gen-c: a numeric a ** b is lowered as an array repeat, /* repeat: a ** b */ {0} #7427

Part of #5980 (self-host P1: the core must refuse what gen-c gets wrong, so gen-c should stop getting it wrong).

What happens

t27c gen-c treats every ** as the array repeat operator. A numeric power is written as a comment and an empty initializer, and gen-c exits 0:

module m;
fn p(a: u32, b: u32) -> u32 { return a ** b; }
fn q(x: f64) -> f64 { return x ** 2.0; }

return /* repeat: a ** b */ {0};

cc rejects return {0}; in a scalar function, so these functions never compile. The language's own compiler specs define ** on numbers as the power operator: specs/compiler/typechecker.t27 lists it in is_arithmetic_op, and specs/compiler/optimizer.t27 folds 2 ** 10 to 1024 in eval_binary_const. Two corpus specs use it: specs/provider/adapters.t27:357 and specs/portable/relay_observer.t27:198.

Expected

A ** whose left operand is not an array literal is a numeric power. gen-c writes it as a call that C can compile: pow/powf/powl for a float operand and an exact integer power for an integer operand. The array repeat [_]T{v} ** n keeps its current lowering.

Check

cargo build --release -p t27c
printf 'module m;\nfn p(a: u32, b: u32) -> u32 { return a ** b; }\ntest t { assert(p(2, 10) == 1024); assert(p(3, 0) == 1); }\n' > /tmp/p.t27
./target/release/t27c gen-c /tmp/p.t27 > /tmp/p.c && cc -std=gnu11 -DT27_TEST_MAIN /tmp/p.c -o /tmp/p -lm && /tmp/p

Today the cc step fails. After the fix the test passes. The regression test is bootstrap/tests/genc_nested_repeat.rs, shared with #7353 because both defects are in the same ** lowering.

Boundary

• specs/compiler/optimizer.t27
• specs/compiler/typechecker.t27
• specs/portable/relay_observer.t27
• specs/provider/adapters.t27
• bootstrap/tests/genc_nested_repeat.rs

User Scenarios

• Given a T27 module with a function returning a ** b where both operands are integer scalars, when t27c gen-c compiles it, then the generated C uses an integer power implementation (not an array repeat) and the resulting binary passes assert(p(2, 10) == 1024).
• Given a T27 module with a function returning x ** 2.0 where the left operand is a float scalar, when t27c gen-c compiles it, then the generated C calls pow/powf/powl and the resulting binary executes without compile errors.

Requirements

• FR-001: The gen-c lowering MUST distinguish numeric ** from array repeat by inspecting whether the left operand is an array literal.
• FR-002: For integer operands, gen-c MUST emit an exact integer power computation (e.g., loop or builtin) that evaluates at runtime without linking -lm.
• FR-003: For floating-point operands, gen-c MUST emit a call to pow, powf, or powl matching the operand width and link with -lm.
• FR-004: The array repeat form [_]T{v} ** n MUST continue to lower to the current repeat initializer.
• FR-005: The typechecker and optimizer specs MUST continue to treat ** as an arithmetic operator on numbers (no spec change required).

Success Criteria

• cargo build --release -p t27c exits 0
• printf 'module m;\nfn p(a: u32, b: u32) -> u32 { return a ** b; }\ntest t { assert(p(2, 10) == 1024); assert(p(3, 0) == 1); }\n' > /tmp/p.t27 && ./target/release/t27c gen-c /tmp/p.t27 > /tmp/p.c && cc -std=gnu11 -DT27_TEST_MAIN /tmp/p.c -o /tmp/p -lm && /tmp/p exits 0
• printf 'module m;\nfn q(x: f64) -> f64 { return x ** 2.0; }\ntest t { assert(q(3.0) == 9.0); }\n' > /tmp/q.t27 && ./target/release/t27c gen-c /tmp/q.t27 > /tmp/q.c && cc -std=gnu11 -DT27_TEST_MAIN /tmp/q.c -o /tmp/q -lm && /tmp/q exits 0
• cargo test --test genc_nested_repeat exits 0 (5 tests)