Bug 23686 – template instance reused with default alias arg

``` void FNfoo() {} void load(string name, alias T = mixin("FN" ~ name))() {} void main() { load!"foo"(); load!"wtf"(); } ``` Compiles without error even though FNwtf does not exist. Smells similar to https://issues.dlang.org/show_bug.cgi?id=12533 and https://issues.dlang.org/show_bug.cgi?id=14501

@SixthDot created dlang/dmd pull request #15679 "fix issue 23686 - TemplateAliasParam default value possibly considere…" fixing this issue: - fix issue 23686 - TemplateAliasParam default value possibly considered equal in different instances The default arg might depend on another TemplateParameter, in which case it is required to copy the AST, otherwise different instances of the template might be falsy considered equal. https://github.com/dlang/dmd/pull/15679

@SixthDot updated dlang/dmd pull request #15681 "fix issue 23865 - ICE on attempt to insert an non overloadable symbol…" fixing this issue: - fix issue 23686 - TemplateAliasParam default value possibly considered equal in different instances (#15679) The default arg might depend on another TemplateParameter, in which case it is required to copy the AST, otherwise different instances of the template might be falsy considered equal. https://github.com/dlang/dmd/pull/15681

dlang/dmd pull request #15771 "merge stable" was merged into master: - 26f706a9ddc760d585e4c0eb9d18dc354158426c by SixthDot: fix issue 23686 - TemplateAliasParam default value possibly considered equal in different instances (#15679) The default arg might depend on another TemplateParameter, in which case it is required to copy the AST, otherwise different instances of the template might be falsy considered equal. https://github.com/dlang/dmd/pull/15771