checker: fix type mismatch checking for assignments with generics (fix #20298) (#20327)

vlib/v/checker/assign.v

@@ -735,8 +735,7 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 				}
 			}
 		}
-		if !is_blank_ident && right_sym.kind != .placeholder && left_sym.kind != .interface_
+		if !is_blank_ident && right_sym.kind != .placeholder && left_sym.kind != .interface_ {
-			&& !right_type.has_flag(.generic) && !left_type.has_flag(.generic) {
 			// Dual sides check (compatibility check)
 			c.check_expected(right_type_unwrapped, left_type_unwrapped) or {
 				// allow literal values to auto deref var (e.g.`for mut v in values { v = 1.0 }`)

vlib/v/checker/tests/assign_type_mismatch_with_generics_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/assign_type_mismatch_with_generics_err.vv:13:9: error: cannot assign to `b`: expected `bool`, not `fn (Bar) bool`
+   11 |     mut b := false
+   12 |     if f.f != none {
+   13 |         b = f.f or { panic(err) }
+      |               ^
+   14 |     } else {
+   15 |         b = true

vlib/v/checker/tests/assign_type_mismatch_with_generics_err.vv

@@ -0,0 +1,28 @@
+type Fn[T] = fn (arg T) bool
+
+struct Foo[T] {
+	f ?Fn[T]
+}
+
+@[params]
+struct Bar {}
+
+fn (mut f Foo[T]) method(arg T) {
+	mut b := false
+	if f.f != none {
+		b = f.f or { panic(err) }
+	} else {
+		b = true
+	}
+	if b {
+	}
+}
+
+fn main() {
+	mut foo := Foo[Bar]{
+		f: fn (arg Bar) bool {
+			return true
+		}
+	}
+	foo.method(Bar{})
+}

vlib/v/tests/generics_chans_select_test.v

@@ -35,8 +35,8 @@ pub fn (mut ec EventController[T]) emit(e T, options EmitOptions) {
 	for i, w in ec.wait_fors {
 		mut b := false
 		if w.check != none {
-			b = (w.check or { panic(err) })
+			func := w.check or { panic(err) }
-			e
+			b = func(e)
 		} else {
 			b = true
 		}