Enhance type error understanding by displaying incompatible constraints

[Gueckmooh]

Sometimes it is complicated to understand errors such as Unable to deduce type for variable a. I tried to add more information on the cause of the error.

For now it is still a work in progress, but I have a few results.

Example

.decl foo(a: number, b: number)
.decl foo(a: symbol, b: number)

foo(1, 2).
bar(a, b) :-
   foo(a, b).

Before

$ souffle test.dl
Error: Unable to deduce type for variable a in file test.dl at line 5
bar(a, b) :-
----^--------
Error: Unable to deduce type for variable a in file test.dl at line 6
    foo(a, b).
--------^------
2 errors generated, evaluation aborted

After

$ souffle test.dl
Error: Unable to deduce type for variable a in file test.dl at line 5
bar(a, b) :-
----^--------
Following constraints are incompatible:
   Type of variable 'a' should be a subtype of 'symbol'
   Type of variable 'a' should be a subtype of 'number' in file test.dl at line 6
    foo(a, b).
----^----------
1 errors generated, evaluation aborted

How

I added an ErrorAnalyzer class that collects the minimal set of incompatible constraints on a problem, similarly to the unsat core of a SAT problem. It is injected into the type analysis to gather those information.

Then when emitting errors, it is used to "explain" why the error occurs.

To do

• Deduplicate errors (merge line 5 and line 6 errors in the example)
• Associate source location information to constraints
• Get user friendly messages to explain failing constraints
• Activate the "error understanding" mechanism on option
• Unit tests
• Remove the option
• Prettify constant type names

Make deduplication less ugly it's ugly but only when an error occurs

Let me know if the change looks too intrusive.

[codecov]

Codecov Report

Merging #2402 (2914f7b) into master (0334b07) will increase coverage by 0.19%.
The diff coverage is 89.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2402      +/-   ##
==========================================
+ Coverage   77.56%   77.75%   +0.19%     
==========================================
  Files         467      474       +7     
  Lines       30785    31164     +379     
==========================================
+ Hits        23877    24232     +355     
- Misses       6908     6932      +24     

Impacted Files	Coverage Δ
src/ast/analysis/typesystem/Type.h	50.00% <ø> (-10.00%)	⬇️
src/parser/ParserDriver.h	100.00% <ø> (ø)
src/synthesiser/Synthesiser.h	100.00% <ø> (ø)
.../ast/transform/SimplifyConstantBinaryConstraints.h	50.00% <50.00%> (ø)
src/MainDriver.cpp	70.39% <70.27%> (+0.15%)	⬆️
src/parser/scanner.ll	78.01% <76.92%> (+5.49%)	⬆️
tests/libsouffle_interface/vfs_overlay_test.cpp	85.93% <85.93%> (ø)
src/ast/analysis/ConstraintSystem.h	96.96% <86.36%> (-3.04%)	⬇️
src/parser/SrcLocation.cpp	91.86% <87.50%> (+7.24%)	⬆️
src/parser/ParserDriver.cpp	89.44% <88.33%> (-0.39%)	⬇️
... and 28 more