From a1e890bc3107dd28b2a28fd8fbc18b2cb6426163 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Wed, 25 Mar 2026 16:42:23 +0300
Subject: [PATCH] cgen: fix incorrect comptime return with $if/$else (fixes
 #17262)

---
 vlib/v/gen/c/cgen.v                           |  4 +-
 vlib/v/gen/c/comptime.v                       | 39 +++++++++++++++++--
 .../comptime_if_return_termux.c.must_have     |  3 ++
 .../c/testdata/comptime_if_return_termux.vv   | 14 +++++++
 .../platform_wrapper_emscripten.c.must_have   |  2 +-
 5 files changed, 55 insertions(+), 7 deletions(-)
 create mode 100644 vlib/v/gen/c/testdata/comptime_if_return_termux.c.must_have
 create mode 100644 vlib/v/gen/c/testdata/comptime_if_return_termux.vv

diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 18013e4a4..5fbbc8350 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -7885,9 +7885,7 @@ fn (mut g Gen) gen_hash_stmts(mut sb strings.Builder, node &ast.HashStmtNode, se
 					} else {
 						sb.write_string('\n#elif ')
 					}
-					// directly use `checker` evaluate results
-					// for `cgen`, we can use `is_true.c_str` or `is_true.value` here
-					sb.writeln('${is_true.c_str}')
+					sb.writeln(g.comptime_if_condition_for_c(branch.cond, is_true))
 					$if debug_comptime_branch_context ? {
 						sb.writeln('/* ${node.branches[i].cond} | generic=[${comptime_branch_context_str}] */')
 					}
diff --git a/vlib/v/gen/c/comptime.v b/vlib/v/gen/c/comptime.v
index 1388679f6..897af3276 100644
--- a/vlib/v/gen/c/comptime.v
+++ b/vlib/v/gen/c/comptime.v
@@ -528,6 +528,41 @@ fn (mut g Gen) gen_branch_context_string() string {
 	return arr.join(',')
 }
 
+fn (g &Gen) can_preserve_comptime_if_condition_in_c(cond ast.Expr) bool {
+	match cond {
+		ast.BoolLiteral {
+			return true
+		}
+		ast.ParExpr {
+			return g.can_preserve_comptime_if_condition_in_c(cond.expr)
+		}
+		ast.PrefixExpr {
+			return cond.op == .not && g.can_preserve_comptime_if_condition_in_c(cond.right)
+		}
+		ast.InfixExpr {
+			return cond.op in [.and, .logical_or]
+				&& g.can_preserve_comptime_if_condition_in_c(cond.left)
+				&& g.can_preserve_comptime_if_condition_in_c(cond.right)
+		}
+		ast.PostfixExpr {
+			return cond.op == .question && cond.expr is ast.Ident
+		}
+		else {
+			return false
+		}
+	}
+}
+
+fn (g &Gen) comptime_if_condition_for_c(cond ast.Expr, result ast.ComptTimeCondResult) string {
+	if g.pref.output_cross_c || g.can_preserve_comptime_if_condition_in_c(cond) {
+		return result.c_str
+	}
+	// For normal C generation, honor the branch result that V already resolved.
+	// Re-evaluating built-in comptime conditions in the C preprocessor can pick
+	// a different branch than the V checker, e.g. `termux` vs `linux`.
+	return if result.val { '1' } else { '0' }
+}
+
 fn (mut g Gen) comptime_if(node ast.IfExpr) {
 	tmp_var := g.new_tmp_var()
 	mut inferred_typ := node.typ
@@ -594,9 +629,7 @@ fn (mut g Gen) comptime_if(node ast.IfExpr) {
 			} else {
 				g.write('#elif ')
 			}
-			// directly use `checker` evaluate results
-			// for `cgen`, we can use `is_true.c_str` or `is_true.value` here
-			g.writeln('${is_true.c_str}')
+			g.writeln(g.comptime_if_condition_for_c(branch.cond, is_true))
 			$if debug_comptime_branch_context ? {
 				g.writeln('/* ${node.branches[i].cond} | generic=[${comptime_branch_context_str}] */')
 			}
diff --git a/vlib/v/gen/c/testdata/comptime_if_return_termux.c.must_have b/vlib/v/gen/c/testdata/comptime_if_return_termux.c.must_have
new file mode 100644
index 000000000..2c18867f4
--- /dev/null
+++ b/vlib/v/gen/c/testdata/comptime_if_return_termux.c.must_have
@@ -0,0 +1,3 @@
+#if 0
+#elif 1
+return _S("linux-aarch64");
diff --git a/vlib/v/gen/c/testdata/comptime_if_return_termux.vv b/vlib/v/gen/c/testdata/comptime_if_return_termux.vv
new file mode 100644
index 000000000..63b3a43e3
--- /dev/null
+++ b/vlib/v/gen/c/testdata/comptime_if_return_termux.vv
@@ -0,0 +1,14 @@
+// vtest vflags: -gc none -os termux
+fn host_arch() string {
+	$if linux {
+		return 'linux-x86_64'
+	} $else $if termux {
+		return 'linux-aarch64'
+	} $else {
+		return 'unknown'
+	}
+}
+
+fn main() {
+	println(host_arch())
+}
diff --git a/vlib/v/gen/c/testdata/platform_wrapper_emscripten.c.must_have b/vlib/v/gen/c/testdata/platform_wrapper_emscripten.c.must_have
index e53842fdc..9c3b04b5b 100644
--- a/vlib/v/gen/c/testdata/platform_wrapper_emscripten.c.must_have
+++ b/vlib/v/gen/c/testdata/platform_wrapper_emscripten.c.must_have
@@ -1,7 +1,7 @@
 #if defined(CUSTOM_DEFINE_emscripten)
 println(_S("> inside then branch of if emscripten"));
 
-#if defined(__EMSCRIPTEN__)
+#if 1
 println(_S("> inside then branch of if wasm32_emscripten"));
 
 v__gen__c__testdata__platform_wrapper__fn_defined_in_wasm32_emscripten();
-- 
2.39.5