Fix location for duplicate function implementation errors

Use only the relevant declarations (by collecting them in the for loop),
and use `declaration` if `getNameOfDeclaration` didn't work (useful for
`export default` with anonymous functions).

Fixes microsoft#39804.

Also, use `nodeIsPresent` once, and a random `?.`.

Author: elibarzilay

src/compiler/checker.ts

@@ -31280,6 +31280,7 @@ namespace ts {
             let duplicateFunctionDeclaration = false;
             let multipleConstructorImplementation = false;
             let hasNonAmbientClass = false;
+            const functionDeclarations = [] as Declaration[];
             for (const current of declarations) {
                 const node = <SignatureDeclaration | ClassDeclaration | ClassExpression>current;
                 const inAmbientContext = node.flags & NodeFlags.Ambient;
@@ -31300,25 +31301,27 @@ namespace ts {
                 }
 
                 if (node.kind === SyntaxKind.FunctionDeclaration || node.kind === SyntaxKind.MethodDeclaration || node.kind === SyntaxKind.MethodSignature || node.kind === SyntaxKind.Constructor) {
+                    functionDeclarations.push(node);
                     const currentNodeFlags = getEffectiveDeclarationFlags(node, flagsToCheck);
                     someNodeFlags |= currentNodeFlags;
                     allNodeFlags &= currentNodeFlags;
                     someHaveQuestionToken = someHaveQuestionToken || hasQuestionToken(node);
                     allHaveQuestionToken = allHaveQuestionToken && hasQuestionToken(node);
+                    const bodyIsPresent = nodeIsPresent((node as FunctionLikeDeclaration).body);
 
-                    if (nodeIsPresent((node as FunctionLikeDeclaration).body) && bodyDeclaration) {
+                    if (bodyIsPresent && bodyDeclaration) {
                         if (isConstructor) {
                             multipleConstructorImplementation = true;
                         }
                         else {
                             duplicateFunctionDeclaration = true;
                         }
                     }
-                    else if (previousDeclaration && previousDeclaration.parent === node.parent && previousDeclaration.end !== node.pos) {
+                    else if (previousDeclaration?.parent === node.parent && previousDeclaration.end !== node.pos) {
                         reportImplementationExpectedError(previousDeclaration);
                     }
 
-                    if (nodeIsPresent((node as FunctionLikeDeclaration).body)) {
+                    if (bodyIsPresent) {
                         if (!bodyDeclaration) {
                             bodyDeclaration = node as FunctionLikeDeclaration;
                         }
@@ -31336,14 +31339,14 @@ namespace ts {
             }
 
             if (multipleConstructorImplementation) {
-                forEach(declarations, declaration => {
+                forEach(functionDeclarations, declaration => {
                     error(declaration, Diagnostics.Multiple_constructor_implementations_are_not_allowed);
                 });
             }
 
             if (duplicateFunctionDeclaration) {
-                forEach(declarations, declaration => {
-                    error(getNameOfDeclaration(declaration), Diagnostics.Duplicate_function_implementation);
+                forEach(functionDeclarations, declaration => {
+                    error(getNameOfDeclaration(declaration) || declaration, Diagnostics.Duplicate_function_implementation);
                 });
             }
 

tests/baselines/reference/exportDefaultInterfaceAndTwoFunctions.errors.txt

@@ -0,0 +1,13 @@
+tests/cases/compiler/exportDefaultInterfaceAndTwoFunctions.ts(2,1): error TS2393: Duplicate function implementation.
+tests/cases/compiler/exportDefaultInterfaceAndTwoFunctions.ts(3,1): error TS2393: Duplicate function implementation.
+
+
+==== tests/cases/compiler/exportDefaultInterfaceAndTwoFunctions.ts (2 errors) ====
+    export default interface A { a: string; }
+    export default function() { return 1; }
+    ~~~~~~
+!!! error TS2393: Duplicate function implementation.
+    export default function() { return 2; }
+    ~~~~~~
+!!! error TS2393: Duplicate function implementation.
+    

tests/baselines/reference/exportDefaultInterfaceAndTwoFunctions.js

@@ -0,0 +1,13 @@
+//// [exportDefaultInterfaceAndTwoFunctions.ts]
+export default interface A { a: string; }
+export default function() { return 1; }
+export default function() { return 2; }
+
+
+//// [exportDefaultInterfaceAndTwoFunctions.js]
+"use strict";
+exports.__esModule = true;
+function default_1() { return 1; }
+exports["default"] = default_1;
+function default_2() { return 2; }
+exports["default"] = default_2;

tests/baselines/reference/exportDefaultInterfaceAndTwoFunctions.symbols

@@ -0,0 +1,8 @@
+=== tests/cases/compiler/exportDefaultInterfaceAndTwoFunctions.ts ===
+export default interface A { a: string; }
+>A : Symbol(A, Decl(exportDefaultInterfaceAndTwoFunctions.ts, 0, 41), Decl(exportDefaultInterfaceAndTwoFunctions.ts, 1, 39), Decl(exportDefaultInterfaceAndTwoFunctions.ts, 0, 0))
+>a : Symbol(A.a, Decl(exportDefaultInterfaceAndTwoFunctions.ts, 0, 28))
+
+export default function() { return 1; }
+export default function() { return 2; }
+

tests/baselines/reference/exportDefaultInterfaceAndTwoFunctions.types

@@ -0,0 +1,10 @@
+=== tests/cases/compiler/exportDefaultInterfaceAndTwoFunctions.ts ===
+export default interface A { a: string; }
+>a : string
+
+export default function() { return 1; }
+>1 : 1
+
+export default function() { return 2; }
+>2 : 2
+

tests/cases/compiler/exportDefaultInterfaceAndTwoFunctions.ts

@@ -0,0 +1,3 @@
+export default interface A { a: string; }
+export default function() { return 1; }
+export default function() { return 2; }