parser: report a clear error for the if if cond { typo (fix #27907) (#27919)

Author: medvednikov

vlib/v/parser/if_match.v

@@ -184,6 +184,17 @@ fn (mut p Parser) if_expr(is_comptime bool, is_expr bool) ast.IfExpr {
 			p.comptime_if_cond = false
 		}
 		comments << p.eat_comments()
+		// catch the common typo `if if cond {` / `} else if if cond {`, where a
+		// second `if` was written by mistake. The inner `if` gets parsed as the
+		// condition expression, so bail out with a helpful message instead of the
+		// confusing `expecting {` error that appears further down the file.
+		if !is_comptime && p.tok.kind != .lcbr {
+			if cond is ast.IfExpr && !cond.has_else {
+				p.error_with_pos('the condition of an `if` should be a boolean expression, not another `if` statement; did you write `if` twice by mistake?',
+					cond.pos)
+				return ast.IfExpr{}
+			}
+		}
 		end_pos := p.prev_tok.pos()
 		body_pos := p.tok.pos()
 		p.inside_if = false

vlib/v/parser/tests/if_double_if_cond_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/if_double_if_cond_err.vv:6:12: error: the condition of an `if` should be a boolean expression, not another `if` statement; did you write `if` twice by mistake?
+    4 |     if x == 1 {
+    5 |         commands << 'a'
+    6 |     } else if if x == 2 {
+      |               ~~
+    7 |         commands << 'b'
+    8 |     }

vlib/v/parser/tests/if_double_if_cond_err.vv

@@ -0,0 +1,14 @@
+fn get_commands() []string {
+	mut commands := []string{}
+	x := 1
+	if x == 1 {
+		commands << 'a'
+	} else if if x == 2 {
+		commands << 'b'
+	}
+	return commands
+}
+
+fn main() {
+	println(get_commands())
+}