fix(errors): error on loading polymorphic function

[acl-cqc]

(Stumbled across this while investigating another issue, but nonetheless this seems an improvement.)

Currently if you assign a polymorphic function to a variable you get an error during compilation, not checking. Turned out we didn't have a test of this at all, so have added one...

(See new tests in first commit which are passing on main)

In a couple of cases this leads to checking issuing a different error, and of course we never get to the one that would be issued later in compilation, so detecting the error earlier leads to a change in message.

I reason that assignment is the only case where we care:

• If the poly-function is used as an argument, type inference will be used, and infer an instantiation. We could do this for assignment (i.e. to find an instantiation where the variable is used only monomorphically), e.g. perhaps by searching across the basic block, so maybe Type inference doesn't work across basic blocks #707 is in fact an understatement (type inference doesn't even work across a single BB yet...)
• For a top-level expression (foo where that's a poly-function), we don't need to find an instantiation, so it seems fine not to.

[acl-cqc]

I mean, maybe this test is entirely redundant given the new one in poly_callable.py ?

[github-actions]

Bencher Report

Project	guppylang
Branch	acl/check_poly_load
Testbed	Linux

Click to view all benchmark results

Benchmark	hugr_bytes	Benchmark Result bytes x 1e3 (Result Δ%)	Upper Boundary bytes x 1e3 (Limit %)	hugr_nodes	Benchmark Result nodes (Result Δ%)	Upper Boundary nodes (Limit %)
tests/benchmarks/test_big_array.py::test_big_array_compile	📈 view plot 🚷 view threshold	62.10 x 1e3 (0.00%) Baseline: 62.10 x 1e3	62.72 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	4,585.00 (0.00%) Baseline: 4,585.00	4,630.85 (99.01%)
tests/benchmarks/test_ctrl_flow.py::test_many_ctrl_flow_compile	📈 view plot 🚷 view threshold	27.83 x 1e3 (0.00%) Baseline: 27.83 x 1e3	28.11 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	1,301.00 (0.00%) Baseline: 1,301.00	1,314.01 (99.01%)
tests/benchmarks/test_queue_push_pop.py::test_queue_push_benchmark_compile	📈 view plot 🚷 view threshold	7.24 x 1e3 (0.00%) Baseline: 7.24 x 1e3	7.32 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	310.00 (0.00%) Baseline: 310.00	313.10 (99.01%)
tests/benchmarks/test_queue_push_pop.py::test_queue_push_pop_benchmark_compile	📈 view plot 🚷 view threshold	9.91 x 1e3 (0.00%) Baseline: 9.91 x 1e3	10.01 x 1e3 (99.01%)	📈 view plot 🚷 view threshold	411.00 (0.00%) Baseline: 411.00	415.11 (99.01%)

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.30%. Comparing base (ec1e2e4) to head (304b55b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2213      +/-   ##
==========================================
+ Coverage   93.29%   93.30%   +0.01%     
==========================================
  Files         160      160              
  Lines       15261    15270       +9     
==========================================
+ Hits        14238    14248      +10     
+ Misses       1023     1022       -1     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 11 untouched benchmarks

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_{Comparing acl/check_poly_load (304b55b) with main (5fb7ce4)}