all: implement comptime if for generic type `$if T is string {` (#7901)

vlib/v/checker/checker.v

@@ -4122,6 +4122,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 	mut nbranches_without_return := 0
 	mut should_skip := false // Whether the current branch should be skipped
 	mut found_branch := false // Whether a matching branch was found- skip the rest
+	mut is_comptime_t_is_expr := false // if `$if T is string`
 	for i in 0 .. node.branches.len {
 		mut branch := node.branches[i]
 		if branch.cond is ast.ParExpr {
@@ -4237,10 +4238,16 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 			// smartcast field type on comptime if
 			if branch.cond is ast.InfixExpr {
 				if branch.cond.op == .key_is {
-					se := branch.cond.left
-					if se is ast.SelectorExpr {
+					left := branch.cond.left
 					got_type := (branch.cond.right as ast.Type).typ
-						c.comptime_fields_type[se.expr.str()] = got_type
+					if left is ast.SelectorExpr {
+						c.comptime_fields_type[left.expr.str()] = got_type
+					} else if left is ast.Type {
+						is_comptime_t_is_expr = true
+						left_type := c.unwrap_generic(left.typ)
+						if left_type != got_type {
+							should_skip = true
+						}
 					}
 				}
 			}
@@ -4255,7 +4262,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 			}
 			if !c.skip_flags || c.pref.output_cross_c {
 				c.stmts(branch.stmts)
-			} else {
+			} else if !is_comptime_t_is_expr {
 				node.branches[i].stmts = []
 			}
 			c.skip_flags = cur_skip_flags
@@ -4386,7 +4393,9 @@ fn (mut c Checker) comp_if_branch(cond ast.Expr, pos token.Position) bool {
 					return l && r // skip (return true) only if both should be skipped
 				}
 				.key_is, .not_is {
-					if cond.left is ast.SelectorExpr && cond.right is ast.Type {
+					if (cond.left is ast.SelectorExpr ||
+						cond.left is ast.Type) &&
+						cond.right is ast.Type {
 						// $if method.@type is string
 					} else {
 						c.error('invalid `\$if` condition: $cond.left', cond.pos)

vlib/v/gen/comptime.v

@@ -231,10 +231,18 @@ fn (mut g Gen) comp_if_expr(cond ast.Expr) {
 					g.comp_if_expr(cond.right)
 				}
 				.key_is, .not_is {
-					se := cond.left as ast.SelectorExpr
-					name := '${se.expr}.$se.field_name'
-					exp_type := g.comptime_var_type_map[name]
+					left := cond.left
+					mut name := ''
+					mut exp_type := table.Type(0)
 					got_type := (cond.right as ast.Type).typ
+					if left is ast.SelectorExpr {
+						name = '${left.expr}.$left.field_name'
+						exp_type = g.comptime_var_type_map[name]
+					} else if left is ast.Type {
+						name = left.str()
+						// this is only allowed for generics currently, otherwise blocked by checker
+						exp_type = g.unwrap_generic(left.typ)
+					}
 					g.write('$exp_type == $got_type')
 				}
 				.eq, .ne {

vlib/v/parser/pratt.v

@@ -31,6 +31,10 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 				node = p.sql_expr()
 				p.inside_match = false
 			} else {
+				if p.inside_if && p.tok.lit == 'T' {
+					// $if T is string {}
+					p.expecting_type = true
+				}
 				node = p.name_expr()
 				p.is_stmt_ident = is_stmt_ident
 			}

vlib/v/tests/comptime_if_expr_test.v

@@ -28,3 +28,40 @@ fn test_ct_expressions() {
 		assert true
 	}
 }
+
+fn generic_t_is<T>() T {
+	$if T is string {
+		return 'It\'s a string!'
+	} $else {
+		return T{}
+	}
+	return T{}
+}
+
+struct GenericTIsTest {}
+
+fn test_generic_t_is() {
+	assert generic_t_is<string>() == 'It\'s a string!'
+	assert generic_t_is<GenericTIsTest>() == GenericTIsTest{}
+}
+
+fn generic_t_is2<T>() ?T {
+	$if T is string {
+		return 'It\'s a string!'
+	} $else {
+		return T{}
+	}
+ }
+
+fn test_generic_t_is2() {
+	res := generic_t_is2<string>() or {
+		assert false
+		''
+	}
+	res2 := generic_t_is2<GenericTIsTest>() or {
+		assert false
+		GenericTIsTest{}
+	}
+	assert res == 'It\'s a string!'
+	assert res2 == GenericTIsTest{}
+}