Fix non-null discriminant narrowing consistency

tsc/internal/checker/flow.go

@@ -707,7 +707,8 @@ func (c *Checker) narrowTypeByDiscriminantProperty(t *Type, access *ast.Node, op
 				candidate := c.getConstituentTypeForKeyType(t, c.getTypeOfExpression(value))
 				if candidate != nil {
 					if assumeTrue && operator == ast.KindEqualsEqualsEqualsToken || !assumeTrue && operator == ast.KindExclamationEqualsEqualsToken {
-						return candidate
+						// Nullable constituents have no key property; preserve them as the general path does.
+						return c.getUnionType([]*Type{candidate, c.filterType(t, func(t *Type) bool { return t.flags&TypeFlagsNullable != 0 })})
 					}
 					if propType := c.getTypeOfPropertyOfType(candidate, keyPropertyName); propType != nil && isUnitType(propType) {
 						return c.removeType(t, candidate)

tsc/testdata/baselines/reference/compiler/narrowingUnionWithBang.types

@@ -83,9 +83,9 @@ if (working.thing!.name !== "Correct") {
 >log : (...data: any[]) => void
 >working.thing!.id : string
 >working.thing! : { name: 'Correct'; id: string; }
->working.thing : { name: 'Correct'; id: string; }
+>working.thing : { name: 'Correct'; id: string; } | undefined
 >working : WorkingType
->thing : { name: 'Correct'; id: string; }
+>thing : { name: 'Correct'; id: string; } | undefined
 >id : string
 }
 

tsc/testdata/tests/cases/compiler/discriminatedUnionNonNullAccessNarrowing.ts

@@ -0,0 +1,35 @@
+// @strict: true
+// @noEmit: true
+// @noTypesAndSymbols: true
+
+// Discriminant narrowing should be consistent regardless of union size (#62511).
+
+type Small =
+    | { type: "1" }
+    | { type: "2" }
+    | undefined;
+
+type Large =
+    | { type: "1" }
+    | { type: "2" }
+    | { type: "3" }
+    | { type: "4" }
+    | { type: "5" }
+    | { type: "6" }
+    | { type: "7" }
+    | { type: "8" }
+    | { type: "9" }
+    | { type: "10" }
+    | undefined;
+
+declare let small: Small;
+if (small!.type === "1") {
+    // @ts-expect-error
+    small.type;
+}
+
+declare let large: Large;
+if (large!.type === "1") {
+    // @ts-expect-error
+    large.type;
+}