checker: have stricter checks for .clone() requirement, disallow empty maps

[crthpl]

Fixes #10019
Fixes #10011
Depends On #10328

[yuyi98]

Personal advices: Array assignments should be Clone by default, If you need direct assignment, you can use unsafe to do that.

[medvednikov]

Build fails:

/tmp/v/v2.exe.8151136170925915032.tmp.c:18205: error: cannot convert 'extern extern struct array *' to 'struct array'

[crthpl]

Personal advices: Array assignments should be Clone by default, If you need direct assignment, you can use unsafe to do that.

@yuyi98 I also support this, but Alex wants clone to be optimized first: #10011 (comment)

[JalonSolov]

I'm confused by the "disallow empty maps" - what does that mean? That you won't be allowed to clone an empty map?

If so... why not?

[crthpl]

I'm confused by the "disallow empty maps" - what does that mean? That you won't be allowed to clone an empty map?

If so... why not?

@JalonSolov No, this PR just fixes 2 completely unrelated bugs. The empty map fix is that doing map{} causes an error, and becomes a void value, instead of becoming the 0 type and crashing the compiler.

[JalonSolov]

Gotcha. So disallow completely empty maps, meaning they don't even specify the key and value types.

[JalonSolov]

ping?