Only enforce err cuddling if the statement above was an assignment

Fixes #78

wsl.go

@@ -398,6 +398,18 @@ func (p *Processor) parseBlockStatements(statements []ast.Stmt) {
 			if !cuddledWithLastStmt {
 				checkingErr := atLeastOneInListsMatch(rightAndLeftHandSide, p.config.ErrorVariableNames)
 				if checkingErr {
+					// We only want to enforce cuddling error checks if the
+					// error was assigned on the line above. See
+					// https://github.com/bombsimon/wsl/issues/78.
+					// This is needed since `assignedOnLineAbove` is not
+					// actually just assignments but everything from LHS in the
+					// previous statement. This means that if previous line was
+					// `if err ...`, `err` will now be in the list
+					// `assignedOnLineAbove`.
+					if _, ok := previousStatement.(*ast.AssignStmt); !ok {
+						continue
+					}
+
 					if checkingErrInitializedInline() {
 						continue
 					}

wsl_test.go

@@ -1674,6 +1674,50 @@ func TestWithConfig(t *testing.T) {
 				}
 			}`),
 		},
+		{
+			description: "only warn about cuddling errors if it's an expression above",
+			customConfig: &Configuration{
+				ForceCuddleErrCheckAndAssign: true,
+				ErrorVariableNames:           []string{"err"},
+			},
+			code: []byte(`package main
+
+			var ErrSomething error
+
+			func validateErr(err error) {
+				if err == nil {
+					return
+				}
+
+				if errors.Is(err, ErrSomething) {
+					return
+				}
+
+				// Should be valid since the if actually is cuddled so the error
+				// check assumes something right is going on here. If
+				// anotherThingToCheck wasn't used in the if statement we would
+				// get a regular if cuddle error.
+				anotherThingToCheck := someFunc()
+				if multiCheck(anotherThingToCheck, err) {
+					fmt.Println("this must be OK, err is not assigned above")
+				}
+
+				notUsedInNextIf := someFunc()
+
+				if multiCheck(anotherThingToCheck, err) {
+					fmt.Println("this should not warn, we didn't assign err above")
+				}
+
+				// This fails under the cuddle if rule.
+				if err != nil {
+					return
+				}
+				if !errors.Is(err, ErrSomething) {
+					return
+				}
+			}`),
+			expectedErrorStrings: []string{reasonOnlyCuddleIfWithAssign},
+		},
 	}
 
 	for _, tc := range cases {