Bug 3072 – tuples can't be aliases ( except when the aliases are templates. )

D1.0 import std.stdio; template Ta(alias t){void Tfn(){writef(t);}} template Tt(t...){void Tfn(){writef(t);}} struct S { int i; int j; int k; mixin Ta!(i); // passes mixin Tt!(i,j); // fails } this causes issues for mixin function that are supposed to work on a subset of the members of the enclosing type. The string mixin work around is just flat ugly. I don't think the spec says this shouldn't work so I'm calling it a bug.

In general template tuples can't contain alias template parameters but it appears that there is an exception to this rule when the parameter is itself a template. DMD 2.049 T foo(T)(T t) { return t; } template map1(fun...) { enum map1 = 42; } template map2(T, U...) { static if(U.length > 0) enum map2 = map2!(U); else enum map2 = 42; } void main() { auto x = map1!(char,foo); // compiles auto y = map2!(char,foo); // doesn't compile auto z = map1!(char,map1); // compiles auto w = map2!(char,map1); // doesn't compile return; } I've added the accepts-invalid, rejects-valid and spec keywords, since tuples of aliases should either work or not-work and the behavior should be documented in the spec. I'd lean towards not-work, since tuples of aliases seem to be buggy (i.e. Issue 5087) not to mention that alias parameters are buggy(i.e. issue 5082)

With 1.076, the OP code in comment#0 compiles successfully. And with 2.067a, dmd accepts the following code. import std.stdio; template Ta(alias t){ void Tfn() { writeln(t); } } template Tt(t...) { void Tfn() { writeln(t); } } struct S { int i; int j; int k; mixin Ta!(i); // passes mixin Tt!(i,j); // also passes } Therefore the original issue is already fixed in D1 and D2.

(In reply to Rob Jacques from comment #1) > In general template tuples can't contain alias template parameters but it > appears that there is an exception to this rule when the parameter is itself > a template. > > DMD 2.049 > > T foo(T)(T t) { return t; } > template map1(fun...) { > enum map1 = 42; > } > template map2(T, U...) { > static if(U.length > 0) enum map2 = map2!(U); > else enum map2 = 42; > } > > void main() { > auto x = map1!(char,foo); // compiles > auto y = map2!(char,foo); // doesn't compile > auto z = map1!(char,map1); // compiles > auto w = map2!(char,map1); // doesn't compile > return; > } > > I've added the accepts-invalid, rejects-valid and spec keywords, since > tuples of aliases should either work or not-work and the behavior should be > documented in the spec. I'd lean towards not-work, since tuples of aliases > seem to be buggy (i.e. Issue 5087) not to mention that alias parameters are > buggy(i.e. issue 5082) This test case is invalid. map2 is recursively instantiated, but in the line: > static if(U.length > 0) enum map2 = map2!(U); the instantiation map2!(foo) or map2!(map1) fails because any template symbols cannot match to the type parameter T.