From 8ea79933021ca0a5d34560669fdf1ff40ccd2f6f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ibrahim=20Numanagic=CC=81?= <ibrahimpasa@gmail.com>
Date: Mon, 30 Jan 2023 17:26:47 -0800
Subject: [PATCH] Fix #155

---
 codon/parser/visitors/typecheck/infer.cpp | 10 ++++++++++
 stdlib/internal/core.codon                |  4 +++-
 test/parser/types.codon                   |  3 +++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/codon/parser/visitors/typecheck/infer.cpp b/codon/parser/visitors/typecheck/infer.cpp
index 88877361..b35f4bbf 100644
--- a/codon/parser/visitors/typecheck/infer.cpp
+++ b/codon/parser/visitors/typecheck/infer.cpp
@@ -838,6 +838,15 @@ TypecheckVisitor::generateSpecialAst(types::FuncType *type) {
     ll.push_back(format("ret {{}} %{}", as.size()));
     items[0] = N<ExprStmt>(N<StringExpr>(combine2(ll, "\n")));
     ast->suite = N<SuiteStmt>(items);
+  } else if (startswith(ast->name, "Union.__new__:0")) {
+    auto unionType = type->funcParent->getUnion();
+    seqassert(unionType, "expected union, got {}", type->funcParent);
+
+    StmtPtr suite = N<ReturnStmt>(
+              N<CallExpr>(N<IdExpr>("__internal__.new_union:0"),
+                          N<IdExpr>(type->ast->args[0].name),
+                          N<IdExpr>(unionType->realizedTypeName())));
+    ast->suite = suite;
   } else if (startswith(ast->name, "__internal__.new_union:0")) {
     // Special case: __internal__.new_union
     // def __internal__.new_union(value, U[T0, ..., TN]):
@@ -876,6 +885,7 @@ TypecheckVisitor::generateSpecialAst(types::FuncType *type) {
     }
     suite->stmts.push_back(N<ExprStmt>(N<CallExpr>(
         N<IdExpr>("compile_error"), N<StringExpr>("invalid union constructor"))));
+    LOG("-> {}", suite->toString(2));
     ast->suite = suite;
   } else if (startswith(ast->name, "__internal__.get_union:0")) {
     // Special case: __internal__.get_union
diff --git a/stdlib/internal/core.codon b/stdlib/internal/core.codon
index d8525436..94130d47 100644
--- a/stdlib/internal/core.codon
+++ b/stdlib/internal/core.codon
@@ -108,7 +108,9 @@ class Ref[T]:
 @__internal__
 @tuple
 class Union[TU]:
-    pass
+    # compiler-generated
+    def __new__(val):
+        TU
 
 # dummy
 @__internal__
diff --git a/test/parser/types.codon b/test/parser/types.codon
index 389b12a0..26d88296 100644
--- a/test/parser/types.codon
+++ b/test/parser/types.codon
@@ -1477,6 +1477,9 @@ x : Union[A,B,C] = A()
 print x.foo(), x.foo().__class__.__name__
 #: 1 Union[List[bool],int,str]
 
+xx = Union[int, str](0)
+print(xx)  #: 0
+
 #%% generator_capture_nonglobal,barebones
 # Issue #49
 def foo(iter):