[clang] [Clang] speed up -Wassign-enum via enumerator caching (PR #176560)

[Erich Keane via cfe-commits]

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@@ -1761,30 +1761,30 @@ Sema::DiagnoseAssignmentEnum(QualType DstType, QualType SrcType,
     return;
   }
 
-  typedef SmallVector<std::pair<llvm::APSInt, EnumConstantDecl *>, 64>
-      EnumValsTy;
-  EnumValsTy EnumVals;
-
-  // Gather all enum values, set their type and sort them,
-  // allowing easier comparison with rhs constant.
-  for (auto *EDI : ED->enumerators()) {
-    llvm::APSInt Val = EDI->getInitVal();
-    AdjustAPSInt(Val, DstWidth, DstIsSigned);
-    EnumVals.emplace_back(Val, EDI);
+  const EnumDecl *Key = ED->getCanonicalDecl();
+  auto [It, Inserted] = AssignEnumCache.try_emplace(Key);
+  auto &Values = It->second;
+
+  if (Inserted) {
+    Values.reserve(std::distance(ED->enumerator_begin(), ED->enumerator_end()));
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erichkeane wrote:

OH!  Oh darn, that is sad :/  I guess the attempt to enforce O(1) is there, I missed the SFINAE.

Alright, I guess your std::distance version at the top was best, still makes me sad tho :( 

https://github.com/llvm/llvm-project/pull/176560