Regression with lambda return type deduction

[AaronBallman]

The following used to compile without diagnostic in Clang 20.1:

template<class T, class U>
struct is_same {
  static constexpr bool value = false;
};
 
template<class T>
struct is_same<T, T> {
  static constexpr bool value = true;
};

template <typename T, int N>
struct A {
      static_assert([]() constexpr { // error: value of type 'void' is not contextually convertible to 'bool'
        if constexpr (is_same<T, int>::value)
          return true;
        else
          return (10 != N);
      }());
};                                                                                                                                            
int f1() {
    if constexpr (false) {
        A<int, 256> a;
    }
    return 0;
} 

but now gives an error on trunk: https://godbolt.org/z/fd5WY4xhY

Note, if you remove the if constexpr (false) from f1(), then the code compiles. If you add -> bool to the lambda, the code also compiles.

The code compiles without diagnostics in GCC, MSVC, and EDG.

[llvmbot]

@llvm/issue-subscribers-c-17

Author: Aaron Ballman (AaronBallman)

The following used to compile without diagnostic in Clang 20.1: ``` template<class T, class U> struct is_same { static constexpr bool value = false; };

template<class T>
struct is_same<T, T> {
static constexpr bool value = true;
};

template <typename T, int N>
struct A {
static_assert( constexpr { // error: value of type 'void' is not contextually convertible to 'bool'
if constexpr (is_same<T, int>::value)
return true;
else
return (10 != N);
}());
};
int f1() {
if constexpr (false) {
A<int, 256> a;
}
return 0;
}

but now gives an error on trunk: https://godbolt.org/z/fd5WY4xhY

Note, if you remove the `if constexpr (false)` from `f1()`, then the code compiles. If you add `-> bool` to the lambda, the code also compiles.

The code compiles without diagnostics in GCC, MSVC, and EDG.
</details>

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Aaron Ballman (AaronBallman)

The following used to compile without diagnostic in Clang 20.1: ``` template<class T, class U> struct is_same { static constexpr bool value = false; };

template<class T>
struct is_same<T, T> {
static constexpr bool value = true;
};

template <typename T, int N>
struct A {
static_assert( constexpr { // error: value of type 'void' is not contextually convertible to 'bool'
if constexpr (is_same<T, int>::value)
return true;
else
return (10 != N);
}());
};
int f1() {
if constexpr (false) {
A<int, 256> a;
}
return 0;
}

but now gives an error on trunk: https://godbolt.org/z/fd5WY4xhY

Note, if you remove the `if constexpr (false)` from `f1()`, then the code compiles. If you add `-> bool` to the lambda, the code also compiles.

The code compiles without diagnostics in GCC, MSVC, and EDG.
</details>

[frederick-vs-ja]

Slightly simplified (Godbolt link):

template <typename>
struct A {
  static_assert([]() constexpr { return true; }());
};

void f1() {
  if constexpr (false) {
    A<int> a;
  }
}

This bug is very similar to #140934. I don't know why #140859 hasn't fixed it.

CC @cor3ntin

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Aaron Ballman (AaronBallman)

The following used to compile without diagnostic in Clang 20.1: ``` template<class T, class U> struct is_same { static constexpr bool value = false; };

template<class T>
struct is_same<T, T> {
static constexpr bool value = true;
};

template <typename T, int N>
struct A {
static_assert( constexpr { // error: value of type 'void' is not contextually convertible to 'bool'
if constexpr (is_same<T, int>::value)
return true;
else
return (10 != N);
}());
};
int f1() {
if constexpr (false) {
A<int, 256> a;
}
return 0;
}

but now gives an error on trunk: https://godbolt.org/z/fd5WY4xhY

Note, if you remove the `if constexpr (false)` from `f1()`, then the code compiles. If you add `-> bool` to the lambda, the code also compiles.

The code compiles without diagnostics in GCC, MSVC, and EDG.
</details>

[efriedma-quic]

Reduced more:

void f1() {
  if constexpr (false) {
    static_assert([]{});
  }
}