Bug 21098 – poor diagnostic when trying to assign a string literal to a `char*`

the following code --- void main() { char* c; c = ""; } --- fails to compile with the error > /tmp/temp_7F155FF5EE70.d(8,9): Error: cannot implicitly convert expression `""` of type `string` to `char*` which is correct. However the compiler could detect that the intent is correct but that `c` must be of type `const(char)*` or `immutable(char)*` and emit a supplemental message accordingly.

@NilsLankila created dlang/dmd pull request #11602 "fix issue 21098 - Add specific diagnostic for failed assign from StringExp" fixing this issue: - fix issue 21098 - Add specific diagnostic for failed assign from StringExp Because assigning a literal to `char*` or a `char[]` is almost correct, add a supplemental error message that suggests to use a const/immtable type. https://github.com/dlang/dmd/pull/11602

The PR did not encounter much success and this is a minor problem anyway.