Alias template CTAD with a member template call in constructor noexcept segfaults

[gwllx]

Issue Summary

EDG segfaults on valid C++20 code that uses CTAD through an alias template when:

• the alias template names a class template with fewer template parameters than itself
• the class template has a constructor whose noexcept specification calls one of the class's static members by passing a template parameter pack of the constructor
• and where an object is declared through the alias without template arguments, such that the compiler must perform CTAD.

The construct causing this crash was found when compiling a project using the libstdc++ std::tuple: libstdc++-v3/include/std/tuple#L984.

Reproducing Source Code

template<typename... E> struct S {
  template<typename... U> static constexpr bool n() { return true; }
  S(const E&...) {}
  template<typename... U> S(U&&...) noexcept(n<U...>()) {}
};
template<typename X, typename T> struct F {};
template<typename X, typename... T> using A = S<F<X, T>...>;
A a{F<int, int>{}};

https://godbolt.org/z/P8zh8bEf9

Command-line Options

cpfe --c++20

Type of Issue

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

Additional Details

make_operand_for_rescanned_identifier substitutes the explicit template arguments of a dependent member template-id, then passes the substituted list to scan_identifier, which substitutes it again. When an alias template deduction guide is created, the substituted arguments refer to the new template parameter list, which has the same depth as the guide's list. The second substitution then indexes past the end of the guide's list, and EDG segfaults in get_template_arg_by_list_pos.

Reproducing (Standard) Configuration(s)

linux-gcc-release

Reproducing (Non-standard) Configuration

No response

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• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
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[nv-cmeerw]

Created #532

[nv-cmeerw]

Created #532