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141de74959
This is yet another one-line patch to fix crashes on constraint
substitution.
```cpp
template <class, class> struct formatter;
template <class, class> struct basic_format_context {};
template <typename CharType>
concept has_format_function = format(basic_format_context<CharType, CharType>());
template <typename ValueType, typename CharType>
requires has_format_function<CharType>
struct formatter<ValueType, CharType> {
template <typename OutputIt>
CharType format(basic_format_context<OutputIt, CharType>);
};
```
In this case, we would build up a `RecoveryExpr` for a call within a
constraint expression due to the absence of viable functions. The
heuristic algorithm attempted to find such a function inside of a
ClassTemplatePartialSpecialization, from which we started to substitute
its requires-expression, and it succeeded with a FunctionTemplate such
that
1) It has only one parameter, which is dependent.
2) The only one parameter depends on two template parameters. They are,
in canonical form, `<template-parameter-1-0>` and
`<template-parameter-0-1>` respectively.
Before we emit an error, we still want to recover the most viable
functions. This goes downhill to deducing template parameters against
its arguments, where we would collect the argument type with the same
depth as the parameter type into a Deduced set. The size of the set is
presumed to be that of function template parameters, which is 1 in this
case. However, since we haven't yet properly set the template depth
before the dance, we'll end up putting the type for
`<template-parameter-0-1>` to the second position of Deduced set, which
is unfortunately an access violation!
The bug seems to appear since clang 12.0.
This fixes [the
case](https://github.com/llvm/llvm-project/issues/58548#issuecomment-1287935336).