diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ffed972ed120d..ab0e75c518471 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -528,6 +528,8 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses ``[[deprecated]]`` attribute usage on local variables (#GH90073).
 
+- Improved diagnostic message for ``__builtin_bit_cast`` size mismatch (#GH115870).
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index 509d45c086759..200dc8bd23835 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -12354,7 +12354,7 @@ def err_preserve_enum_value_not_const: Error<
 def err_bit_cast_non_trivially_copyable : Error<
   "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
 def err_bit_cast_type_size_mismatch : Error<
-  "__builtin_bit_cast source size does not equal destination size (%0 vs %1)">;
+  "size of '__builtin_bit_cast' source type %0 does not match destination type %1 (%2 vs %3 bytes)">;
 
 // SYCL-specific diagnostics
 def warn_sycl_kernel_num_of_template_params : Warning<
diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index 6ac6201843476..3eb013e3612d1 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -3283,7 +3283,8 @@ void CastOperation::CheckBuiltinBitCast() {
   CharUnits SourceSize = Self.Context.getTypeSizeInChars(SrcType);
   if (DestSize != SourceSize) {
     Self.Diag(OpRange.getBegin(), diag::err_bit_cast_type_size_mismatch)
-        << (int)SourceSize.getQuantity() << (int)DestSize.getQuantity();
+        << SrcType << DestType << (int)SourceSize.getQuantity()
+        << (int)DestSize.getQuantity();
     SrcExpr = ExprError();
     return;
   }
diff --git a/clang/test/AST/ByteCode/builtin-bit-cast.cpp b/clang/test/AST/ByteCode/builtin-bit-cast.cpp
index 7d1fcbda10965..60e8c3a615c5e 100644
--- a/clang/test/AST/ByteCode/builtin-bit-cast.cpp
+++ b/clang/test/AST/ByteCode/builtin-bit-cast.cpp
@@ -496,7 +496,7 @@ typedef bool bool32 __attribute__((ext_vector_type(32)));
 typedef bool bool128 __attribute__((ext_vector_type(128)));
 
 static_assert(bit_cast<unsigned char>(bool8{1,0,1,0,1,0,1,0}) == (LITTLE_END ? 0x55 : 0xAA), "");
-constexpr bool8 b8 = __builtin_bit_cast(bool8, 0x55); // both-error {{__builtin_bit_cast source size does not equal destination size (4 vs 1)}}
+constexpr bool8 b8 = __builtin_bit_cast(bool8, 0x55); // both-error {{'__builtin_bit_cast' source type 'int' does not match destination type 'bool8' (vector of 8 'bool' values) (4 vs 1 bytes)}}
 #if 0
 static_assert(check_round_trip<bool8>(static_cast<unsigned char>(0)), "");
 static_assert(check_round_trip<bool8>(static_cast<unsigned char>(1)), "");
@@ -535,5 +535,5 @@ namespace test_complex {
   static_assert(TF.A == 1.0f && TF.B == 2.0f);
 
   constexpr double D = __builtin_bit_cast(double, test_float_complex);
-  constexpr int M = __builtin_bit_cast(int, test_int_complex); // both-error {{__builtin_bit_cast source size does not equal destination size}}
+  constexpr int M = __builtin_bit_cast(int, test_int_complex); // both-error {{size of '__builtin_bit_cast' source type 'const _Complex unsigned int' does not match destination type 'int' (8 vs 4 bytes)}}
 }
diff --git a/clang/test/SemaCXX/builtin-bit-cast.cpp b/clang/test/SemaCXX/builtin-bit-cast.cpp
index 87919d9d9d286..d7f24c7939b55 100644
--- a/clang/test/SemaCXX/builtin-bit-cast.cpp
+++ b/clang/test/SemaCXX/builtin-bit-cast.cpp
@@ -24,7 +24,7 @@ void test1() {
 
 void test2() {
   constexpr int i = 0;
-  // expected-error@+1{{__builtin_bit_cast source size does not equal destination size (4 vs 1)}}
+  // expected-error@+1{{size of '__builtin_bit_cast' source type 'const int' does not match destination type 'char' (4 vs 1 bytes)}}
   constexpr char c = __builtin_bit_cast(char, i);
 }
 
diff --git a/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp b/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp
index 5ddb77b35ff14..425447c5e604e 100644
--- a/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp
+++ b/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp
@@ -525,5 +525,5 @@ namespace test_complex {
   static_assert(TF.A == 1.0f && TF.B == 2.0f);
 
   constexpr double D = __builtin_bit_cast(double, test_float_complex);
-  constexpr int M = __builtin_bit_cast(int, test_int_complex); // expected-error {{__builtin_bit_cast source size does not equal destination size}}
+  constexpr int M = __builtin_bit_cast(int, test_int_complex); // expected-error {{size of '__builtin_bit_cast' source type 'const _Complex unsigned int' does not match destination type 'int' (8 vs 4 bytes)}}
 }