fix(type-inference): for ternary expression with null branch

server/src/TypeInferer.ts

@@ -323,7 +323,20 @@ export class TypeInferer {
             const alternate = node.node.childForFieldName("alternative")
             if (!consequent || !alternate) return null
 
-            return this.inferType(new Expression(consequent, node.file))
+            const trueType = this.inferType(new Expression(consequent, node.file))
+            const falseType = this.inferType(new Expression(alternate, node.file))
+            if (!falseType) return trueType
+            if (!trueType) return falseType
+
+            if (trueType instanceof NullTy) {
+                return new OptionTy(falseType)
+            }
+
+            if (falseType instanceof NullTy) {
+                return new OptionTy(trueType)
+            }
+
+            return trueType
         }
 
         if (node instanceof NamedNode) {

server/src/e2e/suite/testcases/types/basic.test

@@ -133,3 +133,48 @@ fun foo() {
 }
 ------------------------------------------------------------------------
 ok
+
+========================================================================
+ternary expression
+========================================================================
+primitive Int;
+primitive String;
+
+contract Contract {}
+
+fun foo() {
+    let a = true ? 10 : 20;
+    //! ^ Int
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+ternary expression with false null
+========================================================================
+primitive Int;
+primitive String;
+
+contract Contract {}
+
+fun foo() {
+    let a = true ? 10 : null;
+    //! ^ Int?
+}
+------------------------------------------------------------------------
+ok
+
+========================================================================
+ternary expression with true null
+========================================================================
+primitive Int;
+primitive String;
+
+contract Contract {}
+
+fun foo() {
+    let a = true ? null : 10;
+    //! ^ Int?
+}
+------------------------------------------------------------------------
+ok