[clang] [Clang] fix crash with generic lambdas in function default args (PR #222987)

[Oleksandr Tarasiuk via cfe-commits]

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@@ -7756,6 +7756,12 @@ void Parser::ParseParameterDeclarationClause(
           DelayTemplateIdDestructionRAII DontDestructTemplateIds(
               *this, /*DelayTemplateIdDestruction=*/true);
 
+          TemplateParameterDepthRAII CurTemplateDepthTracker(
+              TemplateParameterDepth);
+          unsigned Depth = Actions.getTemplateDepth(getCurScope());
+          if (Depth > TemplateParameterDepth)
+            CurTemplateDepthTracker.addDepth(Depth - TemplateParameterDepth);
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a-tarasyuk wrote:

I reworked this to track the implicit template scope directly and use `getInventedTemplateParameterList` when re-entering it. I also added tests for deduced lambda types and non-type parameters. Defaults before the first `auto` are still outside the scope of this fix... 

https://github.com/llvm/llvm-project/pull/222987