From 010df176b83db90164aa7eb3f1665d3b29a25c0a Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 15:40:26 +0300
Subject: [PATCH] cgen: fix -(-2) translated into --2 (fixes #26813)

---
 .gitignore                                    |  2 ++
 vlib/v/gen/c/cgen.v                           | 29 +++++++++++++++++++
 .../nested_unary_minus_regression.out         |  1 +
 .../testdata/nested_unary_minus_regression.vv |  4 +++
 4 files changed, 36 insertions(+)
 create mode 100644 vlib/v/gen/c/testdata/nested_unary_minus_regression.out
 create mode 100644 vlib/v/gen/c/testdata/nested_unary_minus_regression.vv

diff --git a/.gitignore b/.gitignore
index 1a88525cd..f2d37c36b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,8 @@
 # unignore checker test files
 !vlib/v/checker/tests/*.vv
 !vlib/v/checker/tests/*.out
+!vlib/v/gen/c/testdata/nested_unary_minus_regression.vv
+!vlib/v/gen/c/testdata/nested_unary_minus_regression.out
 
 # unignore vlib/x/markdown
 !vlib/x/markdown/**
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 80e964c90..b043a49e7 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -5834,11 +5834,20 @@ fn (mut g Gen) expr(node_ ast.Expr) {
 				if node.right.is_auto_deref_var() && node.op !in [.amp, .mul, .arrow] {
 					g.write('*')
 				}
+				// Keep nested unary +/- parenthesized so C does not collapse them into ++/--
+				// when the parser has preserved a nested prefix node or folded a signed literal.
+				needs_nested_prefix_parens := prefix_expr_operand_needs_parens(node)
+				if needs_nested_prefix_parens {
+					g.write('(')
+				}
 				if tmp_var == '' {
 					g.expr(node.right)
 				} else {
 					g.write(tmp_var)
 				}
+				if needs_nested_prefix_parens {
+					g.write(')')
+				}
 				if has_slice_call {
 					g.write(')')
 				}
@@ -5936,6 +5945,26 @@ fn (mut g Gen) char_literal(node ast.CharLiteral) {
 	g.write("'${node.val}'")
 }
 
+fn prefix_expr_operand_needs_parens(node ast.PrefixExpr) bool {
+	if node.op !in [.plus, .minus] {
+		return false
+	}
+	return match node.right {
+		ast.PrefixExpr {
+			true
+		}
+		ast.FloatLiteral {
+			node.right.val.starts_with(node.op.str())
+		}
+		ast.IntegerLiteral {
+			node.right.val.starts_with(node.op.str())
+		}
+		else {
+			false
+		}
+	}
+}
+
 // T.name, typeof(expr).name
 fn (mut g Gen) type_name(raw_type ast.Type) {
 	typ := g.get_type(raw_type)
diff --git a/vlib/v/gen/c/testdata/nested_unary_minus_regression.out b/vlib/v/gen/c/testdata/nested_unary_minus_regression.out
new file mode 100644
index 000000000..0cfbf0888
--- /dev/null
+++ b/vlib/v/gen/c/testdata/nested_unary_minus_regression.out
@@ -0,0 +1 @@
+2
diff --git a/vlib/v/gen/c/testdata/nested_unary_minus_regression.vv b/vlib/v/gen/c/testdata/nested_unary_minus_regression.vv
new file mode 100644
index 000000000..2f4591b49
--- /dev/null
+++ b/vlib/v/gen/c/testdata/nested_unary_minus_regression.vv
@@ -0,0 +1,4 @@
+fn main() {
+	i := -(-2)
+	println(i)
+}
-- 
2.39.5