Fix #155

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

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__

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):