Check assigned to in if and match branches

[JSAbrahams]

Relevant issues

• Resolves #449

In future we can perhaps use this logic to also de-nullify types (i.e. Str? -> Str) as we move towards implementing type refinement

• 96

Summary

If we assign to a variable of a class in all possible execution paths of the constructor, we consider it to be assigned to.

Environment intersection and union functions only perform operations on the "must be assigned to" set. All other items in the Environment are left untouched. That is because

• Shadowing logic is partially implemented in the constraint builder
• We don't want full unions of environments because we don't consider a variable to be declared even if it is declared in all branches (even if this is valid in Python). We only consider a variable to be declared if it is declared in scope.

Added Tests

Happy

• Assing to class var in all branches of match
• Assign to class var in all branches of if

Sad

• Definition does not leak out of then branch...
• ...And else branch of if
• Assign to clas var in only then branch...
• ...And else branch of if
• Assigng to class var in only some branches of match

[codecov[bot]]

Codecov Report

Merging #451 (4e2af79) into develop (423f6ce) will increase coverage by 0.01%. The diff coverage is 91.11%.

@@             Coverage Diff             @@
##           develop     #451      +/-   ##
===========================================
+ Coverage    88.19%   88.20%   +0.01%     
===========================================
  Files          105      105              
  Lines        11476    11501      +25     
===========================================
+ Hits         10121    10145      +24     
- Misses        1355     1356       +1     

Impacted Files	Coverage Δ
src/parse/ast/node.rs	81.00% <0.00%> (-0.17%)	:arrow_down:
src/check/constrain/generate/control_flow.rs	91.39% <92.85%> (-0.74%)	:arrow_down:
src/check/constrain/generate/definition.rs	90.62% <100.00%> (+0.56%)	:arrow_up:
src/check/constrain/generate/env.rs	98.97% <100.00%> (+0.26%)	:arrow_up:
src/check/ast/mod.rs	68.22% <0.00%> (+0.93%)	:arrow_up: