diff --git a/NEWS.md b/NEWS.md
index 83ac6d77ddf1d..8790950f080ff 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -84,6 +84,9 @@ New library features
 Standard library changes
 ------------------------
 
+* `codepoint(c)` now succeeds for overlong encodings.  `Base.ismalformed`, `Base.isoverlong`, and
+  `Base.show_invalid` are now `public` and documented (but not exported) ([#55152]).
+
 #### JuliaSyntaxHighlighting
 
 #### LinearAlgebra
diff --git a/base/char.jl b/base/char.jl
index 4ec4163234749..a75019bf429f5 100644
--- a/base/char.jl
+++ b/base/char.jl
@@ -4,18 +4,22 @@ import Core: AbstractChar, Char
 
 """
 The `AbstractChar` type is the supertype of all character implementations
-in Julia. A character represents a Unicode code point, and can be converted
-to an integer via the [`codepoint`](@ref) function in order to obtain the
-numerical value of the code point, or constructed from the same integer.
-These numerical values determine how characters are compared with `<` and `==`,
-for example.  New `T <: AbstractChar` types should define a `codepoint(::T)`
+in Julia. A character normally represents a Unicode codepoint (and can
+also encapsulate other information from an encoded byte sequence as described below),
+and characters can be converted to integer codepoint values via the [`codepoint`](@ref)
+function, or can be constructed from the same integer.  At least for valid,
+properly encoded Unicode characters, these numerical codepoint values
+determine how characters are compared with `<` and `==`, for example.
+New `T <: AbstractChar` types should define a `codepoint(::T)`
 method and a `T(::UInt32)` constructor, at minimum.
 
 A given `AbstractChar` subtype may be capable of representing only a subset
 of Unicode, in which case conversion from an unsupported `UInt32` value
 may throw an error. Conversely, the built-in [`Char`](@ref) type represents
 a *superset* of Unicode (in order to losslessly encode invalid byte streams),
-in which case conversion of a non-Unicode value *to* `UInt32` throws an error.
+in which case conversion of a non-Unicode value *to* `UInt32` throws an error
+(see [`Base.ismalformed`](@ref)), and on the other hand a `Char` can also represent
+a nonstandard "overlong" encoding ([`Base.isoverlong`](@ref)) of a codepoint.
 The [`isvalid`](@ref) function can be used to check which codepoints are
 representable in a given `AbstractChar` type.
 
@@ -77,10 +81,19 @@ end
     codepoint(c::AbstractChar)::Integer
 
 Return the Unicode codepoint (an unsigned integer) corresponding
-to the character `c` (or throw an exception if `c` does not represent
-a valid character). For `Char`, this is a `UInt32` value, but
+to the character `c` (or throw an exception if `c` represents
+a malformed character). For `Char`, this is a `UInt32` value, but
 `AbstractChar` types that represent only a subset of Unicode may
 return a different-sized integer (e.g. `UInt8`).
+
+Should succeed for any non-malformed character, i.e. when
+[`Base.ismalformed(c)`](@ref) returns `false`.   This includes
+invalid Unicode characters (such as unpaired surrogates)
+and overlong encodings.
+
+!!! compat "Julia 1.12"
+    Prior to Julia 1.12, `codepoint(c)` fails for overlong encodings (when
+    [`Base.isoverlong(c)`](@ref) is `true`), and `Base.decode_overlong(c)` was needed.
 """
 function codepoint end
 
@@ -116,10 +129,19 @@ end
 """
     ismalformed(c::AbstractChar)::Bool
 
-Return `true` if `c` represents malformed (non-Unicode) data according to the
+Return `true` if `c` represents malformed (non-codepoint / mis-encoded) data according to the
 encoding used by `c`. Defaults to `false` for non-`Char` types.
 
-See also [`show_invalid`](@ref).
+Any *non*-malformed `c` can be mapped to an integer codepoint
+by [`codepoint(c)`](@ref); this includes codepoints that are
+not valid Unicode characters ([`isvalid(c)`](@ref) is `false`).
+For example, well-formed characters can include invalid Unicode
+codepoints like `'\\U110000'`, unpaired surrogates such as `'\\ud800'`,
+and can also include overlong encodings ([`Base.isoverlong`](@ref)).
+Malformed data, in contrast, cannot be decoded to a codepoint
+(`codepoint` will throw an exception).
+
+See also [`Base.show_invalid`](@ref).
 """
 ismalformed(c::AbstractChar) = false
 
@@ -129,7 +151,7 @@ ismalformed(c::AbstractChar) = false
 Return `true` if `c` represents an overlong UTF-8 sequence. Defaults
 to `false` for non-`Char` types.
 
