REPL: fix brace detection when ' is used for transpose (#56252)

(cherry picked from commit 1c67d0c)

stdlib/REPL/src/REPLCompletions.jl

@@ -400,6 +400,7 @@ function find_start_brace(s::AbstractString; c_start='(', c_end=')')
     in_double_quotes = false
     in_back_ticks = false
     in_comment = 0
+    num_single_quotes_in_string = count('\'', s)
     while i <= ncodeunits(r)
         c, i = iterate(r, i)
         if c == '#' && i <= ncodeunits(r) && iterate(r, i)[1] == '='
@@ -422,7 +423,9 @@ function find_start_brace(s::AbstractString; c_start='(', c_end=')')
                 braces += 1
             elseif c == c_end
                 braces -= 1
-            elseif c == '\''
+            elseif c == '\'' && num_single_quotes_in_string % 2 == 0
+                # ' can be a transpose too, so check if there are even number of 's in the string
+                # TODO: This probably needs to be more robust
                 in_single_quotes = true
             elseif c == '"'
                 in_double_quotes = true
@@ -1079,7 +1082,9 @@ function complete_identifiers!(suggestions::Vector{Completion}, @nospecialize(ff
             if !isinfix
                 # Handle infix call argument completion of the form bar + foo(qux).
                 frange, end_of_identifier = find_start_brace(@view s[1:prevind(s, end)])
-                isinfix = Meta.parse(@view(s[frange[1]:end]), raise=false, depwarn=false) == ex.args[end]
+                if !isempty(frange) # if find_start_brace fails to find the brace just continue
+                    isinfix = Meta.parse(@view(s[frange[1]:end]), raise=false, depwarn=false) == ex.args[end]
+                end
             end
             if isinfix
                 ex = ex.args[end]

stdlib/REPL/test/replcompletions.jl

@@ -325,6 +325,12 @@ end
 # inexistent completion inside a cmd
 @test_nocompletion("run(`lol")
 
+# issue 55856: copy(A').<TAB> errors in the REPL
+let
+    c, r = test_complete("copy(A').")
+    @test isempty(c)
+end
+
 # test latex symbol completions
 let s = "\\alpha"
     c, r = test_bslashcomplete(s)