Fix some Variable alignment propagation in the simplifier.

[mcourteaux]

learn_true(x == y) with x and y vars did not fuse their alignment and bounds info.

Fixes #9328.
Partial resolution for #9327 by adding const which avoids accidental assignment.

Breaking changes

None I'm aware of.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[alexreinking]

Can you add a regression test somewhere?

[alexreinking]

clang-tidy rejects the const-return-value trick. Please drop that commit for this PR; I'm still on the fence about whether I'd like to use a proxy object anyway.

[abadams]

Apparently it's preferred to forbid operator= on rvalues on the type in question, rather than return const things (which breaks moving from them). E.g in struct Expr:

Expr &operator=(const Expr &) & = default;

Note the final &, which prevents assigning to rvalue Exprs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.04%. Comparing base (4b349d8) to head (20ff39e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9333      +/-   ##
==========================================
+ Coverage   69.92%   70.04%   +0.12%     
==========================================
  Files         258      258              
  Lines       78239    78252      +13     
  Branches    19047    19050       +3     
==========================================
+ Hits        54705    54810     +105     
+ Misses      17801    17786      -15     
+ Partials     5733     5656      -77     

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[mcourteaux]

Can you add a regression test somewhere?

Oops. Working on the regression test. Building on top of your correctness_vector_store_alignment.

UPDATE: regression test added in #9337.

[mcourteaux]

I'd argue the clang-tidy warning is just dumb?

/home/runner/work/Halide/Halide/src/Dimension.cpp:47:1: error: return type 'const Expr' is 'const'-qualified at the top level, which may reduce code readability without improving const correctness [readability-const-return-type,-warnings-as-errors]

   47 | const Expr Dimension::stride() const {
      | ^~~~~

Error: return type 'const Expr' is 'const'-qualified at the top level%2C which may reduce code readability without improving const correctness

It does improve const correctness.

Expr &operator=(const Expr &) & = default;

I guess you meant = delete, @abadams ?