Improve discriminated union error messages

src/compiler/checker.ts

@@ -7771,16 +7771,36 @@ namespace ts {
                 if (target.flags & TypeFlags.Union && containsType(targetTypes, source)) {
                     return Ternary.True;
                 }
-                const len = targetTypes.length;
-                for (let i = 0; i < len; i++) {
-                    const related = isRelatedTo(source, targetTypes[i], reportErrors && i === len - 1);
+                for (const type of targetTypes) {
+                    const related = isRelatedTo(source, type, /*reportErrors*/ false);
                     if (related) {
                         return related;
                     }
                 }
+                if (reportErrors) {
+                    const discriminantType = findMatchingDiscriminantType(source, target);
+                    isRelatedTo(source, discriminantType || targetTypes[targetTypes.length - 1], /*reportErrors*/ true);
+                }
                 return Ternary.False;
             }
 
+            function findMatchingDiscriminantType(source: Type, target: UnionOrIntersectionType) {
+                const sourceProperties = getPropertiesOfObjectType(source);
+                if (sourceProperties) {
+                    for (const sourceProperty of sourceProperties) {
+                        if (isDiscriminantProperty(target, sourceProperty.name)) {
+                            const sourceType = getTypeOfSymbol(sourceProperty);
+                            for (const type of target.types) {
+                                const targetType = getTypeOfPropertyOfType(type, sourceProperty.name);
+                                if (targetType && isRelatedTo(sourceType, targetType)) {
+                                    return type;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+
             function typeRelatedToEachType(source: Type, target: UnionOrIntersectionType, reportErrors: boolean): Ternary {
                 let result = Ternary.True;
                 const targetTypes = target.types;

tests/baselines/reference/discriminatedUnionErrorMessage.errors.txt

@@ -0,0 +1,23 @@
+tests/cases/compiler/discriminatedUnionErrorMessage.ts(8,5): error TS2322: Type '{ kind: "sq"; x: number; y: number; }' is not assignable to type 'Shape'.
+  Type '{ kind: "sq"; x: number; y: number; }' is not assignable to type 'Square'.
+    Property 'size' is missing in type '{ kind: "sq"; x: number; y: number; }'.
+
+
+==== tests/cases/compiler/discriminatedUnionErrorMessage.ts (1 errors) ====
+    type Square = { kind: "sq", size: number }
+    type Rectangle = { kind: "rt", x: number, y: number }
+    type Circle = { kind: "cr", radius: number }
+    type Shape =
+        | Square
+        | Rectangle
+        | Circle;
+    let shape: Shape = {
+        ~~~~~
+!!! error TS2322: Type '{ kind: "sq"; x: number; y: number; }' is not assignable to type 'Shape'.
+!!! error TS2322:   Type '{ kind: "sq"; x: number; y: number; }' is not assignable to type 'Square'.
+!!! error TS2322:     Property 'size' is missing in type '{ kind: "sq"; x: number; y: number; }'.
+        kind: "sq",
+        x: 12,
+        y: 13,
+    }
+

tests/baselines/reference/discriminatedUnionErrorMessage.js

@@ -0,0 +1,21 @@
+//// [discriminatedUnionErrorMessage.ts]
+type Square = { kind: "sq", size: number }
+type Rectangle = { kind: "rt", x: number, y: number }
+type Circle = { kind: "cr", radius: number }
+type Shape =
+    | Square
+    | Rectangle
+    | Circle;
+let shape: Shape = {
+    kind: "sq",
+    x: 12,
+    y: 13,
+}
+
+
+//// [discriminatedUnionErrorMessage.js]
+var shape = {
+    kind: "sq",
+    x: 12,
+    y: 13
+};

tests/cases/compiler/discriminatedUnionErrorMessage.ts

@@ -0,0 +1,12 @@
+type Square = { kind: "sq", size: number }
+type Rectangle = { kind: "rt", x: number, y: number }
+type Circle = { kind: "cr", radius: number }
+type Shape =
+    | Square
+    | Rectangle
+    | Circle;
+let shape: Shape = {
+    kind: "sq",
+    x: 12,
+    y: 13,
+}