fmt: fix parser failing on duplicate methods in $if/$else blocks (fix #26271) (#26278)

Author: fleximus

vlib/v/fmt/tests/comptime_if_duplicate_method_keep.vv

@@ -0,0 +1,35 @@
+// Test for issue #26271: v fmt fails on mutually exclusive $if blocks with duplicate method definitions
+module main
+
+struct MyType {
+	value int
+}
+
+$if foo ? {
+	pub fn (x MyType) str() string {
+		return 'foo mode'
+	}
+} $else {
+	pub fn (x MyType) str() string {
+		return 'normal mode'
+	}
+}
+
+$if bar ? {
+	fn (m MyType) debug() string {
+		return 'bar debug'
+	}
+} $else $if baz ? {
+	fn (m MyType) debug() string {
+		return 'baz debug'
+	}
+} $else {
+	fn (m MyType) debug() string {
+		return 'default debug'
+	}
+}
+
+fn main() {
+	t := MyType{42}
+	println(t.str())
+}

vlib/v/parser/fn.v

@@ -658,7 +658,9 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 					is_duplicate = !type_sym.info.has_method(name)
 				}
 			}
-			if is_duplicate {
+			// when formatting, methods in mutually exclusive $if/$else branches
+			// may appear as duplicates since all branches are parsed
+			if is_duplicate && !p.pref.is_fmt {
 				if type_sym.kind == .enum
 					&& name in ['is_empty', 'has', 'all', 'set', 'set_all', 'clear', 'clear_all', 'toggle', 'zero', 'from'] {
 					if enum_fn := type_sym.find_method(name) {