Simplify member access into a compound literal

[tautschnig]

simplify_member now folds member(compound_literal(v), .m) by unwrapping the compound literal to its single initializer operand v and recursing, so the existing struct/union member-extraction rules apply.

This fixes a constant-folding gap that made the C front-end reject valid constant static initializers nesting a compound literal: the kernel's dynamic-debug _ddebug descriptor initialises its .key union from the STATIC_KEY_FALSE_INIT compound literal (under CONFIG_JUMP_LABEL), which projected to member(compound_literal{...}, .key.enabled.counter). That member-of-compound-literal was not simplified, so make_constant's is_compile_time_constantt rejected it with "expected constant expression" -- blocking goto-cc on every linux-6.12 translation unit that uses dynamic debug (sound/usb, fs/hfsplus, fs/jfs, ...). With the fix sound/usb/format.c (and the rest) goto-cc cleanly.

regression/ansi-c/compound_literal_member_static reproduces the _ddebug shape (a static struct whose union field is initialised from a compound literal) and checks the projected member folds to its constant.

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[Copilot]

Pull request overview

Note

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Fixes a constant-folding gap in the C front-end by simplifying member access on compound literals so nested static initializers (e.g., Linux kernel dynamic-debug _ddebug patterns) typecheck and constant-fold correctly.

Changes:

• Extend simplify_member to unwrap single-operand compound_literal expressions and recurse, enabling existing struct/union member folding.
• Add a regression test reproducing a static initializer that projects through a union member from a compound literal.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/util/simplify_expr_struct.cpp	Unwraps member(compound_literal(v), .m) into member(v, .m) and reuses existing folding logic.
regression/ansi-c/compound_literal_member_static/test.desc	Registers a new CORE regression test for the compound-literal member folding case.
regression/ansi-c/compound_literal_member_static/main.c	Adds a C repro mirroring the kernel _ddebug initializer shape and asserts the folded constant.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.69%. Comparing base (321ba11) to head (3c0f01c).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9060      +/-   ##
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+ Coverage    80.68%   80.69%   +0.01%     
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  Files         1714     1714              
  Lines       189501   189505       +4     
  Branches        73       73              
===========================================
+ Hits        152902   152925      +23     
+ Misses       36599    36580      -19     

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