Issue 8262 - ICE(mtype.c) alias this to alias of an expression tuple

[rainers]

https://issues.dlang.org/show_bug.cgi?id=8262

This PR doesn't change anything about what should happen with a tuple being aliased to "this", but merely avoids the crash. The net result seems to be that implicite conversions don't take place.

This is blocking dlang/druntime#1057 ATM.

[WalterBright]

I am not comfortable with changes that "merely avoid the crash". It suggests not understanding what the actual problem is. I'd like Kenji to review this, the RECtracing thing is his design.

[rainers]

I am not comfortable with changes that "merely avoid the crash". It suggests not understanding what the actual problem is. I'd like Kenji to review this, the RECtracing thing is his design.

I'm not happy with it, either. The problem is that alias this to expression or mixed tuples is not well defined yet, though it sometimes works. When compiled with dlang/druntime#1057, this test crashes: https://github.com/D-Programming-Language/dmd/blob/master/test/runnable/aliasthis.d#L1269

What is supposed to happen when aliasing to Tuple!(1,int,"foo")? In pull request #4025, you proposed " the tuple must be one element, and it is resolved to that one element, otherwise an error.", but aliasthis.d shows examples that work differently, e.g. test9873.

[0joshuaolson1]

alias this to expression or mixed tuples is not well defined yet

Is this a comment about the D language specification?

[andralex]

@rainers status?

[dlang-bot]

Thanks for your pull request, @rainers!

Bugzilla references

Auto-close	Bugzilla	Description
✓	8262	ICE(mtype.c) alias this to alias of an expression tuple

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

[rainers]

@rainers status?

redone now and added test case. This still just follows the example of other code ignoring alias this if aliasthisof() returns null instead of crashing.

[andralex]

@rainers please mind the test failures thx!