C++: Remove decimal floating point types

cpp/downgrades/d2d611b3fdcc7c4fe370f0d115200a3aa6ad5837/upgrade.properties

@@ -0,0 +1,2 @@
+description: Remove _Decimal{32,64,128} types
+compatibility: full

cpp/ql/lib/change-notes/2026-01-02-decimal-removal.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `_Decimal32`, `_Decimal64`, and `_Decimal128` types are no longer exposed as builtin types. Support for these gcc-specific types was incomplete, and are generally not used in C/C++ codebases.

cpp/ql/lib/semmle/code/cpp/Type.qll

@@ -802,15 +802,6 @@ private predicate floatingPointTypeMapping(
   // _Complex __float128
   kind = 39 and base = 2 and domain = TComplexDomain() and realKind = 38 and extended = false
   or
-  // _Decimal32
-  kind = 40 and base = 10 and domain = TRealDomain() and realKind = 40 and extended = false
-  or
-  // _Decimal64
-  kind = 41 and base = 10 and domain = TRealDomain() and realKind = 41 and extended = false
-  or
-  // _Decimal128
-  kind = 42 and base = 10 and domain = TRealDomain() and realKind = 42 and extended = false
-  or
   // _Float32
   kind = 45 and base = 2 and domain = TRealDomain() and realKind = 45 and extended = false
   or
@@ -871,9 +862,8 @@ private predicate floatingPointTypeMapping(
 
 /**
  * The C/C++ floating point types. See 4.5.  This includes `float`, `double` and `long double`, the
- * fixed-size floating-point types like `_Float32`, the extended-precision floating-point types like
- * `_Float64x`, and the decimal floating-point types like `_Decimal32`. It also includes the complex
- * and imaginary versions of all of these types.
+ * fixed-size floating-point types like `_Float32`, and the extended-precision floating-point types
+ * like `_Float64x`. It also includes the complex and imaginary versions of all of these types.
  */
 class FloatingPointType extends ArithmeticType {
   final int base;
@@ -991,42 +981,6 @@ class Float128Type extends RealNumberType, BinaryFloatingPointType {
   override string getAPrimaryQlClass() { result = "Float128Type" }
 }
 
-/**
- * The GNU C `_Decimal32` primitive type.  This is not standard C/C++.
- * ```
- * _Decimal32 d32;
- * ```
- */
-class Decimal32Type extends RealNumberType, DecimalFloatingPointType {
-  Decimal32Type() { builtintypes(underlyingElement(this), _, 40, _, _, _) }
-
-  override string getAPrimaryQlClass() { result = "Decimal32Type" }
-}
-
-/**
- * The GNU C `_Decimal64` primitive type.  This is not standard C/C++.
- * ```
- * _Decimal64 d64;
- * ```
- */
-class Decimal64Type extends RealNumberType, DecimalFloatingPointType {
-  Decimal64Type() { builtintypes(underlyingElement(this), _, 41, _, _, _) }
-
-  override string getAPrimaryQlClass() { result = "Decimal64Type" }
-}
-
-/**
- * The GNU C `_Decimal128` primitive type.  This is not standard C/C++.
- * ```
- * _Decimal128 d128;
- * ```
- */
-class Decimal128Type extends RealNumberType, DecimalFloatingPointType {
-  Decimal128Type() { builtintypes(underlyingElement(this), _, 42, _, _, _) }
-
-  override string getAPrimaryQlClass() { result = "Decimal128Type" }
-}
-
 /**
  * The C/C++ `void` type. See 4.7.
  * ```

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -617,9 +617,9 @@ case @builtintype.kind of
 | 37 = @signed_int128         // signed __int128
 | 38 = @float128              // __float128
 | 39 = @complex_float128      // _Complex __float128
-| 40 = @decimal32             // _Decimal32
-| 41 = @decimal64             // _Decimal64
-| 42 = @decimal128            // _Decimal128
+// ... 40 _Decimal32
+// ... 41 _Decimal64
+// ... 42 _Decimal128
 | 43 = @char16_t
 | 44 = @char32_t
 | 45 = @std_float32           // _Float32