fix(typescript): don't report redeclaration of imported variable

If an import imports a type, then re-declaring that imported type as a
function is valid in TypeScript. Similarly, if an import imports a
variable, then re-declaring that imported variable as an interface is
valid in TypeScript. Ensure quick-lint-js doesn't report false
diagnostics in these cases.

docs/CHANGELOG.md

@@ -84,6 +84,10 @@ Semantic Versioning.
     directly in its own definition") when using `extends ? :`.
   * Making a type alias and a function with the same name no longer falsely
     reports [E0034][] ("redeclaration of variable").
+  * Importing a type then declaring a function or variable with the same name no
+    longer falsely reports [E0034][] ("redeclaration of variable").
+  * Importing a function or variable then declaring a type with the same name no
+    longer falsely reports [E0034][] ("redeclaration of variable").
   * `T extends () => RT ? A : B` no longer falsely reports [E0348][]
     ("unexpected '?' in type; use '| void' to make an optional type").
   * `<T extends />` is now correctly parsed as a JSX element.

src/quick-lint-js/fe/linter.cpp

@@ -39,6 +39,7 @@ void parse_and_lint(Padded_String_View code, Diag_Reporter& reporter,
           .eval_can_declare_variables = !options.typescript,
           // TODO(strager): Deduplicate with typescript_var_options in tests.
           .can_assign_to_class = !options.typescript,
+          .import_variable_can_be_runtime_or_type = options.typescript,
       });
 
 #if defined(__EMSCRIPTEN__)

src/quick-lint-js/fe/variable-analyzer.cpp

@@ -101,6 +101,7 @@ namespace quick_lint_js {
 namespace {
 bool is_runtime(Variable_Kind);
 bool is_type(Variable_Kind);
+bool is_runtime_and_type(Variable_Kind);
 }
 
 Variable_Analyzer::Variable_Analyzer(
@@ -1122,7 +1123,6 @@ bool Variable_Analyzer::report_error_if_variable_declaration_conflicts(
       (kind == VK::_import_type        && other_kind == VK::_var) ||
       (kind == VK::_infer_type         && other_kind == VK::_infer_type) ||
       (kind == VK::_interface          && other_kind == VK::_class) ||
-      (kind == VK::_interface          && other_kind == VK::_import) ||
       (kind == VK::_interface          && other_kind == VK::_import_alias) ||
       (kind == VK::_interface          && other_kind == VK::_interface) ||
       (kind == VK::_interface          && other_kind == VK::_namespace) ||
@@ -1160,6 +1160,11 @@ bool Variable_Analyzer::report_error_if_variable_declaration_conflicts(
       (kind_is_parameter               && other_kind_is_parameter) ||
       (!quick_lint_js::is_type(kind)   && other_kind == VK::_interface) ||
       // clang-format on
+      (this->options_.import_variable_can_be_runtime_or_type &&
+       ((kind == VK::_import &&
+         !quick_lint_js::is_runtime_and_type(other_kind)) ||
+        (other_kind == VK::_import &&
+         !quick_lint_js::is_runtime_and_type(kind)))) ||
       (other_kind == VK::_namespace &&
        (kind == VK::_class || kind == VK::_function) &&
        !(already_declared_var.flags &
@@ -1404,6 +1409,10 @@ bool is_type(Variable_Kind kind) {
   }
   QLJS_UNREACHABLE();
 }
+
+bool is_runtime_and_type(Variable_Kind kind) {
+  return is_type(kind) && is_runtime(kind);
+}
 }
 }
 

src/quick-lint-js/fe/variable-analyzer.h

@@ -35,6 +35,14 @@ struct Variable_Analyzer_Options {
   // If false, assigning to a 'class'-declared variable is invalid, like in
   // TypeScript.
   bool can_assign_to_class = true;
+
+  // If true, imported variables can be run-time variables (e.g. function, let)
+  // or type-only variables (e.g. TypeScript type alias or interface) or both
+  // (e.g. class).
+  //
+  // If false, imported variables can only be run-time variables, like in
+  // vanilla JavaScript.
+  bool import_variable_can_be_runtime_or_type = false;
 };
 
 // A variable_analyzer is a parse_visitor which implements variable lookup

test/quick-lint-js/variable-analyzer-support.h

@@ -27,6 +27,7 @@ constexpr Variable_Analyzer_Options typescript_var_options =
         .allow_deleting_typescript_variable = false,
         .eval_can_declare_variables = false,
         .can_assign_to_class = false,
+        .import_variable_can_be_runtime_or_type = true,
     };
 
 struct Test_Parse_And_Analyze_Options {

test/test-variable-analyzer-multiple-declarations.cpp

@@ -273,6 +273,71 @@ TEST(Test_Variable_Analyzer_Multiple_Declarations,
       javascript_analyze_options, default_globals);
 }
 
