Consider ${foo:=bar} an assignment.

ShellCheck/Analytics.hs

@@ -2139,6 +2139,7 @@ prop_checkUnassignedReferences23= verifyTree checkUnassignedReferences "declare
 prop_checkUnassignedReferences24= verifyNotTree checkUnassignedReferences "declare -A foo; foo[bar]=42;"
 prop_checkUnassignedReferences25= verifyNotTree checkUnassignedReferences "declare -A foo=(); foo[bar]=42;"
 prop_checkUnassignedReferences26= verifyNotTree checkUnassignedReferences "a::b() { foo; }; readonly -f a::b"
+prop_checkUnassignedReferences27= verifyNotTree checkUnassignedReferences ": ${foo:=bar}"
 checkUnassignedReferences params t = warnings
   where
     (readMap, writeMap) = execState (mapM tally $ variableFlow params) (Map.empty, Map.empty)

ShellCheck/AnalyzerLib.hs

@@ -334,6 +334,12 @@ getModifiedVariables t =
             name <- getLiteralString lhs
             return (t, t, name, DataString $ SourceFrom [rhs])
 
+        T_DollarBraced _ l -> maybeToList $ do
+            let string = bracedString t
+            let modifier = getBracedModifier string
+            guard $ ":=" `isPrefixOf` modifier
+            return (t, t, getBracedReference string, DataString $ SourceFrom [l])
+
         t@(T_FdRedirect _ ('{':var) op) -> -- {foo}>&2 modifies foo
             [(t, t, takeWhile (/= '}') var, DataString SourceInteger) | not $ isClosingFileOp op]