checker: fix generic inference in if expressions used in assignments (#21781)

Author: felipensp

vlib/v/checker/assign.v

@@ -53,6 +53,14 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				right_len = node.right_types.len
 			} else if right_type == ast.void_type {
 				right_len = 0
+				if mut right is ast.IfExpr {
+					last_branch := right.branches.last()
+					last_stmts := last_branch.stmts.filter(it is ast.ExprStmt)
+					if last_stmts.any((it as ast.ExprStmt).typ.has_flag(.generic)) {
+						right_len = last_branch.stmts.len
+						node.right_types = last_stmts.map((it as ast.ExprStmt).typ)
+					}
+				}
 			}
 		}
 		if mut right is ast.InfixExpr {

vlib/v/checker/if.v

@@ -473,6 +473,10 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 						c.error('mismatched types `${c.table.type_to_str(node.typ)}` and `${c.table.type_to_str(stmt.typ)}`',
 							node.pos)
 					} else {
+						if node.is_expr == false && c.comptime.is_generic_param_var(stmt.expr) {
+							// generic variable no yet type bounded
+							node.is_expr = true
+						}
 						if c.inside_assign && node.is_expr && !node.typ.has_flag(.shared_f)
 							&& stmt.typ != ast.voidptr_type {
 							if stmt.typ.is_ptr() != node.typ.is_ptr() {

vlib/v/tests/generic_if_ret_test.v

@@ -0,0 +1,8 @@
+fn clamp[T](a T, x T, b T) T {
+	min := if x < b { x } else { b }
+	return if min < a { a } else { min }
+}
+
+fn test_main() {
+	assert dump(clamp(1, 2, 3)) == 2
+}