+TEST(Test_Variable_Analyzer_Multiple_Declarations,
+     typescript_import_does_not_conflict_with_runtime_only_variables) {
+  for (String8_View other_thing : {
+           u8"const x = null;"_sv,
+           u8"function x() {}"_sv,
+           u8"let x;"_sv,
+           u8"var x;"_sv,
+       }) {
+    test_parse_and_analyze(concat(u8"import x from ''; "_sv, other_thing),
+                           no_diags, typescript_analyze_options,
+                           default_globals);
+    test_parse_and_analyze(concat(other_thing, u8" import x from '';"_sv),
+                           no_diags, typescript_analyze_options,
+                           default_globals);
+  }
+}
+
+TEST(Test_Variable_Analyzer_Multiple_Declarations,
+     typescript_import_does_not_conflict_with_type_only_variables) {
+  for (String8_View other_thing : {
+           u8"interface x {}"_sv,
+           u8"type x = null;"_sv,
+           u8"import {type x} from 'othermod';"_sv,
+       }) {
+    test_parse_and_analyze(concat(u8"import x from ''; "_sv, other_thing),
+                           no_diags, typescript_analyze_options,
+                           default_globals);
+    test_parse_and_analyze(concat(other_thing, u8" import x from '';"_sv),
+                           no_diags, typescript_analyze_options,
+                           default_globals);
+  }
+}
+
+TEST(Test_Variable_Analyzer_Multiple_Declarations,
+     typescript_import_conflicts_with_mixed_runtime_and_type_variables) {
+  for (String8_View other_thing : {
+           u8"class x {}"_sv,
+           u8"enum x {}"_sv,
+       }) {
+    test_parse_and_analyze(concat(u8"import x from ''; "_sv, other_thing),
+                           u8"Diag_Redeclaration_Of_Variable"_diag,
+                           typescript_analyze_options, default_globals);
+    test_parse_and_analyze(concat(other_thing, u8" import x from '';"_sv),
+                           u8"Diag_Redeclaration_Of_Variable"_diag,
+                           typescript_analyze_options, default_globals);
+  }
+}
+
+TEST(Test_Variable_Analyzer_Multiple_Declarations,
+     typescript_import_always_conflicts_with_another_import) {
+  for (String8_View other_thing : {
+           u8"import {x} from 'othermod';"_sv,
+           u8"import x from 'othermod';"_sv,
+           u8"import * as x from 'othermod';"_sv,
+           u8"import x = require('othermod');"_sv,
+       }) {
+    test_parse_and_analyze(concat(u8"import x from ''; "_sv, other_thing),
+                           u8"Diag_Redeclaration_Of_Variable"_diag,
+                           typescript_analyze_options, default_globals);
+    test_parse_and_analyze(concat(other_thing, u8" import x from '';"_sv),
+                           u8"Diag_Redeclaration_Of_Variable"_diag,
+                           typescript_analyze_options, default_globals);
+  }
+}
+
 TEST(Test_Variable_Analyzer_Multiple_Declarations,
      import_alias_does_not_conflict_with_most_other_things) {
   for (String8_View other_thing : {
@@ -355,28 +420,6 @@ TEST(Test_Variable_Analyzer_Multiple_Declarations,
       typescript_analyze_options, default_globals);
 }
 
-TEST(Test_Variable_Analyzer_Multiple_Declarations,
-     imported_type_always_conflicts_with_things_declaring_types) {
-  for (String8_View other_thing : {
-           u8"class T {}"_sv,
-           u8"enum T {}"_sv,
-           u8"import T from 'othermod';"_sv,
-           u8"import T = require('othermod');"_sv,
-           u8"import {type T} from 'othermod';"_sv,
-           u8"interface T {}"_sv,
-           u8"type T = null;"_sv,
-       }) {
-    test_parse_and_analyze(
-        concat(u8"import {type T} from 'mod'; "_sv, other_thing),
-        u8"Diag_Redeclaration_Of_Variable"_diag, typescript_analyze_options,
-        default_globals);
-    test_parse_and_analyze(
-        concat(other_thing, u8" import {type T} from 'mod';"_sv),
-        u8"Diag_Redeclaration_Of_Variable"_diag, typescript_analyze_options,
-        default_globals);
-  }
-}
-
 TEST(Test_Variable_Analyzer_Multiple_Declarations,
      imported_type_might_not_conflict_with_runtime_only_declarations) {
   // These things conflict when importing a class, but do not conflict when