Enable bugprone-unhandled-self-assignment

This check looks for suspicious assignment operators that might not
correctly handle self-assignment.  In practice it is a bit prone to
false-positives, but we have so few assignment operators in general
that's not a big deal.

Also enable cert-oop54-cpp which is essentially the same check.  Tweak
its configuration so that it doesn't have more false positives.

Suppress these warnings in one place and fix them in another.

.clang-tidy

@@ -51,8 +51,6 @@ readability-*,\
 -bugprone-narrowing-conversions,\
 -bugprone-redundant-branch-condition,\
 -bugprone-signed-char-misuse,\
--bugprone-unhandled-self-assignment,\
--cert-oop54-cpp,\
 -cert-str34-c,\
 -clang-analyzer-core.CallAndMessage,\
 -clang-analyzer-deadcode.DeadStores,\
@@ -86,9 +84,11 @@ WarningsAsErrors: '*'
 HeaderFilterRegex: '(src|test|tools).*'
 FormatStyle:     none
 CheckOptions:
-  - key: readability-uppercase-literal-suffix.NewSuffixes
-    value: 'L;UL;LL;ULL'
   - key: cata-text-style.EscapeUnicode
     value: 0
+  - key: cert-oop54-cpp.WarnOnlyIfThisHasSuspiciousField
+    value: true
+  - key: readability-uppercase-literal-suffix.NewSuffixes
+    value: 'L;UL;LL;ULL'
 
 # vim:tw=0

src/json.cpp

@@ -486,6 +486,7 @@ JsonArray::JsonArray( const JsonArray &ja )
     final_separator = ja.final_separator;
 }
 
+// NOLINTNEXTLINE(bugprone-unhandled-self-assignment,cert-oop54-cpp)
 JsonArray &JsonArray::operator=( const JsonArray &ja )
 {
     jsin = ja.jsin;

src/safe_reference.cpp

@@ -19,9 +19,11 @@ safe_reference_anchor::safe_reference_anchor( safe_reference_anchor && ) noexcep
     safe_reference_anchor()
 {}
 
-safe_reference_anchor &safe_reference_anchor::operator=( const safe_reference_anchor & )
+safe_reference_anchor &safe_reference_anchor::operator=( const safe_reference_anchor &other )
 {
-    impl = std::make_shared<empty>();
+    if( this != &other ) {
+        impl = std::make_shared<empty>();
+    }
     return *this;
 }