[LLVMbugs] [Bug 24252] New: Produce more information when template substitution fails

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=24252

            Bug ID: 24252
           Summary: Produce more information when template substitution
                    fails
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dnovillo at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

For the following code:

template <class T> typename T::inner_type Foo() {}
template <class T> decltype(Foo<T>()) Bar() {}
template <class T> decltype(Bar<T>()) Baz() {}

int main() {
  Baz<int>();
}

Clang produces:

$ clang -c -std=c++11 foo.cc
16947782.cpp:6:3: error: no matching function for call to 'Baz'
  Baz<int>();
  ^~~~~~~~
foo.cc:3:39: note: candidate template ignored: substitution failure [with T =
int]: no matching function for call to 'Bar'
template <class T> decltype(Bar<T>()) Baz() {}
                            ~~~       ^

That's not very helpful because it doesn't say why there was no matching
function to call 'Bar'. GCC says this:

error: no matching function for call to 'Baz()'
note: candidate is:
note: template<class T> decltype (Bar<T>()) n::{anonymous}::Baz()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> decltype (Bar<T>())
n::{anonymous}::Baz() [with T = int]':
  required from here
error: no matching function for call to 'Bar()'
note: candidate is:
note: template<class T> decltype (Foo<T>()) n::{anonymous}::Bar()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> decltype (Foo<T>())
n::{anonymous}::Bar() [with T = int]':
  required by substitution of 'template<class T> decltype (Bar<T>())
n::{anonymous}::Baz() [with T = int]'
  required from here
error: no matching function for call to 'Foo()'
note: candidate is:
note: template<class T> typename T::inner_type n::{anonymous}::Foo()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> typename T::inner_type
n::{anonymous}::Foo() [with T = int]':
  required by substitution of 'template<class T> decltype (Foo<T>())
n::{anonymous}::Bar() [with T = int]'
  required by substitution of 'template<class T> decltype (Bar<T>())
n::{anonymous}::Baz() [with T = int]'
  required from here
error: 'int' is not a class, struct, or union type

That's better. We can't call Baz<int>() because we can't call Bar<int>()
because we can't call Foo<int>() because 'int' is not a class. Would be nice if
clang could produce a similar error message.

Note that GCC will print a substitution stack for every overload that gets
disabled by SFINAE, which effectively becomes a substitution tree. This is very
useful. Here's an example:

template <class T> typename T::inner_type Foo() {}
template <class T> decltype(Foo<T>()) Bar() {}

// Baz is overloaded.
template <class T> decltype(Bar<T>()) Baz() {}
template <class T> decltype(Bar<const T>()) Baz() {}

int main() {
  Baz<int>();
}

GCC says:

error: no matching function for call to 'Baz()'
note: candidates are:
note: template<class T> decltype (Bar<T>()) n::{anonymous}::Baz()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> decltype (Bar<T>())
n::{anonymous}::Baz() [with T = int]':
  required from here
error: no matching function for call to 'Bar()'
note: candidate is:
note: template<class T> decltype (Foo<T>()) n::{anonymous}::Bar()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> decltype (Foo<T>())
n::{anonymous}::Bar() [with T = int]':
  required by substitution of 'template<class T> decltype (Bar<T>())
n::{anonymous}::Baz() [with T = int]'
  required from here
error: no matching function for call to 'Foo()'
note: candidate is:
note: template<class T> typename T::inner_type n::{anonymous}::Foo()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> typename T::inner_type
n::{anonymous}::Foo() [with T = int]':
  required by substitution of 'template<class T> decltype (Foo<T>())
n::{anonymous}::Bar() [with T = int]'
  required by substitution of 'template<class T> decltype (Bar<T>())
n::{anonymous}::Baz() [with T = int]'
  required from here
error: 'int' is not a class, struct, or union type
note: template<class T> decltype (Bar<const T>()) n::{anonymous}::Baz()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> decltype (Bar<const T>())
n::{anonymous}::Baz() [with T = int]':
  required from here
error: no matching function for call to 'Bar()'
note: candidate is:
note: template<class T> decltype (Foo<T>()) n::{anonymous}::Bar()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> decltype (Foo<T>())
n::{anonymous}::Bar() [with T = const int]':
  required by substitution of 'template<class T> decltype (Bar<const T>())
n::{anonymous}::Baz() [with T = int]'
  required from here
error: no matching function for call to 'Foo()'
note: candidate is:
note: template<class T> typename T::inner_type n::{anonymous}::Foo()
note:   template argument deduction/substitution failed:
 In substitution of 'template<class T> typename T::inner_type
n::{anonymous}::Foo() [with T = const int]':
  required by substitution of 'template<class T> decltype (Foo<T>())
n::{anonymous}::Bar() [with T = const int]'
  required by substitution of 'template<class T> decltype (Bar<const T>())
n::{anonymous}::Baz() [with T = int]'
  required from here
error: 'const int' is not a class, struct, or union type

Note how GCC explains why neither of the two overloads of Baz() can be called.

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