Improve type inferrence for binary expressions

[turbolent]

Description

I wrote a Base64 encoding function for Cadence here:
https://forum.onflow.org/t/base64-encode-in-cadence/1915/7

This currently requires a lot of static type annotations and casts and could be much simplified through the following observations and changes.

• Currently the contextually expected type (if any) is only used for arithmetic binary expressions.

  However, bitwise binary expressions have the same property that the resulting type must be compatible with the operands.

• Currently the left expression and the right expression are type checked individually, and each side is provided with the contextually expected type (if any).

  However, the both sides of arithmetic, logical, etc. expressions must have the same type, so if no contextually expected type is present, use the left type as the expected type when checking the right side.

• The contextually expected type might be less specific. For example, the indexing type for arrays and strings is Integer. This causes an expression like let x = 1 as UInt8; "abc"[x + 1] to fail to type check.

With the following changes, this portion of the example program can be significantly simplified:

• Before:

  let n = (UInt32(data[i]) << 16 as UInt32)
      + (UInt32(data[i + 1 as Int]) << 8 as UInt32)
      + UInt32(data[i + 2 as Int])
  
  let n1 = (n >> 18 as UInt32) & 63 as UInt32
  let n2 = (n >> 12 as UInt32) & 63 as UInt32
  let n3 = (n >> 6 as UInt32) & 63 as UInt32
  let n4 = n & 63 as UInt32

• After:

  let n = UInt32(data[i]) << 16
      + UInt32(data[i + 1]) << 8
      + UInt32(data[i + 2])
  
  let n1 = (n >> 18) & 63
  let n2 = (n >> 12) & 63
  let n3 = (n >> 6) & 63
  let n4 = n & 63

Also, improve the tests by

• Removing the wrapper functions
• Adding assertions for the resulting types

---

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[SupunS]

This enforcement seems sensible 👍

[SupunS]

👍

[codecov-commenter]

Codecov Report

Merging #957 (9b9f123) into master (8e3c230) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
+ Coverage   75.36%   75.42%   +0.06%     
==========================================
  Files         268      268              
  Lines       32666    32729      +63     
==========================================
+ Hits        24618    24686      +68     
+ Misses       6915     6911       -4     
+ Partials     1133     1132       -1     

Flag	Coverage Δ
unittests	75.42% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
runtime/sema/check_binary_expression.go	87.89% <100.00%> (+0.22%)	⬆️
runtime/sema/type.go	88.38% <100.00%> (-0.01%)	⬇️
runtime/sema/ranges.go	100.00% <0.00%> (ø)
runtime/stdlib/builtin.go	95.52% <0.00%> (ø)
runtime/ast/programindices.go	100.00% <0.00%> (ø)
runtime/sema/check_function.go	99.54% <0.00%> (+<0.01%)	⬆️
runtime/sema/check_composite_declaration.go	96.13% <0.00%> (+0.01%)	⬆️
runtime/sema/checker.go	89.96% <0.00%> (+0.01%)	⬆️
runtime/sema/variable_activations.go	94.91% <0.00%> (+0.04%)	⬆️
... and 11 more

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