parser: give a friendly error when misusing if over $if (#19810)

vlib/v/parser/if_match.v

@@ -72,6 +72,7 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 				p.check(.dollar)
 			}
 		}
+		if_pos := p.tok.pos()
 		// `if` or `else if`
 		p.check(.key_if)
 		if p.tok.kind == .key_match {
@@ -139,6 +140,17 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 			p.comptime_if_cond = true
 			p.inside_if_cond = true
 			cond = p.expr(0)
+			if mut cond is ast.InfixExpr && !is_comptime {
+				if cond.op in [.key_is, .not_is] {
+					if mut cond.left is ast.Ident {
+						if cond.left.name.len == 1 && cond.left.name.is_capital()
+							&& cond.right is ast.TypeNode {
+							p.error_with_pos('use `\$if` instead of `if`', if_pos)
+							return ast.IfExpr{}
+						}
+					}
+				}
+			}
 			p.inside_if_cond = false
 			if p.if_cond_comments.len > 0 {
 				comments << p.if_cond_comments

vlib/v/parser/tests/not_using_if_comptime_in_comptime_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/not_using_if_comptime_in_comptime_err.vv:10:2: error: use `$if` instead of `if`
+    8 | 
+    9 | fn check[T]() bool {
+   10 |     if T is Checkable {
+      |     ~~
+   11 |         return true
+   12 |     } else {

vlib/v/parser/tests/not_using_if_comptime_in_comptime_err.vv

@@ -0,0 +1,17 @@
+interface Checkable {
+	test int
+}
+
+struct Test {
+	test int
+}
+
+fn check[T]() bool {
+	if T is Checkable {
+		return true
+	} else {
+		return false
+	}
+}
+
+println(check[Test]())