567
+
+namespace Issue50891 {
+template <typename T>
+concept Numeric =
+    requires(T a) {
+        foo(a);
+    };
+
+struct Deferred {
+    friend void foo(Deferred);
+    template <Numeric TO> operator TO();
+};
+
+static_assert(Numeric<Deferred>); // should not crash clang
+} // namespace Issue50891