-See also [`decode_overlong`](@ref) and [`show_invalid`](@ref).
+See also [`Base.show_invalid`](@ref).
 """
 isoverlong(c::AbstractChar) = false
 
@@ -140,7 +162,7 @@ isoverlong(c::AbstractChar) = false
     l1 = leading_ones(u)
     t0 = trailing_zeros(u) & 56
     (l1 == 1) | (8l1 + t0 > 32) |
-    ((((u & 0x00c0c0c0) ⊻ 0x00808080) >> t0 != 0) | is_overlong_enc(u)) &&
+    (((u & 0x00c0c0c0) ⊻ 0x00808080) >> t0 != 0) &&
         throw_invalid_char(c)
     u &= 0xffffffff >> l1
     u >>= t0
@@ -152,20 +174,18 @@ end
     decode_overlong(c::AbstractChar)::Integer
 
 When [`isoverlong(c)`](@ref) is `true`, `decode_overlong(c)` returns
-the Unicode codepoint value of `c`. `AbstractChar` implementations
-that support overlong encodings should implement `Base.decode_overlong`.
+the Unicode codepoint value of `c`.   Deprecated in favor of
+`codepoint(c)`.
+
+!!! compat "Julia 1.12"
+    In Julia 1.12 or later, `decode_overlong(c)` simply calls
+    `codepoint(c)`, which should now work for overlong encodings.
+    `AbstractChar` implementations that support overlong encodings
+    should implement `Base.decode_overlong` on older releases.
 """
 function decode_overlong end
 
-@constprop :aggressive function decode_overlong(c::Char)
-    u = bitcast(UInt32, c)
-    l1 = leading_ones(u)
-    t0 = trailing_zeros(u) & 56
-    u &= 0xffffffff >> l1
-    u >>= t0
-    ((u & 0x0000007f) >> 0) | ((u & 0x00007f00) >> 2) |
-    ((u & 0x007f0000) >> 4) | ((u & 0x7f000000) >> 6)
-end
+@constprop :aggressive decode_overlong(c::AbstractChar) = codepoint(c)
 
 @constprop :aggressive function Char(u::UInt32)
     u < 0x80 && return bitcast(Char, u << 24)
@@ -277,7 +297,7 @@ function show_invalid(io::IO, c::Char)
 end
 
 """
-    show_invalid(io::IO, c::AbstractChar)
+    Base.show_invalid(io::IO, c::AbstractChar)
 
 Called by `show(io, c)` when [`isoverlong(c)`](@ref) or
 [`ismalformed(c)`](@ref) return `true`.   Subclasses
@@ -330,7 +350,7 @@ function show(io::IO, ::MIME"text/plain", c::T) where {T<:AbstractChar}
         print(io, ": ")
         if isoverlong(c)
             print(io, "[overlong] ")
-            u = decode_overlong(c)
+            u = decode_overlong(c) # backwards compat Julia < 1.12
             c = T(u)
         else
             u = codepoint(c)
diff --git a/base/public.jl b/base/public.jl
index 52b0dd5233e39..afbb77c382659 100644
--- a/base/public.jl
+++ b/base/public.jl
@@ -112,6 +112,11 @@ public
 # Strings
     escape_raw_string,
 
+# Chars
+    ismalformed,
+    isoverlong,
+    show_invalid,
+
 # IO
     # types
     BufferStream,
diff --git a/doc/src/base/strings.md b/doc/src/base/strings.md
index 15a7c0531de4a..3b415202559f4 100644
--- a/doc/src/base/strings.md
+++ b/doc/src/base/strings.md
@@ -35,6 +35,9 @@ Base.Docs.@text_str
 Base.isvalid(::Any)
 Base.isvalid(::Any, ::Any)
 Base.isvalid(::AbstractString, ::Integer)
+Base.ismalformed
+Base.isoverlong
+Base.show_invalid
 Base.match
 Base.eachmatch
 Base.RegexMatch
diff --git a/test/char.jl b/test/char.jl
index cd7997d4a37f5..a576ad072b1d8 100644
--- a/test/char.jl
+++ b/test/char.jl
@@ -244,8 +244,8 @@ end
 
 @testset "overlong codes" begin
     function test_overlong(c::Char, n::Integer, rep::String)
-        if isvalid(c)
-            @test Int(c) == n
+        if !Base.ismalformed(c)
+            @test Int(c) == n == codepoint(c)
         else
             @test_throws Base.InvalidCharError UInt32(c)
         end
@@ -357,6 +357,15 @@ end
         "'\\xc0': Malformed UTF-8 (category Ma: Malformed, bad data)"
 end
 
+@testset "overlong, non-malformed chars" begin
+    c = ['\xc0\xa0', '\xf0\x8e\x80\x80']
+    @test all(Base.isoverlong, c)
+    @test !any(Base.ismalformed, c)
+    @test repr("text/plain", c[1]) == "'\\xc0\\xa0': [overlong] ASCII/Unicode U+0020 (category Zs: Separator, space)"
+    @test codepoint.(c) == [0x20, 0xE000]
+    @test isuppercase(c[1]) == isuppercase(c[2]) == false # issue #54343
+end
+
 @testset "More fallback tests" begin
     @test length(ASCIIChar('x')) == 1
     @test firstindex(ASCIIChar('x')) == 1