[clang] [Clang][Modules] Fix generic lambda local capture mapping mismatch in template instantiation (PR #202248)

[Ivo Popov via cfe-commits]

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@@ -3454,9 +3454,21 @@ uint64_t ASTWriter::WriteDeclContextLexicalBlock(ASTContext &Context,
   if (DC->decls_empty())
     return 0;
 
-  // In reduced BMI, we don't care the declarations in functions.
-  if (GeneratingReducedBMI && DC->isFunctionOrMethod())
-    return 0;
+  // In reduced BMI, we don't care the declarations in functions, unless they
+  // are templated functions, because their bodies may contain nested class/lambda
+  // definitions that are canonicalized and need mapping of local declarations.
+  if (GeneratingReducedBMI && DC->isFunctionOrMethod()) {
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ipopov wrote:

You're right about the existing test - sorry... (as Reduced BMI is C++20 modules-specific.)

But I think that the underlying deserialization bug will recur when templates containing generic lambdas are defined in C++20 named modules (compiled with Reduced BMI) and then imported or mixed with header modules.

I added a separate C++20 named modules test (`modules-generic-lambda-local-mapping-bmi.cpp`) that compiles using `-emit-reduced-module-interface`. It crashes on the assertion without the `ASTWriter` fix, and passes with it.

LMK if this kind of mixing of C++20 modules with Clang header modules is unrealistic. I figured since it can trigger an alert, it's worth fixing.

Thanks for having a look!

https://github.com/llvm/llvm-project/pull/202248