Skip leading signature incompatability flattening

Author: weswigham

src/compiler/checker.ts

@@ -12343,7 +12343,7 @@ namespace ts {
             ignoreReturnTypes: boolean,
             reportErrors: boolean,
             errorReporter: ErrorReporter | undefined,
-            incompatibleErrorReporter: (() => void) | undefined,
+            incompatibleErrorReporter: ((source: Type, target: Type) => void) | undefined,
             compareTypes: TypeComparer): Ternary {
             // TODO (drosen): De-duplicate code between related functions.
             if (source === target) {
@@ -12461,7 +12461,7 @@ namespace ts {
                     result &= callbackCheck === CallbackCheck.Bivariant && compareTypes(targetReturnType, sourceReturnType, /*reportErrors*/ false) ||
                         compareTypes(sourceReturnType, targetReturnType, reportErrors);
                     if (!result && reportErrors && incompatibleErrorReporter) {
-                        incompatibleErrorReporter();
+                        incompatibleErrorReporter(sourceReturnType, targetReturnType);
                     }
                 }
 
@@ -12766,6 +12766,7 @@ namespace ts {
                 // The first error will be the innermost, while the last will be the outermost - so by popping off the end,
                 // we can build from left to right
                 let path = "";
+                const secondaryRootErrors: typeof incompatibleStack = [];
                 while (stack.length) {
                     const [msg, ...args] = stack.pop()!;
                     switch (msg.code) {
@@ -12793,27 +12794,52 @@ namespace ts {
                             }
                             break;
                         }
-                        case Diagnostics.Call_signature_return_types_are_incompatible.code: {
-                            path = path.length === 0 ? "(...)" : `${path}(...)`;
-                            break;
-                        }
-                        case Diagnostics.Construct_signature_return_types_are_incompatible.code: {
-                            path = path.length === 0 ? "new (...)" : `new ${path}(...)`;
-                            break;
-                        }
-                        case Diagnostics.Call_signatures_with_no_arguments_have_incompatible_return_types.code: {
-                            path = path.length === 0 ? "()" : `${path}()`;
-                            break;
-                        }
-                        case Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types.code: {
-                            path = path.length === 0 ? "new ()" : `new ${path}()`;
+                        case Diagnostics.Call_signature_return_types_0_and_1_are_incompatible.code:
+                        case Diagnostics.Construct_signature_return_types_0_and_1_are_incompatible.code:
+                        case Diagnostics.Call_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code:
+                        case Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code: {
+                            if (path.length === 0) {
+                                // Don't flatten signature compatability errors at the start of a chain - instead prefer
+                                // to unify (the with no arguments bit is excessive for printback) and print them back
+                                let mappedMsg = msg;
+                                if (msg.code === Diagnostics.Call_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code) {
+                                    mappedMsg = Diagnostics.Call_signature_return_types_0_and_1_are_incompatible;
+                                }
+                                else if (msg.code === Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code) {
+                                    mappedMsg = Diagnostics.Construct_signature_return_types_0_and_1_are_incompatible;
+                                }
+                                secondaryRootErrors.unshift([mappedMsg, args[0], args[1]]);
+                            }
+                            else {
+                                const prefix = (msg.code === Diagnostics.Construct_signature_return_types_0_and_1_are_incompatible.code ||
+                                    msg.code === Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code)
+                                        ? "new "
+                                        : "";
+                                const params = (msg.code === Diagnostics.Call_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code ||
+                                    msg.code === Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types_0_and_1.code)
+                                        ? ""
+                                        : "...";
+                                path = `${prefix}${path}(${params})`;
+                            }
                             break;
                         }
                         default:
                             return Debug.fail(`Unhandled Diagnostic: ${msg.code}`);
                     }
                 }
-                reportError(Diagnostics.The_types_of_0_are_incompatible_between_these_types, path);
+                if (path) {
+                    reportError(Diagnostics.The_types_of_0_are_incompatible_between_these_types, path);
+                }
+                else {
+                    // Remove the innermost secondary error as it will duplicate the error already reported by `reportRelationError` on entry
+                    secondaryRootErrors.shift();
+                }
+                for (const [msg, ...args] of secondaryRootErrors) {
+                    const originalValue = msg.elidedInCompatabilityPyramid;
+                    msg.elidedInCompatabilityPyramid = false; // Teporarily override elision to ensure error is reported
+                    reportError(msg, ...args);
+                    msg.elidedInCompatabilityPyramid = originalValue;
+                }
                 if (info) {
                     // Actually do the last relation error
                     reportRelationError(/*headMessage*/ undefined, ...info);
@@ -14304,22 +14330,22 @@ namespace ts {
 
             function reportIncompatibleCallSignatureReturn(siga: Signature, sigb: Signature) {
                 if (siga.parameters.length === 0 && sigb.parameters.length === 0) {
-                    return () => reportIncompatibleError(Diagnostics.Call_signatures_with_no_arguments_have_incompatible_return_types);
+                    return (source: Type, target: Type) => reportIncompatibleError(Diagnostics.Call_signatures_with_no_arguments_have_incompatible_return_types_0_and_1, typeToString(source), typeToString(target));
                 }
-                return () => reportIncompatibleError(Diagnostics.Call_signature_return_types_are_incompatible);
+                return (source: Type, target: Type) => reportIncompatibleError(Diagnostics.Call_signature_return_types_0_and_1_are_incompatible, typeToString(source), typeToString(target));
             }
 
