Index: clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
===================================================================
--- clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
+++ clang-tools-extra/clang-tidy/readability/UppercaseLiteralSuffixCheck.cpp
@@ -134,6 +134,11 @@
       CharSourceRange::getTokenRange(*Range), SM, LO, &Invalid);
   assert(!Invalid && "Failed to retrieve the source text.");
 
+  // Make sure the first character is actually a digit
+  // https://bugs.llvm.org/show_bug.cgi?id=51790
+  if (!std::isdigit(static_cast<unsigned char>(LiteralSourceText.front())))
+    return llvm::None;
+
   size_t Skip = 0;
 
   // Do we need to ignore something before actually looking for the suffix?
Index: clang-tools-extra/test/clang-tidy/checkers/readability-uppercase-literal-suffix-integer.cpp
===================================================================
--- clang-tools-extra/test/clang-tidy/checkers/readability-uppercase-literal-suffix-integer.cpp
+++ clang-tools-extra/test/clang-tidy/checkers/readability-uppercase-literal-suffix-integer.cpp
@@ -270,3 +270,17 @@
 void d();
 void d() { c<b>(); }
 } // namespace
+
+// Check that non-type template parameters do not cause any diags.
+// https://bugs.llvm.org/show_bug.cgi?id=51790
+template <int capacity>
+struct Vector {
+  static constexpr int kCapacity = capacity;
+};
+
+template <int capacity>
+constexpr int Vector<capacity>::kCapacity;
+
+void test_vector() {
+  int x = Vector<10>::kCapacity;
+}