check if import collides with exported local name

Author: vladima

src/compiler/checker.ts

@@ -15063,10 +15063,16 @@ namespace ts {
             const symbol = getSymbolOfNode(node);
             const target = resolveAlias(symbol);
             if (target !== unknownSymbol) {
+                // For external modules symbol represent local symbol for an alias. 
+                // This local symbol will merge any other local declarations (excluding other aliases)
+                // and symbol.flags will contains combined representation for all merged declaration.
+                // Based on symbol.flags we can compute a set of excluded meanings (meaning that resolved alias should not have,
+                // otherwise it will conflict with some local declaration). Note that in addition to normal flags we include matching SymbolFlags.Export* 
+                // in order to prevent collisions with declarations that were exported from the current module (they still contribute to local names). 
                 const excludedMeanings =
-                    (symbol.flags & SymbolFlags.Value ? SymbolFlags.Value : 0) |
-                    (symbol.flags & SymbolFlags.Type ? SymbolFlags.Type : 0) |
-                    (symbol.flags & SymbolFlags.Namespace ? SymbolFlags.Namespace : 0);
+                    (symbol.flags & (SymbolFlags.Value | SymbolFlags.ExportValue) ? SymbolFlags.Value : 0) |
+                    (symbol.flags & (SymbolFlags.Type | SymbolFlags.ExportType) ? SymbolFlags.Type : 0) |
+                    (symbol.flags & (SymbolFlags.Namespace | SymbolFlags.ExportNamespace) ? SymbolFlags.Namespace : 0);
                 if (target.flags & excludedMeanings) {
                     const message = node.kind === SyntaxKind.ExportSpecifier ?
                         Diagnostics.Export_declaration_conflicts_with_exported_declaration_of_0 :

tests/baselines/reference/functionAndImportNameConflict.errors.txt

@@ -0,0 +1,13 @@
+tests/cases/compiler/f2.ts(1,9): error TS2440: Import declaration conflicts with local declaration of 'f'
+
+
+==== tests/cases/compiler/f1.ts (0 errors) ====
+    export function f() {
+    }
+    
+==== tests/cases/compiler/f2.ts (1 errors) ====
+    import {f} from './f1';
+            ~
+!!! error TS2440: Import declaration conflicts with local declaration of 'f'
+    export function f() {
+    }

tests/baselines/reference/functionAndImportNameConflict.js

@@ -0,0 +1,21 @@
+//// [tests/cases/compiler/functionAndImportNameConflict.ts] ////
+
+//// [f1.ts]
+export function f() {
+}
+
+//// [f2.ts]
+import {f} from './f1';
+export function f() {
+}
+
+//// [f1.js]
+"use strict";
+function f() {
+}
+exports.f = f;
+//// [f2.js]
+"use strict";
+function f() {
+}
+exports.f = f;

tests/cases/compiler/functionAndImportNameConflict.ts

@@ -0,0 +1,9 @@
+// @module: commonjs
+// @filename: f1.ts
+export function f() {
+}
+
+// @filename: f2.ts
+import {f} from './f1';
+export function f() {
+}