[llvm] [AsmParser][NFCI] Restructure DiagnosticPredicate (PR #126653)

[Sam Elliott via llvm-commits]

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@@ -198,21 +192,36 @@ enum class DiagnosticPredicateTy {
 // This is a light-weight alternative to the 'NearMissInfo' approach
 // below which collects *all* possible diagnostics. This alternative
 // is optional and fully backward compatible with existing
-// PredicateMethods that return a 'bool' (match or no match).
-struct DiagnosticPredicate {
-  DiagnosticPredicateTy Type;
-
-  explicit DiagnosticPredicate(bool Match)
-      : Type(Match ? DiagnosticPredicateTy::Match
-                   : DiagnosticPredicateTy::NearMatch) {}
-  DiagnosticPredicate(DiagnosticPredicateTy T) : Type(T) {}
-  DiagnosticPredicate(const DiagnosticPredicate &) = default;
+// PredicateMethods that return a 'bool' (match or near match).
+class DiagnosticPredicate {
+  enum class PredicateTy {
+    Match,     // Matches
+    NearMatch, // Close Match: use Specific Diagnostic
+    NoMatch,   // No Match: use `InvalidOperand`
+  } Predicate;
+
+public:
+#if __cplusplus >= 202002L
+  using enum PredicateTy;
+#else
+  static constexpr PredicateTy Match = PredicateTy::Match;
+  static constexpr PredicateTy NearMatch = PredicateTy::NearMatch;
+  static constexpr PredicateTy NoMatch = PredicateTy::NoMatch;
+#endif
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lenary wrote:

> [DiagnosticPredicate] is not used often and doesn't seem to require conversions from bool or other types.

This is not true. It's used a lot in the asm matcher table-generated code (for the returns from `PredicateMethod` on AsmOperands), and almost all backends depend on the conversion from bool. AArch64 is one of the few backends to not depend on the conversions. See here: https://github.com/llvm/llvm-project/blob/d6cf04c401c029cf96b001ae5782693c78c91469/llvm/utils/TableGen/AsmMatcherEmitter.cpp#L2496



https://github.com/llvm/llvm-project/pull/126653