Error: template instance does not match template declaration

Steven Schveighoffer schveiguy at
Wed Nov 17 18:00:59 UTC 2021

On 11/17/21 7:55 AM, Vitalii wrote:
> Thank you for response, Tejas!
> What I intended to do was make a class with only two states ("Inspect" - 
> do some analysis, "Execute" - do some stuff). That's why I tried to use 
> template specialization.

Template parameters are of 3 types:

1. A type parameter. This has a single symbol name, and represents a 
type *provided by the caller*.

enum Mode { a, b }
class Foo(Mode) {
   // inside here, `Mode` is a locally named type, not the enum
// usage

2. A template value. This is specified by using 2 names, the type of the 
value, and the name for the value to be used internally.

class Foo(Mode mode) {
    // NOW `Mode` is referring to the type of `mode`, and
    // `Mode` is the external type, not a local type

// usage

3. An alias to a symbol, type, or value. This is specified using the 
keyword `alias` and then a name for the aliased symbol or value. This is 
probably the most versatile, and you can use this as a last resort if 
you aren't specifically interested in types or values, or you want a 
value that is of unknown type.

class Foo(alias mode) {
    // `mode` can be anything.

// usage

Aside from those 3 basic template parameter mechanisms, there is 
specialization which provide a *specialized* template implementation *if 
the arguments match or implicitly can convert to the specialization*. I 
know there are some languages that use this syntax to specify the type 
of a parameter, but D does not. Note that alias specialization is not 

enum Mode { a, b}
class Example(T : int) { pragma(msg, "specialized ", typeof(this)); }
class Example(T : Mode) { pragma(msg, "specialized2 ", typeof(this)); }
class Example(T) { pragma(msg, "unspecialized ", typeof(this)); }

Example!int a; // => "specialized Example!int"
Example!short b; // => "specialized Example!short"
Example!string c; // => "unspecialized Example!string"
Example!Mode d; // => "specialized2 Example!Mode"

class Example2(Mode mode : Mode.a) { pragma(msg, "specialized ", 
typeof(this)); }
class Example2(Mode mode) { pragma(msg, "unspecialized ", typeof(this)); }

Example2!(Mode.a) e; // specialized Example2!Mode.a
Example2!(Mode.b) f; // unspecialized Example2!Mode.b

I hope this helps you understand how to properly write these.


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