Error when explicitly specifying return type of generic lambda in constraint?

[Eczbek]

https://godbolt.org/z/Yfsdrnn4E

struct foo {
	foo(const foo&) = default;

	foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
};

struct bar { foo x; };

int main() {}

<source>:4:21: error: substitution into constraint expression resulted in a non-constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:1:8: note: while checking constraint satisfaction for template 'foo<foo>' required here
    1 | struct foo {
      |        ^~~
<source>:1:8: note: in instantiation of function template specialization 'foo::foo<foo>' requested here
<source>:7:8: note: while declaring the implicit copy constructor for 'bar'
    7 | struct bar { foo x; };
      |        ^
<source>:4:21: note: undefined function 'operator()<0>' cannot be used in a constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^
<source>:4:21: note: declared here
<source>:4:21: error: substitution into constraint expression resulted in a non-constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:1:8: note: while checking constraint satisfaction for template 'foo<foo>' required here
    1 | struct foo {
      |        ^~~
<source>:1:8: note: while substituting deduced template arguments into function template 'foo' [with auto:1 = foo]
<source>:7:8: note: while declaring the implicit move constructor for 'bar'
    7 | struct bar { foo x; };
      |        ^
<source>:4:21: note: undefined function 'operator()<0>' cannot be used in a constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^
<source>:4:21: note: declared here
2 errors generated.
Compiler returned: 1

[Eczbek]

This compiles just fine if I remove -> bool:
https://godbolt.org/z/aqGjcrd35

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Eczbek)

https://godbolt.org/z/Yfsdrnn4E

struct foo {
	foo(const foo&) = default;

	foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
};

struct bar { foo x; };

int main() {}

<source>:4:21: error: substitution into constraint expression resulted in a non-constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:1:8: note: while checking constraint satisfaction for template 'foo<foo>' required here
    1 | struct foo {
      |        ^~~
<source>:1:8: note: in instantiation of function template specialization 'foo::foo<foo>' requested here
<source>:7:8: note: while declaring the implicit copy constructor for 'bar'
    7 | struct bar { foo x; };
      |        ^
<source>:4:21: note: undefined function 'operator()<0>' cannot be used in a constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^
<source>:4:21: note: declared here
<source>:4:21: error: substitution into constraint expression resulted in a non-constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:1:8: note: while checking constraint satisfaction for template 'foo<foo>' required here
    1 | struct foo {
      |        ^~~
<source>:1:8: note: while substituting deduced template arguments into function template 'foo' [with auto:1 = foo]
<source>:7:8: note: while declaring the implicit move constructor for 'bar'
    7 | struct bar { foo x; };
      |        ^
<source>:4:21: note: undefined function 'operator()<0>' cannot be used in a constant expression
    4 |         foo(auto) requires([]<int = 0> -> bool { return true; }()) {}
      |                            ^
<source>:4:21: note: declared here
2 errors generated.
Compiler returned: 1

[zyn0217]

This is basically #35052

In Sema::MarkFunctionReferenced(), we incorrectly perceived the lambda class as a "local" class because the parent of the lambda is the CXXConstructorDecl, so we deferred its body instantiation until the end of the function body, which is too late for the ongoing constraint evaluation.