             function reportIncompatibleConstructSignatureReturn(siga: Signature, sigb: Signature) {
                 if (siga.parameters.length === 0 && sigb.parameters.length === 0) {
-                    return () => reportIncompatibleError(Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types);
+                    return (source: Type, target: Type) => reportIncompatibleError(Diagnostics.Construct_signatures_with_no_arguments_have_incompatible_return_types_0_and_1, typeToString(source), typeToString(target));
                 }
-                return () => reportIncompatibleError(Diagnostics.Construct_signature_return_types_are_incompatible);
+                return (source: Type, target: Type) => reportIncompatibleError(Diagnostics.Construct_signature_return_types_0_and_1_are_incompatible, typeToString(source), typeToString(target));
             }
 
             /**
              * See signatureAssignableTo, compareSignaturesIdentical
              */
-            function signatureRelatedTo(source: Signature, target: Signature, erase: boolean, reportErrors: boolean, incompatibleReporter: () => void): Ternary {
+            function signatureRelatedTo(source: Signature, target: Signature, erase: boolean, reportErrors: boolean, incompatibleReporter: (source: Type, target: Type) => void): Ternary {
                 return compareSignaturesRelated(erase ? getErasedSignature(source) : source, erase ? getErasedSignature(target) : target,
                     CallbackCheck.None, /*ignoreReturnTypes*/ false, reportErrors, reportError, incompatibleReporter, isRelatedTo);
             }

src/compiler/diagnosticMessages.json

@@ -1044,22 +1044,22 @@
         "category": "Error",
         "code": 2200
     },
-    "Call signature return types are incompatible.": {
+    "Call signature return types '{0}' and '{1}' are incompatible.": {
         "category": "Error",
         "code": 2201,
         "elidedInCompatabilityPyramid": true
     },
-    "Construct signature return types are incompatible.": {
+    "Construct signature return types '{0}' and '{1}' are incompatible.": {
         "category": "Error",
         "code": 2202,
         "elidedInCompatabilityPyramid": true
     },
-    "Call signatures with no arguments have incompatible return types.": {
+    "Call signatures with no arguments have incompatible return types '{0}' and '{1}'.": {
         "category": "Error",
         "code": 2203,
         "elidedInCompatabilityPyramid": true
     },
-    "Construct signatures with no arguments have incompatible return types.": {
+    "Construct signatures with no arguments have incompatible return types '{0}' and '{1}'.": {
         "category": "Error",
         "code": 2204,
         "elidedInCompatabilityPyramid": true

tests/baselines/reference/asyncFunctionDeclaration15_es5.errors.txt

@@ -5,8 +5,9 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(8,23): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(9,23): error TS1055: Type 'PromiseLike' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(10,23): error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
-  The types of '(new (...)).then(...)' are incompatible between these types.
-    Type 'void' is not assignable to type 'PromiseLike<TResult1 | TResult2>'.
+  Construct signature return types 'Thenable' and 'PromiseLike<T>' are incompatible.
+    The types of 'then(...)' are incompatible between these types.
+      Type 'void' is not assignable to type 'PromiseLike<TResult1 | TResult2>'.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(17,16): error TS1058: The return type of an async function must either be a valid promise or must not contain a callable 'then' member.
 tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration15_es5.ts(23,25): error TS1320: Type of 'await' operand must either be a valid promise or must not contain a callable 'then' member.
 
@@ -36,8 +37,9 @@ tests/cases/conformance/async/es5/functionDeclarations/asyncFunctionDeclaration1
     async function fn6(): Thenable { } // error
                           ~~~~~~~~
 !!! error TS1055: Type 'typeof Thenable' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
-!!! error TS1055:   The types of '(new (...)).then(...)' are incompatible between these types.
-!!! error TS1055:     Type 'void' is not assignable to type 'PromiseLike<TResult1 | TResult2>'.
+!!! error TS1055:   Construct signature return types 'Thenable' and 'PromiseLike<T>' are incompatible.
+!!! error TS1055:     The types of 'then(...)' are incompatible between these types.
+!!! error TS1055:       Type 'void' is not assignable to type 'PromiseLike<TResult1 | TResult2>'.
     async function fn7() { return; } // valid: Promise<void>
     async function fn8() { return 1; } // valid: Promise<number>
     async function fn9() { return null; } // valid: Promise<any>

