Bug 22325 – ReplaceType fails on templated type instantiated with void-returning function

As of DMD 2.097.2, the following program fails to compile: --- import std.typecons; struct Foo(alias f) {} void bar() {} alias _ = ReplaceType!(int, int, Foo!bar); --- The error message is: --- /usr/include/dmd/phobos/std/typecons.d(9067): Error: variable `std.typecons.ReplaceTypeUnless!(false_, int, int, Foo!(bar)).F!(bar).replaceTemplateArgs` type `void` is inferred from initializer `bar()`, and variables cannot be of type `void` /usr/include/dmd/phobos/std/meta.d-mixin-794(794): Error: template instance `std.typecons.ReplaceTypeUnless!(false_, int, int, Foo!(bar)).F!(bar)` error instantiating /usr/include/dmd/phobos/std/typecons.d(9071): instantiated from here: `staticMap!(replaceTemplateArgs, bar)` /usr/include/dmd/phobos/std/typecons.d(9012): instantiated from here: `ReplaceTypeUnless!(false_, int, int, Foo!(bar))` bug.d(5): instantiated from here: `ReplaceType!(int, int, Foo!(bar))` ---

@pbackus created dlang/phobos pull request #8248 "Fix Issue 22325 - ReplaceType fails on templated type instantiated wi…" fixing this issue: - Fix Issue 22325 - ReplaceType fails on templated type instantiated with void-returning function Previously, when recursing into a type's template arguments, ReplaceTypeUnless would mistakenly attempt to evaluate a template alias parameter as an expression. In cases where the alias parameter was not a valid expression or could not be evaluated at compile time, this caused a compilation failure; in other cases, it caused ReplaceTypeUnless to give an incorrect result. This change makes ReplaceTypeUnless correctly treat template alias parameters as aliases. https://github.com/dlang/phobos/pull/8248

dlang/phobos pull request #8248 "Fix Issue 22325 - ReplaceType fails on templated type instantiated wi…" was merged into master: - 44257f132ca99a16a34e475f844e93e3822d231c by Paul Backus: Fix Issue 22325 - ReplaceType fails on templated type instantiated with void-returning function Previously, when recursing into a type's template arguments, ReplaceTypeUnless would mistakenly attempt to evaluate a template alias parameter as an expression. In cases where the alias parameter was not a valid expression or could not be evaluated at compile time, this caused a compilation failure; in other cases, it caused ReplaceTypeUnless to give an incorrect result. This change makes ReplaceTypeUnless correctly treat template alias parameters as aliases. https://github.com/dlang/phobos/pull/8248