58f97fff10
lowerNullCoalesce fed resolveOptionalInner's .unresolved (returned for a non-optional lhs) into the merge-block params / optionalUnwrap / RHS target type, reaching codegen and panicking 'unresolved type reached LLVM emission'. Guard: when inferExprType(nc.lhs) is a resolved non-optional type, emit a located diagnostic and bail; an .unresolved lhs (prior error) is excluded to avoid double-report. ?? is optional-only per specs.md (error unions use or/catch), so rejecting a failable lhs is correct; comptime panic closed too. Regression: examples/diagnostics/1200-diagnostics-null-coalesce-non-optional.sx. Verified by 3 adversarial reviews, suite 790/0. Filed adjacent bug 0180 (?? lowering defects for generic/alias/tuple optional lhs).
57 lines
2.9 KiB
Markdown
57 lines
2.9 KiB
Markdown
# 0172 — `??` with a non-optional left-hand side panics instead of diagnosing
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> **RESOLVED.** `lowerNullCoalesce` fed `resolveOptionalInner`'s `.unresolved`
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> (returned for a non-optional lhs) into the merge-block / `optionalUnwrap` / RHS
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> target type → codegen panic. Fix (`src/ir/lower/expr.zig`): before computing
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> `inner_ty`, if `inferExprType(nc.lhs)` is a RESOLVED non-optional type, emit a
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> located diagnostic ("left operand of '??' must be an optional, but has type
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> '<T>'") and bail; an `.unresolved` lhs (prior error) is excluded to avoid
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> double-report. `??` is optional-only per specs.md (error unions use
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> `or`/`catch`), so rejecting a failable lhs is correct. Comptime panic closed
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> too. Verified by 3 adversarial reviews; suite 790/0. Regression:
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> `examples/diagnostics/1200-diagnostics-null-coalesce-non-optional.sx`. (Adjacent
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> pre-existing `??`-lowering defects found + filed: 0180 — generic / alias /
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> tuple optional lhs.)
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## Symptom
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Using `??` where the left operand is NOT an optional panics the compiler:
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`panic: unresolved type reached LLVM emission` (in `emitStructInit` for a struct
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default, or generally), exit 134. `??` is defined to operate on an optional lhs;
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a non-optional lhs is malformed user input that must be a clean type error, not a
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crash. Pre-existing (reproduces independent of the issue 0166 fix).
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## Reproduction
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```sx
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#import "modules/std.sx";
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T :: struct { a: i64 = 0; }
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main :: () {
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x := 5 ?? .{ a = 1 }; // panic: unresolved type reached LLVM emission, exit 134
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}
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```
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Also panics: `5 ?? 7` (scalar default), `some_non_optional_struct ?? .{ a = 1 }`,
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and nested `mk() ?? (5 ?? .{ a = 3 })`. Expected: a located diagnostic like
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`error: left operand of '??' must be an optional, but has type 'i64'`, exit 1.
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## Investigation prompt
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`src/ir/lower/expr.zig` `lowerNullCoalesce`: `resolveOptionalInner` (~expr.zig:1900)
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returns `.unresolved` when `nc.lhs` is not optional, and the function proceeds to
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feed that `.unresolved` into the merge-block params, `optionalUnwrap`, and the
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RHS target type — which then reaches codegen and panics. Add a guard: after
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inferring the lhs type, if it is not an optional (or `resolveOptionalInner`
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yields `.unresolved` for a resolved-but-non-optional lhs), emit
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`self.diagnostics.addFmt(.err, nc.lhs.span, "left operand of '??' must be an
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optional, but has type '{s}'", .{ formatTypeName(lhs_ty) })` and bail (return a
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placeholder), mirroring the non-pointer `.*` deref diagnostic at
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`lowerDerefExpr` (~expr.zig:1839). Be careful to still allow the legitimate
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cases: optional lhs (incl. `a?.b` chains returning optional), and make sure an
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already-`.unresolved` lhs from a PRIOR error (undefined name) doesn't
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double-report (that path already diagnoses via name resolution).
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Verify: `5 ?? .{a=1}`, `5 ?? 7`, non-optional-struct `?? ...` all exit 1 with the
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diagnostic and no panic; existing optional `??` cases still work. Add an
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`examples/diagnostics/11xx-null-coalesce-non-optional.sx` negative regression.
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