Do not warn on switches over enums that do not use [[maybe_unused]] enumerators

[dblaikie]

Summary

PR36231, [dcl.attr.unused]p3

[aaron.ballman]

LGTM!

lib/Sema/SemaStmt.cpp

if (!CaseListIsErroneous && !CaseListIsIncomplete && !HasConstantCond &&
 
         case AR_NotYetIntroduced:
           // Partially available enum constants should be present. Note that we
           // suppress -Wunguarded-availability diagnostics for such uses.
         case AR_Available:
           break;
         }
 
+        if (EI->second->hasAttr<UnusedAttr>())
+          continue;
+
         // Drop unneeded case values
         while (CI != CaseVals.end() && CI->first < EI->first)
           CI++;
 
         if (CI != CaseVals.end() && CI->first == EI->first)
           continue;
 
         // Drop unneeded case ranges

test/CXX/dcl.dcl/dcl.attr/dcl.attr.unused/p3.cpp

 // RUN: %clang_cc1 -fsyntax-only -Wunused -Wused-but-marked-unused -std=c++17 -Wc++17-extensions -verify %s
 // RUN: %clang_cc1 -fsyntax-only -Wunused -Wused-but-marked-unused -std=c++11 -Wc++17-extensions -verify -DEXT %s
 
 static_assert(__has_cpp_attribute(maybe_unused) == 201603, "");
 
 struct [[maybe_unused]] S {};
 
+enum E1 {
+  EnumVal [[maybe_unused]],
+  UsedEnumVal,
+};
+
 void f() {
   int x; // expected-warning {{unused variable}}
   typedef int I; // expected-warning {{unused typedef 'I'}}
+  E1 e;
+  switch (e) { // expected-warning {{enumeration value 'UsedEnumVal' not handled in switch}}
+  }
 
   // Should not warn about these due to not being used.
   [[maybe_unused]] int y;
   typedef int maybe_unused_int [[maybe_unused]];
 
   // Should not warn about these uses.
   S s;
   maybe_unused_int test;
   y = 12;
+  switch (e) {
+  case UsedEnumVal:
+    break;
+  }
 }
 
 #ifdef EXT
 // expected-warning@6 {{use of the 'maybe_unused' attribute is a C++17 extension}}
-// expected-warning@13 {{use of the 'maybe_unused' attribute is a C++17 extension}}
-// expected-warning@14 {{use of the 'maybe_unused' attribute is a C++17 extension}}
+// expected-warning@9 {{use of the 'maybe_unused' attribute is a C++17 extension}}
+// expected-warning@9 {{attributes on an enumerator declaration are a C++17 extension}}
+// expected-warning@21 {{use of the 'maybe_unused' attribute is a C++17 extension}}
+// expected-warning@22 {{use of the 'maybe_unused' attribute is a C++17 extension}}
 #endif