[clang] [clang] Share normalized constraints between decls with same constraint expressions (PR #226620)

[Younan Zhang via cfe-commits]

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@@ -15186,6 +15186,11 @@ class Sema final : public SemaBase {
   /// here.
   llvm::DenseMap<ConstrainedDeclOrNestedRequirement, NormalizedConstraint *>
       NormalizationCache;
+  /// Caches the normal form of constraint expressions (and their pack
+  /// substitution index). These are shared by e.g. the members of all
+  /// specializations of a class template. Used to fill NormalizationCache.
+  llvm::DenseMap<std::pair<const Expr *, unsigned>, NormalizedConstraint *>
+      NormalizedConstraintExprCache;
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zyn0217 wrote:

> So the constraints of e.g. the constrained constructors of std::optional, std::span, std::pair or of the members of range adaptors were normalized from scratch for every single specialization of those classes that a TU uses. 

Instead of introducing a new cache kv, can we instead use the primary template as the key for NormalizationCache?

https://github.com/llvm/llvm-project/pull/226620