Skip to non null targets in error messages

src/compiler/checker.ts

@@ -10631,6 +10631,23 @@ namespace ts {
                     target = getSimplifiedType(target);
                 }
 
+                // Try to see if we're relating something like `Foo` -> `Bar | null | undefined`.
+                // If so, reporting the `null` and `undefined` in the type is hardly useful.
+                // First, see if we're even relating an object type to a union.
+                // Then see if the target is stripped down to a single non-union type.
+                // Note
+                //  * We actually want to remove null and undefined naively here (rather than using getNonNullableType),
+                //    since we don't want to end up with a worse error like "`Foo` is not assignable to `NonNullable<T>`"
+                //    when dealing with generics.
+                //  * We also don't deal with primitive source types, since we already halt elaboration below.
+                if (target.flags & TypeFlags.Union && source.flags & TypeFlags.Object &&
+                    (target as UnionType).types.length <= 3 && maybeTypeOfKind(target, TypeFlags.Nullable)) {
+                    const nullStrippedTarget = extractTypesOfKind(target, ~TypeFlags.Nullable);
+                    if (!(nullStrippedTarget.flags & (TypeFlags.Union | TypeFlags.Never))) {
+                        target = nullStrippedTarget;
+                    }
+                }
+
                 // both types are the same - covers 'they are the same primitive type or both are Any' or the same type parameter cases
                 if (source === target) return Ternary.True;
 
@@ -12223,7 +12240,7 @@ namespace ts {
             if (deferredGlobalNonNullableTypeAlias !== unknownSymbol) {
                 return getTypeAliasInstantiation(deferredGlobalNonNullableTypeAlias, [type]);
             }
-            return getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull); // Type alias unavailable, fall back to non-higherorder behavior
+            return getTypeWithFacts(type, TypeFacts.NEUndefinedOrNull); // Type alias unavailable, fall back to non-higher-order behavior
         }
 
         function getNonNullableType(type: Type): Type {

tests/baselines/reference/elaboratedErrorsOnNullableTargets01.errors.txt

@@ -0,0 +1,31 @@
+tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts(4,1): error TS2322: Type '{ foo: { bar: number; }; }' is not assignable to type '{ foo: { bar: string; }; }'.
+  Types of property 'foo' are incompatible.
+    Type '{ bar: number; }' is not assignable to type '{ bar: string; }'.
+      Types of property 'bar' are incompatible.
+        Type 'number' is not assignable to type 'string'.
+tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts(6,1): error TS2322: Type '{ foo: { bar: string; }; }' is not assignable to type '{ foo: { bar: number; }; }'.
+  Types of property 'foo' are incompatible.
+    Type '{ bar: string; }' is not assignable to type '{ bar: number; }'.
+      Types of property 'bar' are incompatible.
+        Type 'string' is not assignable to type 'number'.
+
+
+==== tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts (2 errors) ====
+    export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
+    export declare let y: { foo: { bar: number | undefined } };
+
+    x = y;
+    ~
+!!! error TS2322: Type '{ foo: { bar: number; }; }' is not assignable to type '{ foo: { bar: string; }; }'.
+!!! error TS2322:   Types of property 'foo' are incompatible.
+!!! error TS2322:     Type '{ bar: number; }' is not assignable to type '{ bar: string; }'.
+!!! error TS2322:       Types of property 'bar' are incompatible.
+!!! error TS2322:         Type 'number' is not assignable to type 'string'.
+
+    y = x;
+    ~
+!!! error TS2322: Type '{ foo: { bar: string; }; }' is not assignable to type '{ foo: { bar: number; }; }'.
+!!! error TS2322:   Types of property 'foo' are incompatible.
+!!! error TS2322:     Type '{ bar: string; }' is not assignable to type '{ bar: number; }'.
+!!! error TS2322:       Types of property 'bar' are incompatible.
+!!! error TS2322:         Type 'string' is not assignable to type 'number'.

tests/baselines/reference/elaboratedErrorsOnNullableTargets01.js

@@ -0,0 +1,13 @@
+//// [elaboratedErrorsOnNullableTargets01.ts]
+export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
+export declare let y: { foo: { bar: number | undefined } };
+
+x = y;
+
+y = x;
+
+//// [elaboratedErrorsOnNullableTargets01.js]
+"use strict";
+exports.__esModule = true;
+exports.x = exports.y;
+exports.y = exports.x;

