Improve error span:duplicate symbols cross-js/ts

when the JS symbol is a JS initializer

Author: sandersn

src/compiler/checker.ts

@@ -911,10 +911,12 @@ namespace ts {
                         ? Diagnostics.Cannot_redeclare_block_scoped_variable_0
                         : Diagnostics.Duplicate_identifier_0;
                 forEach(source.declarations, node => {
-                    error(getNameOfDeclaration(node) || node, message, symbolToString(source));
+                    const errorNode = (getJavascriptInitializer(node, /*isPrototypeAssignment*/ false) ? getOuterNameOfJsInitializer(node) : getNameOfDeclaration(node)) || node;
+                    error(errorNode, message, symbolToString(source));
                 });
                 forEach(target.declarations, node => {
-                    error(getNameOfDeclaration(node) || node, message, symbolToString(source));
+                    const errorNode = (getJavascriptInitializer(node, /*isPrototypeAssignment*/ false) ? getOuterNameOfJsInitializer(node) : getNameOfDeclaration(node)) || node;
+                    error(errorNode, message, symbolToString(source));
                 });
             }
         }

src/compiler/utilities.ts

@@ -1522,13 +1522,13 @@ namespace ts {
      *
      * This function returns the provided initializer, or undefined if it is not valid.
      */
-    export function getJavascriptInitializer(initializer: Expression, isPrototypeAssignment: boolean) {
+    export function getJavascriptInitializer(initializer: Node, isPrototypeAssignment: boolean): Expression {
         if (isCallExpression(initializer)) {
             const e = skipParentheses(initializer.expression);
             return e.kind === SyntaxKind.FunctionExpression || e.kind === SyntaxKind.ArrowFunction ? initializer : undefined;
         }
         if (initializer.kind === SyntaxKind.FunctionExpression || initializer.kind === SyntaxKind.ClassExpression) {
-            return initializer;
+            return initializer as Expression;
         }
         if (isObjectLiteralExpression(initializer) && (initializer.properties.length === 0 || isPrototypeAssignment)) {
             return initializer;
@@ -1550,6 +1550,19 @@ namespace ts {
         }
     }
 
+    /** Given a Javascript initializer, return the outer name. That is, the lhs of the assignment or the declaration name. */
+    export function getOuterNameOfJsInitializer(node: Declaration): DeclarationName | undefined {
+        if (isBinaryExpression(node.parent)) {
+            const parent = (node.parent.operatorToken.kind === SyntaxKind.BarBarToken && isBinaryExpression(node.parent.parent)) ? node.parent.parent : node.parent;
+            if (parent.operatorToken.kind === SyntaxKind.EqualsToken && isIdentifier(parent.left)) {
+                return parent.left;
+            }
+        }
+        else if (isVariableDeclaration(node.parent)) {
+            return node.parent.name;
+        }
+    }
+
     /**
      * Is the 'declared' name the same as the one in the initializer?
      * @return true for identical entity names, as well as ones where the initializer is prefixed with

tests/baselines/reference/jsContainerMergeTsDeclaration.errors.txt

@@ -1,14 +1,14 @@
 error TS5055: Cannot write file 'tests/cases/conformance/salsa/a.js' because it would overwrite input file.
   Adding a tsconfig.json file will help organize projects that contain both TypeScript and JavaScript files. Learn more at https://aka.ms/tsconfig.
-tests/cases/conformance/salsa/a.js(1,23): error TS2300: Duplicate identifier 'x'.
+tests/cases/conformance/salsa/a.js(1,10): error TS2300: Duplicate identifier 'x'.
 tests/cases/conformance/salsa/b.ts(1,5): error TS2300: Duplicate identifier 'x'.
 
 
 !!! error TS5055: Cannot write file 'tests/cases/conformance/salsa/a.js' because it would overwrite input file.
 !!! error TS5055:   Adding a tsconfig.json file will help organize projects that contain both TypeScript and JavaScript files. Learn more at https://aka.ms/tsconfig.
 ==== tests/cases/conformance/salsa/a.js (1 errors) ====
     var /*1*/x = function foo() {
-                          ~~~
+             ~
 !!! error TS2300: Duplicate identifier 'x'.
     }
     x.a = function bar() {