tests/baselines/reference/generatorTypeCheck25.errors.txt

@@ -1,10 +1,11 @@
 tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck25.ts(4,5): error TS2322: Type '() => Generator<Bar | Baz, void, undefined>' is not assignable to type '() => Iterable<Foo>'.
-  The types of '()[Symbol.iterator]().next(...)' are incompatible between these types.
-    Type 'IteratorResult<Bar | Baz, void>' is not assignable to type 'IteratorResult<Foo, any>'.
-      Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorResult<Foo, any>'.
-        Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorYieldResult<Foo>'.
-          Type 'Bar | Baz' is not assignable to type 'Foo'.
-            Property 'x' is missing in type 'Baz' but required in type 'Foo'.
+  Call signature return types 'Generator<Bar | Baz, void, undefined>' and 'Iterable<Foo>' are incompatible.
+    The types of '[Symbol.iterator]().next(...)' are incompatible between these types.
+      Type 'IteratorResult<Bar | Baz, void>' is not assignable to type 'IteratorResult<Foo, any>'.
+        Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorResult<Foo, any>'.
+          Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorYieldResult<Foo>'.
+            Type 'Bar | Baz' is not assignable to type 'Foo'.
+              Property 'x' is missing in type 'Baz' but required in type 'Foo'.
 
 
 ==== tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck25.ts (1 errors) ====
@@ -14,12 +15,13 @@ tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck25.ts(4,5): error
     var g3: () => Iterable<Foo> = function* () {
         ~~
 !!! error TS2322: Type '() => Generator<Bar | Baz, void, undefined>' is not assignable to type '() => Iterable<Foo>'.
-!!! error TS2322:   The types of '()[Symbol.iterator]().next(...)' are incompatible between these types.
-!!! error TS2322:     Type 'IteratorResult<Bar | Baz, void>' is not assignable to type 'IteratorResult<Foo, any>'.
-!!! error TS2322:       Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorResult<Foo, any>'.
-!!! error TS2322:         Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorYieldResult<Foo>'.
-!!! error TS2322:           Type 'Bar | Baz' is not assignable to type 'Foo'.
-!!! error TS2322:             Property 'x' is missing in type 'Baz' but required in type 'Foo'.
+!!! error TS2322:   Call signature return types 'Generator<Bar | Baz, void, undefined>' and 'Iterable<Foo>' are incompatible.
+!!! error TS2322:     The types of '[Symbol.iterator]().next(...)' are incompatible between these types.
+!!! error TS2322:       Type 'IteratorResult<Bar | Baz, void>' is not assignable to type 'IteratorResult<Foo, any>'.
+!!! error TS2322:         Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorResult<Foo, any>'.
+!!! error TS2322:           Type 'IteratorYieldResult<Bar | Baz>' is not assignable to type 'IteratorYieldResult<Foo>'.
+!!! error TS2322:             Type 'Bar | Baz' is not assignable to type 'Foo'.
+!!! error TS2322:               Property 'x' is missing in type 'Baz' but required in type 'Foo'.
 !!! related TS2728 tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck25.ts:1:13: 'x' is declared here.
         yield;
         yield new Bar;

tests/baselines/reference/generatorTypeCheck63.errors.txt

@@ -1,9 +1,10 @@
 tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck63.ts(24,61): error TS2345: Argument of type '(state: State) => Generator<number, State, undefined>' is not assignable to parameter of type '(a: State) => IterableIterator<State>'.
-  The types of '(...).next(...)' are incompatible between these types.
-    Type 'IteratorResult<number, State>' is not assignable to type 'IteratorResult<State, any>'.
-      Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorResult<State, any>'.
-        Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorYieldResult<State>'.
-          Type 'number' is not assignable to type 'State'.
+  Call signature return types 'Generator<number, State, undefined>' and 'IterableIterator<State>' are incompatible.
+    The types of 'next(...)' are incompatible between these types.
+      Type 'IteratorResult<number, State>' is not assignable to type 'IteratorResult<State, any>'.
+        Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorResult<State, any>'.
+          Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorYieldResult<State>'.
+            Type 'number' is not assignable to type 'State'.
 
 
 ==== tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck63.ts (1 errors) ====
@@ -33,11 +34,12 @@ tests/cases/conformance/es6/yieldExpressions/generatorTypeCheck63.ts(24,61): err
     export const Nothing: Strategy<State> = strategy("Nothing", function* (state: State) {
                                                                 ~~~~~~~~
 !!! error TS2345: Argument of type '(state: State) => Generator<number, State, undefined>' is not assignable to parameter of type '(a: State) => IterableIterator<State>'.
-!!! error TS2345:   The types of '(...).next(...)' are incompatible between these types.
-!!! error TS2345:     Type 'IteratorResult<number, State>' is not assignable to type 'IteratorResult<State, any>'.
-!!! error TS2345:       Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorResult<State, any>'.
-!!! error TS2345:         Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorYieldResult<State>'.
-!!! error TS2345:           Type 'number' is not assignable to type 'State'.
+!!! error TS2345:   Call signature return types 'Generator<number, State, undefined>' and 'IterableIterator<State>' are incompatible.
+!!! error TS2345:     The types of 'next(...)' are incompatible between these types.
+!!! error TS2345:       Type 'IteratorResult<number, State>' is not assignable to type 'IteratorResult<State, any>'.
+!!! error TS2345:         Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorResult<State, any>'.
+!!! error TS2345:           Type 'IteratorYieldResult<number>' is not assignable to type 'IteratorYieldResult<State>'.
+!!! error TS2345:             Type 'number' is not assignable to type 'State'.
         yield 1;
         return state;
     });