tests/baselines/reference/elaboratedErrorsOnNullableTargets01.symbols

@@ -0,0 +1,19 @@
+=== tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts ===
+export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
+>x : Symbol(x, Decl(elaboratedErrorsOnNullableTargets01.ts, 0, 18))
+>foo : Symbol(foo, Decl(elaboratedErrorsOnNullableTargets01.ts, 0, 30))
+>bar : Symbol(bar, Decl(elaboratedErrorsOnNullableTargets01.ts, 0, 37))
+
+export declare let y: { foo: { bar: number | undefined } };
+>y : Symbol(y, Decl(elaboratedErrorsOnNullableTargets01.ts, 1, 18))
+>foo : Symbol(foo, Decl(elaboratedErrorsOnNullableTargets01.ts, 1, 23))
+>bar : Symbol(bar, Decl(elaboratedErrorsOnNullableTargets01.ts, 1, 30))
+
+x = y;
+>x : Symbol(x, Decl(elaboratedErrorsOnNullableTargets01.ts, 0, 18))
+>y : Symbol(y, Decl(elaboratedErrorsOnNullableTargets01.ts, 1, 18))
+
+y = x;
+>y : Symbol(y, Decl(elaboratedErrorsOnNullableTargets01.ts, 1, 18))
+>x : Symbol(x, Decl(elaboratedErrorsOnNullableTargets01.ts, 0, 18))
+

tests/baselines/reference/elaboratedErrorsOnNullableTargets01.types

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts ===
+export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
+>x : { foo: { bar: string; }; }
+>null : null
+>foo : { bar: string; }
+>bar : string
+>null : null
+
+export declare let y: { foo: { bar: number | undefined } };
+>y : { foo: { bar: number; }; }
+>foo : { bar: number; }
+>bar : number
+
+x = y;
+>x = y : { foo: { bar: number; }; }
+>x : { foo: { bar: string; }; }
+>y : { foo: { bar: number; }; }
+
+y = x;
+>y = x : { foo: { bar: string; }; }
+>y : { foo: { bar: number; }; }
+>x : { foo: { bar: string; }; }
+

tests/baselines/reference/nestedFreshLiteral.errors.txt

@@ -1,10 +1,9 @@
 tests/cases/compiler/nestedFreshLiteral.ts(12,21): error TS2322: Type '{ nested: { prop: { colour: string; }; }; }' is not assignable to type 'NestedCSSProps'.
   Types of property 'nested' are incompatible.
-    Type '{ prop: { colour: string; }; }' is not assignable to type 'NestedSelector | undefined'.
-      Type '{ prop: { colour: string; }; }' is not assignable to type 'NestedSelector'.
-        Types of property 'prop' are incompatible.
-          Type '{ colour: string; }' is not assignable to type 'CSSProps'.
-            Object literal may only specify known properties, but 'colour' does not exist in type 'CSSProps'. Did you mean to write 'color'?
+    Type '{ prop: { colour: string; }; }' is not assignable to type 'NestedSelector'.
+      Types of property 'prop' are incompatible.
+        Type '{ colour: string; }' is not assignable to type 'CSSProps'.
+          Object literal may only specify known properties, but 'colour' does not exist in type 'CSSProps'. Did you mean to write 'color'?
 
 
 ==== tests/cases/compiler/nestedFreshLiteral.ts (1 errors) ====
@@ -23,9 +22,8 @@ tests/cases/compiler/nestedFreshLiteral.ts(12,21): error TS2322: Type '{ nested:
                         ~~~~~~~~~~~~~
 !!! error TS2322: Type '{ nested: { prop: { colour: string; }; }; }' is not assignable to type 'NestedCSSProps'.
 !!! error TS2322:   Types of property 'nested' are incompatible.
-!!! error TS2322:     Type '{ prop: { colour: string; }; }' is not assignable to type 'NestedSelector | undefined'.
-!!! error TS2322:       Type '{ prop: { colour: string; }; }' is not assignable to type 'NestedSelector'.
-!!! error TS2322:         Types of property 'prop' are incompatible.
-!!! error TS2322:           Type '{ colour: string; }' is not assignable to type 'CSSProps'.
-!!! error TS2322:             Object literal may only specify known properties, but 'colour' does not exist in type 'CSSProps'. Did you mean to write 'color'?
+!!! error TS2322:     Type '{ prop: { colour: string; }; }' is not assignable to type 'NestedSelector'.
+!!! error TS2322:       Types of property 'prop' are incompatible.
+!!! error TS2322:         Type '{ colour: string; }' is not assignable to type 'CSSProps'.
+!!! error TS2322:           Object literal may only specify known properties, but 'colour' does not exist in type 'CSSProps'. Did you mean to write 'color'?
     }

tests/cases/compiler/elaboratedErrorsOnNullableTargets01.ts

@@ -0,0 +1,7 @@
+
+export declare let x: null | { foo: { bar: string | null } | undefined } | undefined;
+export declare let y: { foo: { bar: number | undefined } };
+
+x = y;
+
+y = x;