C++: error: template argument deduction/substitution failed / couldn't deduce template parameter

Meaning

Deduction looks only at function arguments. It fails when two arguments give conflicting types (max(1, 2.5)), when the type appears only in the return type, with braced-init-lists, or when an overload or a lambda is passed where a type is expected. Specify the argument explicitly (f<int>(x)) or unify the types.

Common causes

• Conflicting argument types for the same template parameter (int and double)
• Template parameter appears only in the return type
• Braced list {1,2} passed to a deduced parameter
• Passing an overloaded function or generic lambda directly
• A requires clause or enable_if rejected the type

Detailed fix by platform

Code examples

#include <algorithm>

int main() {
    int a = 1; double b = 2.5;
    // auto m = std::max(a, b);          // error: no matching function; deduced conflicting types 'int' and 'double'
    auto m = std::max<double>(a, b);     // OK
    return static_cast<int>(m);
}

How to diagnose

1. Arguments — Do all arguments of one template parameter have the same type?
2. Return — Does the type appear in the parameters?
3. Constraint — Is there a requires/enable_if?

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