fix: type assertion can reuse existing error variable

This is an edge case that can lead to invalid code.

This commit is basic, it prevents the linter to provide a fix that
cannot work.

Author: ccoVeille

errorlint/lint.go

@@ -648,15 +648,28 @@ func LintErrorTypeAssertions(fset *token.FileSet, info *TypesInfoExt) []analysis
 		// Get the error variable being type-switched on
 		errExpr := typeAssert.X
 
+		// a flag to know if we can fix the issue with a [analysis.SuggestedFix]
+		canFix := true
+
 		// Determine if this is a type switch with assignment (switch e := err.(type))
 		var assignIdent *ast.Ident
 		var useShadowVar bool
 		if assignStmt, ok := typeSwitch.Assign.(*ast.AssignStmt); ok {
 			// This is a type switch with assignment like: switch e := err.(type)
 			if len(assignStmt.Lhs) == 1 {
 				if id, ok := assignStmt.Lhs[0].(*ast.Ident); ok {
-					assignIdent = id
-					useShadowVar = true
+					if exprToString(errExpr) == id.Name {
+						// the switch with assignment is like switch err := err.(type)
+						// we cannot reuse err, otherwise it would lead to errors(err, &err)
+						canFix = false
+
+						// TODO - suggest a fix with a new variable name instead?
+						// the issue is with the fact each branch should have a new variable name
+					} else {
+						// the variable names are different, we can reuse the assigned variable
+						assignIdent = id
+						useShadowVar = true
+					}
 				}
 			}
 		}
@@ -814,20 +827,21 @@ func LintErrorTypeAssertions(fset *token.FileSet, info *TypesInfoExt) []analysis
 			newSwitchStmt.Body.List[i] = newCaseClause
 		}
 
-		// Print the resulting block to get the fix text.
-		var buf bytes.Buffer
-		printer.Fprint(&buf, token.NewFileSet(), blockStmt)
-		fixText := buf.String()
-
-		diagnostic.SuggestedFixes = []analysis.SuggestedFix{{
-			Message: "Convert type switch to use errors.As",
-			TextEdits: []analysis.TextEdit{{
-				Pos:     typeSwitch.Pos(),
-				End:     typeSwitch.End(),
-				NewText: []byte(fixText),
-			}},
-		}}
+		if canFix {
+			// Print the resulting block to get the fix text.
+			var buf bytes.Buffer
+			printer.Fprint(&buf, token.NewFileSet(), blockStmt)
+			fixText := buf.String()
 
+			diagnostic.SuggestedFixes = []analysis.SuggestedFix{{
+				Message: "Convert type switch to use errors.As",
+				TextEdits: []analysis.TextEdit{{
+					Pos:     typeSwitch.Pos(),
+					End:     typeSwitch.End(),
+					NewText: []byte(fixText),
+				}},
+			}}
+		}
 		lints = append(lints, diagnostic)
 	}
 

errorlint/testdata/src/errorassert/errorassert.go

@@ -155,6 +155,17 @@ func TypeSwitchWithAssignment() {
 	}
 }
 
+// This should be flagged - type switch with assignment reuse
+func TypeSwitchWithAssignmentReuse() {
+	err := doSomething()
+	switch err := err.(type) { // want "type switch on error will fail on wrapped errors. Use errors.As to check for specific errors"
+	case *MyError:
+		fmt.Println("my error:", err)
+	default:
+		fmt.Println("unknown error:", err)
+	}
+}
+
 // This should NOT be flagged - using errors.As
 func UsingErrorsAs() {
 	err := doSomethingWrapped()

errorlint/testdata/src/errorassert/errorassert.go.golden

@@ -176,6 +176,17 @@ func TypeSwitchWithAssignment() {
 	}
 }
 
+// This should be flagged - type switch with assignment reuse
+func TypeSwitchWithAssignmentReuse() {
+	err := doSomething()
+	switch err := err.(type) { // want "type switch on error will fail on wrapped errors. Use errors.As to check for specific errors"
+	case *MyError:
+		fmt.Println("my error:", err)
+	default:
+		fmt.Println("unknown error:", err)
+	}
+}
+
 // This should NOT be flagged - using errors.As
 func UsingErrorsAs() {
 	err := doSomethingWrapped()