feat(typeck): support negative integer literal coercion

Implement proper handling of negative integer literals in the type
checker. Previously, negative literals like -42 were broken because:

1. The parser represents them as unary negation applied to a positive
   literal, so type_of_lit only sees the positive value
2. The negativity flag in IntLiteral was always false

This commit fixes the issue by:

- Allowing unary negation on IntLiteral types (they can always be
  negated since the result is just a negative literal)
- Propagating the negativity flag when applying unary negation to an
  IntLiteral, flipping neg from false to true
- Not propagating the expected type through negation when targeting
  signed types, to avoid premature type mismatch errors on the inner
  expression

Also simplifies the coercion rule to use strict inequality for both
positive and negative literals, since TypeSize rounding means we can't
reliably distinguish edge cases in either direction.

Author: onbjerg

crates/sema/src/typeck/checker.rs

@@ -482,13 +482,25 @@ impl<'gcx> TypeChecker<'gcx> {
                 self.gcx.mk_ty(TyKind::Type(self.gcx.type_of_hir_ty(ty)))
             }
             hir::ExprKind::Unary(op, expr) => {
+                // For negation, don't propagate expected type to the inner expression
+                // because we'll modify the type (flipping the sign for int literals).
+                let propagate_expected = op.kind != hir::UnOpKind::Neg
+                    || !matches!(expected, Some(ty) if ty.is_signed());
                 let ty = if op.kind.has_side_effects() {
                     self.require_lvalue(expr)
-                } else {
+                } else if propagate_expected {
                     self.check_expr_with(expr, expected)
+                } else {
+                    self.check_expr(expr)
                 };
                 // TODO: custom operators
                 if valid_unop(ty, op.kind) {
+                    // Propagate negativity for integer literals under unary negation.
+                    if op.kind == hir::UnOpKind::Neg
+                        && let TyKind::IntLiteral(neg, size) = ty.kind
+                    {
+                        return self.gcx.mk_ty(TyKind::IntLiteral(!neg, size));
+                    }
                     ty
                 } else {
                     let msg = format!(
@@ -1001,15 +1013,25 @@ fn valid_unop(ty: Ty<'_>, op: hir::UnOpKind) -> bool {
 
     let ty = ty.peel_refs();
     match ty.kind {
-        TyKind::Elementary(hir::ElementaryType::Int(_) | hir::ElementaryType::UInt(_))
-        | TyKind::IntLiteral(..) => match op {
-            hir::UnOpKind::Neg => ty.is_signed(),
-            hir::UnOpKind::Not => false,
-            hir::UnOpKind::PreInc
+        TyKind::Elementary(hir::ElementaryType::Int(_) | hir::ElementaryType::UInt(_)) => {
+            match op {
+                hir::UnOpKind::Neg => ty.is_signed(),
+                hir::UnOpKind::Not => false,
+                hir::UnOpKind::PreInc
+                | hir::UnOpKind::PreDec
+                | hir::UnOpKind::BitNot
+                | hir::UnOpKind::PostInc
+                | hir::UnOpKind::PostDec => true,
+            }
+        }
+        // IntLiteral can always be negated (it becomes a negative literal).
+        TyKind::IntLiteral(..) => match op {
+            hir::UnOpKind::Neg | hir::UnOpKind::BitNot => true,
+            hir::UnOpKind::Not
+            | hir::UnOpKind::PreInc
             | hir::UnOpKind::PreDec
-            | hir::UnOpKind::BitNot
             | hir::UnOpKind::PostInc
-            | hir::UnOpKind::PostDec => true,
+            | hir::UnOpKind::PostDec => false,
         },
         TyKind::Elementary(hir::ElementaryType::FixedBytes(_)) => op == hir::UnOpKind::BitNot,
         TyKind::Elementary(hir::ElementaryType::Bool) => op == hir::UnOpKind::Not,

tests/ui/typeck/implicit_int_literal.sol

@@ -41,4 +41,25 @@ function f() {
     int256 zero_i256 = 0;
     int8 one_i8 = 1;
     int256 one_i256 = 1;
+
+    // === Negative literals to int ===
+    // Negative literals can only coerce to signed int types.
+    // TypeSize stores actual bits. Negative int_literal[N] can fit in int(N) since
+    // the negative range uses all N bits (e.g., -128 needs 8 bits, fits in int8).
+
+    // int_literal[1] negative (1-8 bits) -> int8+ works
+    int8 neg_1_i8 = -1;
+    int8 neg_128_i8 = -128;
+    int256 neg_1_i256 = -1;
+
+    // int_literal[2] negative (9-16 bits) -> int16+ works
+    int16 neg_129_i16 = -129;
+    int16 neg_32768_i16 = -32768;
+
+    // int_literal[3] negative (17-24 bits) -> int24+ works
+    int32 neg_32769_i32 = -32769;
+
+    // Negative literals cannot coerce to unsigned types
+    uint8 neg_to_uint8 = -1; //~ ERROR: mismatched types
+    uint256 neg_to_uint256 = -42; //~ ERROR: mismatched types
 }

tests/ui/typeck/implicit_int_literal.stderr

@@ -16,5 +16,17 @@ error: mismatched types
 LL │     int16 i16_overflow = 65536;
    ╰╴                         ━━━━━ expected `int16`, found `int_literal[3]`
 
-error: aborting due to 3 previous errors
+error: mismatched types
+   ╭▸ ROOT/tests/ui/typeck/implicit_int_literal.sol:LL:CC
+   │
+LL │     uint8 neg_to_uint8 = -1;
+   ╰╴                         ━━ expected `uint8`, found `int_literal[1]`
+
+error: mismatched types
+   ╭▸ ROOT/tests/ui/typeck/implicit_int_literal.sol:LL:CC
+   │
+LL │     uint256 neg_to_uint256 = -42;
+   ╰╴                             ━━━ expected `uint256`, found `int_literal[1]`
+
+error: aborting due to 5 previous errors