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

[Younan Zhang via cfe-commits]

================
@@ -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();
----------------
zyn0217 wrote:

> 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.

It's all about the collection of explicit template arguments: `PackDeductionScope::finishConstruction` requires a depth-level-match of template parameters.

And I don't think we can add template depth to deduction to the code path that performs a deduction on a function call? we seem to always assume their depths are 0.

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