Bug 15623 – is(M!N) evaluates to true for M!N that fails to instantiate.

struct HasFoo { void foo() {} } struct NoFoo {} struct CallsFoo(T) { T t; void bar() { t.foo(); } } static assert(is(CallsFoo!HasFoo)); static assert(is(typeof({ alias Baz = CallsFoo!HasFoo; return Baz.init; }()))); static assert(__traits(compiles, { alias Baz = CallsFoo!HasFoo; return Baz.init; }())); alias Bar = CallsFoo!HasFoo; // this should fail: static assert(is(CallsFoo!NoFoo)); // this should fail too: static assert(is(typeof({ alias Baz = CallsFoo!NoFoo; return Baz.init; }()))); // this too: static assert(__traits(compiles, { alias Baz = CallsFoo!NoFoo; return Baz.init; }())); // only this fails: //alias Baz = CallsFoo!NoFoo; // (1)

Interestingly, they all fail when moved inside a function. So, they seem to be broken only at module scope.

It's not only module scope - it's broken also inside templates, e.g.: --- template canBeInstantiated(alias StructTemplate, Parameters...) { static if (is(StructTemplate!Parameters)) enum canBeInstantiated = true; else enum canBeInstantiated = false; } static assert( canBeInstantiated!(CallsFoo, HasFoo)); //static assert(!canBeInstantiated!(CallsFoo, NoFoo)); //fails --- or: --- template canBeInstantiated(alias StructTemplate, Parameters...) { enum canBeInstantiated = is(StructTemplate!Parameters); } static assert( canBeInstantiated!(CallsFoo, HasFoo)); //static assert(!canBeInstantiated!(CallsFoo, NoFoo)); //fails --- However, the function trick indeed works: --- bool canBeInstantiated(alias StructTemplate, Parameters...)() { return is(StructTemplate!Parameters); } static assert( canBeInstantiated!(CallsFoo, HasFoo)); static assert(!canBeInstantiated!(CallsFoo, NoFoo)); // hooray! ---

*** Issue 15600 has been marked as a duplicate of this issue. ***

Commits pushed to master at https://github.com/D-Programming-Language/dmd https://github.com/D-Programming-Language/dmd/commit/87c7837a859216be29400874f9528a9f36c6d2cc fix Issue 15623 - is(M!N) evaluates to true for M!N that fails to instantiate Add a new scope flag, and check it to see whether full template instantiation is necessary. https://github.com/D-Programming-Language/dmd/commit/f87e138bb998c19c2a1fa4ecdbc1fca63bf7051d Merge pull request #5477 from 9rnsr/fix15623 Issue 15623 - is(M!N) evaluates to true for M!N that fails to instantiate