Merge pull request #3168 from 9rnsr/fix12016

[REG2.065a] Issue 12016 - implicit immutable upcast becomes null in CTFE
Merge pull request #3163 from 9rnsr/fix12008

[REG2.065a] Issue 12008 - alias this and "unable to resolve forward reference" error
Merge pull request #3174 from 9rnsr/fix12023

[REG2.065a] Issue 12023 - template mixin fails within template class
Merge pull request #3177 from 9rnsr/fix12037

[REG2.065a] Issue 12037 - Link-failure with std.numeric.CustomFloat
Merge pull request #3179 from 9rnsr/fix12038

Issue 12038 - alias this and &this cause ICE
Merge pull request #3169 from 9rnsr/fix12017

[REG2.065a] Issue 12017 - DDoc leaves out the majority of documentation
Merge pull request #3178 from 9rnsr/fix12040

Issue 12040 - Compiler segfault with circular reference in variable type