Alias template CTAD with a constructor trailing requires clause causes segfault

[gwllx]

Issue Summary

EDG segfaults (in ASan or debug builds) on valid C++20 code that uses CTAD through an alias template when:

• the alias names a class template with a template parameter pack
• the class template's constructor has a trailing requires clause referring to a member of the class
• and where an object is declared through the alias causing the compiler to perform CTAD.

The construct causing this crash was found when compiling a project using the libstdc++ std::tuple (which uses trailing requires clauses) via an alias, e.g.: template<class T, class... Ts> using A = std::tuple<S<T, Ts>...>;.

The issue is similar to but distinct from #522.

Reproducing Source Code

// note: visible errors only appear with debug or asan build
template<typename... E> struct S {
  static constexpr bool c = true;
  S(E...) requires c {}
};
template<typename... T> using A = S<T...>;
A a{1};

Note: this does not crash with an optimized/release build. An ASan or debug build is required.

Command-line Options

cpfe --c++20

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

create_transformed_deduction_guide_for_alias_template declares pack_mapping inside an inner block. ctws_state.alias_parameter_pack_mapping still points to it when the guide requires-clause is substituted after that block ends, so EDG reads an object after it has been freed.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

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[nv-cmeerw]

Created #533