diff --git a/clang-tools-extra/clang-tidy/modernize/UseNoexceptCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseNoexceptCheck.cpp
--- a/clang-tools-extra/clang-tidy/modernize/UseNoexceptCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseNoexceptCheck.cpp
@@ -77,13 +77,16 @@
                   .getExceptionSpecRange();
   }
 
+  assert(FnTy && "FunctionProtoType is null.");
+  if (isUnresolvedExceptionSpec(FnTy->getExceptionSpecType()))
+    return;
+
   assert(Range.isValid() && "Exception Source Range is invalid.");
 
   CharSourceRange CRange = Lexer::makeFileCharRange(
       CharSourceRange::getTokenRange(Range), *Result.SourceManager,
       Result.Context->getLangOpts());
 
-  assert(FnTy && "FunctionProtoType is null.");
   bool IsNoThrow = FnTy->isNothrow();
   StringRef ReplacementStr =
       IsNoThrow
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize-use-noexcept-opt.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize-use-noexcept-opt.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize-use-noexcept-opt.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize-use-noexcept-opt.cpp
@@ -4,6 +4,7 @@
 // This test is not run in C++17 or later because dynamic exception
 // specifications were removed in C++17.
 
+using size_t = __SIZE_TYPE__;
 class A {};
 class B {};
 
@@ -19,6 +20,11 @@
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: dynamic exception specification 'throw(int(int))' is deprecated; consider removing it instead [modernize-use-noexcept]
 // CHECK-FIXES: void k() ;
 
+// Shouldn't crash due to llvm_unreachable in canThrow() on EST_Uninstantiated
+template <int> class c { void *operator new(size_t) throw (int);};
+void s() { c<1> doesnt_crash; }
+// CHECK-MESSAGES: :[[@LINE-2]]:53: warning: dynamic exception specification 'throw (int)' is deprecated; consider removing it instead [modernize-use-noexcept]
+
 void foobar() throw(A, B)
 {}
 // CHECK-MESSAGES: :[[@LINE-2]]:15: warning: dynamic exception specification 'throw(A, B)' is deprecated; consider removing it instead [modernize-use-noexcept]