fix(lint): preserve explicit switch coverage semantics

Real defaults were hiding finite missing members while singleton and mixed open
unions escaped the checker walk. Align switch coverage with the upstream scalar
policies and keep generated branches as explicit editor suggestions.

Constraint: Validation is delegated to targeted local experiments after rebasing and GitHub CI
Rejected: Treat every default as exhaustive | violates the upstream default and issue reproduction
Rejected: Reuse Finding.Fix for missing cases | would leak throw stubs into automatic fix-all
Confidence: high
Scope-risk: moderate
Directive: Keep finite-member coverage and open-type default policy independent
Tested: Exhaustive source review across production, shim, LSP, tests, fixtures, and docs
Not-tested: Executable validation had not run at commit time
Related: #416

Author: samchon

packages/lint/README.md

@@ -445,7 +445,7 @@ Source: [`typescript-eslint`](https://github.com/typescript-eslint/typescript-es
 - [`typescript/return-await`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/return-await.ts): reject `return promise` inside `try`, `catch`, or `finally`; require `return await promise`.
 - [`typescript/sort-type-constituents`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/typescript-sort-type-constituents.ts): sort the members of union (`A | B | C`) and intersection (`A & B & C`) types into a canonical order so reorderings don't show up as diffs.
 - [`typescript/strict-boolean-expressions`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/typescript-strict-boolean-expressions.ts): rejects non-boolean values used in a boolean context such as `if`, `&&`, `||`, or `!` (type-aware).
-- [`typescript/switch-exhaustiveness-check`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/typescript-switch-exhaustiveness-check.ts): requires every member of a union or `enum` discriminant to be covered by a `case`, or a `default` clause to be present (type-aware).
+- [`typescript/switch-exhaustiveness-check`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/typescript-switch-exhaustiveness-check.ts): requires every enumerable discriminant member to have an explicit `case` by default; typed options control real/comment defaults and open types (type-aware).
 - [`typescript/triple-slash-reference`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/triple-slash-reference/violation.ts): rejects triple-slash reference directives.
 - [`typescript/unbound-method`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/typescript-unbound-method.ts): reject referencing a class instance method as a value instead of calling it (`obj.method` passed as a callback, aliased to a variable, or stored on another object).
 - [`typescript/use-unknown-in-catch-callback-variable`](https://github.com/samchon/ttsc/blob/master/tests/test-lint/src/cases/use-unknown-in-catch-callback-variable.ts): require the callback parameter of `.catch(...)` and the second argument of `.then(...)` to be typed `unknown`.

packages/lint/linthost/engine.go

@@ -120,17 +120,18 @@ func (c *Context) DecodeOptions(out interface{}) error {
 // both categories — no filter — because `ttsc fix` is the
 // run-everything entry point.
 type Finding struct {
-  Rule     string
-  Severity Severity
-  File     *shimast.SourceFile
-  Pos      int
-  End      int
-  Message  string
-  Fix      []TextEdit
-  IsFormat bool
-}
-
-// TextEdit is one byte-range replacement offered by an autofixable finding.
+  Rule        string
+  Severity    Severity
+  File        *shimast.SourceFile
+  Pos         int
+  End         int
+  Message     string
+  Fix         []TextEdit
+  Suggestions []Suggestion
+  IsFormat    bool
+}
+
+// TextEdit is one byte-range source replacement used by a fix or suggestion.
 // Positions use the same byte offsets as shim AST nodes and must point inside
 // the finding's source file.
 type TextEdit struct {
@@ -139,6 +140,14 @@ type TextEdit struct {
   Text string
 }
 
+// Suggestion is an opt-in editor action attached to a finding. Unlike Fix,
+// suggestion edits are never consumed by `ttsc fix` or source.fixAll.ttsc;
+// the LSP host exposes them as individual quick fixes selected by the user.
+type Suggestion struct {
+  Title string
+  Edits []TextEdit
+}
+
 // Report records a finding at the given node's source range. The pos is
 // trimmed past leading trivia (whitespace + comments) so the renderer's
 // `path:line:col` banner points at the offending token, not the start of
@@ -170,6 +179,29 @@ func (c *Context) ReportFix(node *shimast.Node, message string, edits ...TextEdi
   })
 }
 
+// ReportSuggestion records a node-scoped finding with one opt-in editor
+// action. The diagnostic is still reported when edits is empty, but no quick
+// fix is advertised.
+func (c *Context) ReportSuggestion(node *shimast.Node, message string, title string, edits ...TextEdit) {
+  if c.Severity == SeverityOff || node == nil {
+    return
+  }
+  pos := node.Pos()
+  if c.File != nil {
+    pos = shimscanner.SkipTrivia(c.File.Text(), pos)
+  }
+  c.collect(&Finding{
+    Rule:        c.rule.Name(),
+    Severity:    c.Severity,
+    File:        c.File,
+    Pos:         pos,
+    End:         node.End(),
+    Message:     message,
+    Suggestions: newSuggestions(title, edits),
+    IsFormat:    c.isFormat,
+  })
+}
+
 // ReportRange records a finding at an explicit byte range inside the
 // current file. Use this when the rule wants to highlight a sub-token of
 // a node (e.g. an operator inside a BinaryExpression).
@@ -210,6 +242,14 @@ func cloneTextEdits(edits []TextEdit) []TextEdit {
   return out
 }
 
+func newSuggestions(title string, edits []TextEdit) []Suggestion {
+  cloned := cloneTextEdits(edits)
+  if title == "" || len(cloned) == 0 {
+    return nil
+  }
+  return []Suggestion{{Title: title, Edits: cloned}}
+}
+
 // registry stores the package-global rule list keyed by name. Tests can
 // also reach into it via `LookupRule`.
 type registry struct {