Handle infix operators in REPL completion

stdlib/REPL/src/REPLCompletions.jl

@@ -966,7 +966,7 @@ function complete_identifiers!(suggestions::Vector{Completion}, @nospecialize(ff
         append!(suggestions, complete_keyval(name))
     end
     if dotpos > 1 && string[dotpos] == '.'
-        s = string[1:dotpos-1]
+        s = string[1:prevind(string, dotpos)]
         # First see if the whole string up to `pos` is a valid expression. If so, use it.
         ex = Meta.parse(s, raise=false, depwarn=false)
         if isexpr(ex, :incomplete)
@@ -1005,6 +1005,17 @@ function complete_identifiers!(suggestions::Vector{Completion}, @nospecialize(ff
                 ex = Meta.parse(lookup_name, raise=false, depwarn=false)
             end
             isexpr(ex, :incomplete) && (ex = nothing)
+        elseif isexpr(ex, :call) && length(ex.args) > 1
+            isinfix = s[end] != ')'
+            # A complete call expression that does not finish with ')' is an infix call.
+            if !isinfix
+                # Handle infix call argument completion of the form bar + foo(qux).
+                frange, end_of_identifier = find_start_brace(@view s[1:end-1])
+                isinfix = Meta.parse(@view(s[frange[1]:end]), raise=false, depwarn=false) == ex.args[end]
+            end
+            if isinfix
+                ex = ex.args[end]
+            end
         end
     end
     append!(suggestions, complete_symbol(ex, name, ffunc, context_module))

stdlib/REPL/test/replcompletions.jl

@@ -23,6 +23,7 @@ let ex = quote
         end
         type_test = Test_x(Test_y(1))
         (::Test_y)() = "", ""
+        Base.:(+)(x::Test_x, y::Test_y) = Test_x(Test_y(x.xx.yy + y.yy))
         module CompletionFoo2
 
         end
@@ -1910,3 +1911,16 @@ end
     # If this last test starts failing, that's okay, just pick a new example symbol:
     @test !Base.isexported(Base, :ispublic)
 end
+
+# issue #51194
+for (s, compl) in (("2*CompletionFoo.nam", "named"),
+                   (":a isa CompletionFoo.test!1", "test!12"),
+                   ("-CompletionFoo.Test_y(3).", "yy"),
+                   ("99 ⨷⁻ᵨ⁷ CompletionFoo.type_test.", "xx"),
+                   ("CompletionFoo.type_test + CompletionFoo.Test_y(2).", "yy"),
+                   ("foo'CompletionFoo.test!1", "test!12"))
+    c, r = test_complete(s)
+    @test only(c) == compl
+end
+
+@test isempty(first(test_complete("ℯ.")))