isolatedModules error on alias merging with local value (#56354)

src/compiler/checker.ts

@@ -45427,6 +45427,19 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     Diagnostics.Import_declaration_conflicts_with_local_declaration_of_0;
                 error(node, message, symbolToString(symbol));
             }
+            else if (node.kind !== SyntaxKind.ExportSpecifier) {
+                // Look at 'compilerOptions.isolatedModules' and not 'getIsolatedModules(...)' (which considers 'verbatimModuleSyntax')
+                // here because 'verbatimModuleSyntax' will already have an error for importing a type without 'import type'.
+                const appearsValueyToTranspiler = compilerOptions.isolatedModules && !findAncestor(node, isTypeOnlyImportOrExportDeclaration);
+                if (appearsValueyToTranspiler && symbol.flags & (SymbolFlags.Value | SymbolFlags.ExportValue)) {
+                    error(
+                        node,
+                        Diagnostics.Import_0_conflicts_with_local_value_so_must_be_declared_with_a_type_only_import_when_isolatedModules_is_enabled,
+                        symbolToString(symbol),
+                        isolatedModulesLikeFlagName,
+                    );
+                }
+            }
 
             if (
                 getIsolatedModules(compilerOptions)

src/compiler/diagnosticMessages.json

@@ -3711,6 +3711,10 @@
         "category": "Error",
         "code": 2864
     },
+    "Import '{0}' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.": {
+        "category": "Error",
+        "code": 2865
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

src/testRunner/compilerRunner.ts

@@ -140,6 +140,7 @@ class CompilerTest {
         "importHelpers",
         "downlevelIteration",
         "isolatedModules",
+        "verbatimModuleSyntax",
         "strict",
         "noImplicitAny",
         "strictNullChecks",

tests/baselines/reference/isolatedModulesSketchyAliasLocalMerge(isolatedmodules=false,verbatimmodulesyntax=true).errors.txt

@@ -0,0 +1,19 @@
+bad.ts(1,10): error TS1286: ESM syntax is not allowed in a CommonJS module when 'verbatimModuleSyntax' is enabled.
+bad.ts(1,10): error TS1484: 'FC' is a type and must be imported using a type-only import when 'verbatimModuleSyntax' is enabled.
+
+
+==== types.ts (0 errors) ====
+    export type FC = () => void;
+
+==== bad.ts (2 errors) ====
+    import { FC } from "./types";
+             ~~
+!!! error TS1286: ESM syntax is not allowed in a CommonJS module when 'verbatimModuleSyntax' is enabled.
+             ~~
+!!! error TS1484: 'FC' is a type and must be imported using a type-only import when 'verbatimModuleSyntax' is enabled.
+    let FC: FC | null = null;
+
+==== good.ts (0 errors) ====
+    import type { FC } from "./types";
+    let FC: FC | null = null;
+

tests/baselines/reference/isolatedModulesSketchyAliasLocalMerge(isolatedmodules=true,verbatimmodulesyntax=false).errors.txt

@@ -0,0 +1,16 @@
+bad.ts(1,10): error TS2865: Import 'FC' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.
+
+
+==== types.ts (0 errors) ====
+    export type FC = () => void;
+
+==== bad.ts (1 errors) ====
+    import { FC } from "./types";
+             ~~
+!!! error TS2865: Import 'FC' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.
+    let FC: FC | null = null;
+
+==== good.ts (0 errors) ====
+    import type { FC } from "./types";
+    let FC: FC | null = null;
+

tests/baselines/reference/isolatedModulesSketchyAliasLocalMerge(isolatedmodules=true,verbatimmodulesyntax=true).errors.txt

@@ -0,0 +1,22 @@
+bad.ts(1,10): error TS1286: ESM syntax is not allowed in a CommonJS module when 'verbatimModuleSyntax' is enabled.
+bad.ts(1,10): error TS1484: 'FC' is a type and must be imported using a type-only import when 'verbatimModuleSyntax' is enabled.
+bad.ts(1,10): error TS2865: Import 'FC' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.
+
+
+==== types.ts (0 errors) ====
+    export type FC = () => void;
+
+==== bad.ts (3 errors) ====
+    import { FC } from "./types";
+             ~~
+!!! error TS1286: ESM syntax is not allowed in a CommonJS module when 'verbatimModuleSyntax' is enabled.
+             ~~
+!!! error TS1484: 'FC' is a type and must be imported using a type-only import when 'verbatimModuleSyntax' is enabled.
+             ~~
+!!! error TS2865: Import 'FC' conflicts with local value, so must be declared with a type-only import when 'isolatedModules' is enabled.
+    let FC: FC | null = null;
+
+==== good.ts (0 errors) ====
+    import type { FC } from "./types";
+    let FC: FC | null = null;
+

tests/cases/compiler/isolatedModulesSketchyAliasLocalMerge.ts

@@ -0,0 +1,15 @@
+// @isolatedModules: false, true
+// @verbatimModuleSyntax: false, true
+// @noEmit: true
+// @noTypesAndSymbols: true
+
+// @Filename: types.ts
+export type FC = () => void;
+
+// @Filename: bad.ts
+import { FC } from "./types";
+let FC: FC | null = null;
+
+// @Filename: good.ts
+import type { FC } from "./types";
+let FC: FC | null = null;