regard TemplateLiteral as discriminant property (#44569)

src/compiler/checker.ts

@@ -22031,7 +22031,7 @@ namespace ts {
                     if ((prop as TransientSymbol).isDiscriminantProperty === undefined) {
                         (prop as TransientSymbol).isDiscriminantProperty =
                             ((prop as TransientSymbol).checkFlags & CheckFlags.Discriminant) === CheckFlags.Discriminant &&
-                            !maybeTypeOfKind(getTypeOfSymbol(prop), TypeFlags.Instantiable);
+                            !maybeTypeOfKind(getTypeOfSymbol(prop), TypeFlags.Instantiable & ~TypeFlags.TemplateLiteral);
                     }
                     return !!(prop as TransientSymbol).isDiscriminantProperty;
                 }

tests/baselines/reference/discriminatedUnionTypes3.js

@@ -0,0 +1,26 @@
+//// [discriminatedUnionTypes3.ts]
+// Repro from #44435
+
+type Correct = {
+	code: string
+	property: true
+	err: undefined
+}
+type Err = {
+	err: `${string} is wrong!`
+}
+type SomeReturnType = Correct | Err;
+
+const example: SomeReturnType = {} as SomeReturnType;
+
+if (example.err === undefined) {
+	example.property;  // true
+}
+
+//// [discriminatedUnionTypes3.js]
+"use strict";
+// Repro from #44435
+var example = {};
+if (example.err === undefined) {
+    example.property; // true
+}

tests/baselines/reference/discriminatedUnionTypes3.symbols

@@ -0,0 +1,42 @@
+=== tests/cases/conformance/types/union/discriminatedUnionTypes3.ts ===
+// Repro from #44435
+
+type Correct = {
+>Correct : Symbol(Correct, Decl(discriminatedUnionTypes3.ts, 0, 0))
+
+	code: string
+>code : Symbol(code, Decl(discriminatedUnionTypes3.ts, 2, 16))
+
+	property: true
+>property : Symbol(property, Decl(discriminatedUnionTypes3.ts, 3, 13))
+
+	err: undefined
+>err : Symbol(err, Decl(discriminatedUnionTypes3.ts, 4, 15))
+}
+type Err = {
+>Err : Symbol(Err, Decl(discriminatedUnionTypes3.ts, 6, 1))
+
+	err: `${string} is wrong!`
+>err : Symbol(err, Decl(discriminatedUnionTypes3.ts, 7, 12))
+}
+type SomeReturnType = Correct | Err;
+>SomeReturnType : Symbol(SomeReturnType, Decl(discriminatedUnionTypes3.ts, 9, 1))
+>Correct : Symbol(Correct, Decl(discriminatedUnionTypes3.ts, 0, 0))
+>Err : Symbol(Err, Decl(discriminatedUnionTypes3.ts, 6, 1))
+
+const example: SomeReturnType = {} as SomeReturnType;
+>example : Symbol(example, Decl(discriminatedUnionTypes3.ts, 12, 5))
+>SomeReturnType : Symbol(SomeReturnType, Decl(discriminatedUnionTypes3.ts, 9, 1))
+>SomeReturnType : Symbol(SomeReturnType, Decl(discriminatedUnionTypes3.ts, 9, 1))
+
+if (example.err === undefined) {
+>example.err : Symbol(err, Decl(discriminatedUnionTypes3.ts, 4, 15), Decl(discriminatedUnionTypes3.ts, 7, 12))
+>example : Symbol(example, Decl(discriminatedUnionTypes3.ts, 12, 5))
+>err : Symbol(err, Decl(discriminatedUnionTypes3.ts, 4, 15), Decl(discriminatedUnionTypes3.ts, 7, 12))
+>undefined : Symbol(undefined)
+
+	example.property;  // true
+>example.property : Symbol(property, Decl(discriminatedUnionTypes3.ts, 3, 13))
+>example : Symbol(example, Decl(discriminatedUnionTypes3.ts, 12, 5))
+>property : Symbol(property, Decl(discriminatedUnionTypes3.ts, 3, 13))
+}

tests/baselines/reference/discriminatedUnionTypes3.types

@@ -0,0 +1,42 @@
+=== tests/cases/conformance/types/union/discriminatedUnionTypes3.ts ===
+// Repro from #44435
+
+type Correct = {
+>Correct : Correct
+
+	code: string
+>code : string
+
+	property: true
+>property : true
+>true : true
+
+	err: undefined
+>err : undefined
+}
+type Err = {
+>Err : Err
+
+	err: `${string} is wrong!`
+>err : `${string} is wrong!`
+}
+type SomeReturnType = Correct | Err;
+>SomeReturnType : SomeReturnType
+
+const example: SomeReturnType = {} as SomeReturnType;
+>example : SomeReturnType
+>{} as SomeReturnType : SomeReturnType
+>{} : {}
+
+if (example.err === undefined) {
+>example.err === undefined : boolean
+>example.err : `${string} is wrong!` | undefined
+>example : SomeReturnType
+>err : `${string} is wrong!` | undefined
+>undefined : undefined
+
+	example.property;  // true
+>example.property : true
+>example : Correct
+>property : true
+}

tests/cases/conformance/types/union/discriminatedUnionTypes3.ts

@@ -0,0 +1,19 @@
+// @strict: true
+
+// Repro from #44435
+
+type Correct = {
+	code: string
+	property: true
+	err: undefined
+}
+type Err = {
+	err: `${string} is wrong!`
+}
+type SomeReturnType = Correct | Err;
+
+const example: SomeReturnType = {} as SomeReturnType;
+
+if (example.err === undefined) {
+	example.property;  // true
+}