[clang] [Clang] Transform lambda's constraints when instantiating parameter mapping (PR #195995)

[Matheus Izvekov via cfe-commits]

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@@ -1876,11 +1901,25 @@ namespace {
                               TemplateParameterList *OrigTPL)  {
       if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
 
+      std::optional<MultiLevelTemplateArgumentList> OldMLTAL;
+      // We need to preserve the lambda depth in parameter mapping.
+      // Otherwise the template argument deduction would fail, if we reduced the
+      // depth too early.
+      if (SemaRef.inParameterMappingSubstitution() &&
+          getDepthAndIndex(OrigTPL->getParam(0)).first >=
+              TemplateArgs.getNumSubstitutedLevels())
+        OldMLTAL = ForgetSubstitution();
----------------
mizvekov wrote:

Okay, I think I understand now what you are trying to do. I initially misunderstood the problem as you trying to avoid getting the lambda's depth to go below zero.

I am not sure I understand why changing the depth is a problem for deduction, because deduction accepts the template depth as a parameter and this should be easy to figure out for the lambda since it has template parameters.

Anyway, you can use the `getDepth` method on the template parameter list directly:
```suggestion
      std::optional<MultiLevelTemplateArgumentList> OldMLTAL;
      // We need to preserve the lambda depth in parameter mapping.
      // Otherwise the template argument deduction would fail, if we reduced the
      // depth too early.
      if (SemaRef.inParameterMappingSubstitution() &&
         OrigTPL->getDepth() >= TemplateArgs.getNumSubstitutedLevels())
        OldMLTAL = ForgetSubstitution();
```


https://github.com/llvm/llvm-project/pull/195995