[Version 9.0] Feature support for target-typed conditional expression

standard/conversions.md

@@ -25,7 +25,7 @@
 > ```csharp
 > enum Color { Red, Blue, Green }
 >
 > // The expression 0 converts implicitly to enum types
 > Color c0 = 0;
 >
 > // Other int expressions need explicit conversion
@@ -390,6 +390,15 @@
 
 There is an implicit conversion from a *switch_expression* ([§12.11](expressions.md#1211-switch-expression)) to every type `T` for which there exists an implicit conversion from each *switch_expression_arm*’s *switch_expression_arm_expression*’s to `T`.
 
+### §imp-cond-expr-conv Implicit conditional expression conversions
+
+For a *conditional_expression* `c ? e1 : e2`, when
+
+1. there is no common type for `e1` and `e2`, or
+1. for which a common type exists, but one of the expressions `e1` or `e2` has no implicit conversion to that type
+
+an implicit ***conditional expression conversion*** exists that permits an implicit conversion from *conditional_expression* to any type `T` for which there is a conversion-from-expression from `e1` to `T` and also from `e2` to `T`.  It is an error if *conditional_expression* neither has a common type between `e1` and `e2` nor is subject to a conditional expression conversion.
+
 ## 10.3 Explicit conversions
 
 ### 10.3.1 General

standard/expressions.md

@@ -1,15 +1,15 @@
 # 12 Expressions
 
 ## 12.1 General
 
 An expression is a sequence of operators and operands. This clause defines the syntax, order of evaluation of operands and operators, and meaning of expressions.
 
 ## 12.2 Expression classifications
 
 ### 12.2.1 General
 
 The result of an expression is classified as one of the following:
 
 - A value. Every value has an associated type.
 - A variable. Unless otherwise specified, a variable is explicitly typed and has an associated type, namely the declared type of the variable. An implicitly typed variable has no associated type.
 - A null literal. An expression with this classification can be implicitly converted to a reference type or nullable value type.
@@ -1114,10 +1114,11 @@
 
 #### 12.6.4.5 Better conversion from expression
 
-Given an implicit conversion `C₁` that converts from an expression `E` to a type `T₁`, and an implicit conversion `C₂` that converts from an expression `E` to a type `T₂`, `C₁` is a ***better conversion*** than `C₂` if one of the following holds:
+Given an implicit conversion `C₁` that converts from an expression `E` to a type `T₁`, and an implicit conversion `C₂` that converts from an expression `E` to a type `T₂`, `C₁` is a ***better conversion*** than `C₂` if `E` does not exactly match `T₂` and at least one of the following holds:
 
 - `E` exactly matches `T₁` and `E` does not exactly match `T₂` ([§12.6.4.6](expressions.md#12646-exactly-matching-expression))
-- `E` exactly matches both or neither of `T₁` and `T₂`, and `T₁` is a better conversion target than `T₂` ([§12.6.4.7](expressions.md#12647-better-conversion-target))
+- `C₁` is not a conditional expression conversion and `C₂` is a conditional expression conversion.
+- `E` exactly matches both or neither of `T₁` and `T₂`, and `T₁` is a better conversion target than `T₂` ([§12.6.4.7](expressions.md#12647-better-conversion-target)) and either `C₁` and `C₂` are both conditional expression conversions or neither is a conditional expression conversion.
 - `E` is a method group ([§12.2](expressions.md#122-expression-classifications)), `T₁` is compatible ([§21.4](delegates.md#214-delegate-compatibility)) with the single best method from the method group for conversion `C₁`, and `T₂` is not compatible with the single best method from the method group for conversion `C₂`
 
 #### 12.6.4.6 Exactly matching expression
@@ -3738,7 +3739,7 @@
     ;
 ```
 
-A *cast_expression* of the form `(T)E`, where `T` is a type and `E` is a *unary_expression*, performs an explicit conversion ([§10.3](conversions.md#103-explicit-conversions)) of the value of `E` to type `T`. If no explicit conversion exists from `E` to `T`, a binding-time error occurs. Otherwise, the result is the value produced by the explicit conversion. The result is always classified as a value, even if `E` denotes a variable.
+A *cast_expression* of the form `(T)E`, where `T` is a type and `E` is a *unary_expression*, performs an explicit conversion ([§10.3](conversions.md#103-explicit-conversions)) of the value of `E` to type `T`. In the presence of a conditional expression conversion (§imp-cond-expr-conv) there may be more than one possible conversion from `E` to `T`, in which case, the conditional expression conversion shall only be used as a last resort. If no explicit conversion exists from `E` to `T`, a binding-time error occurs. Otherwise, the result is the value produced by the explicit conversion. The result is always classified as a value, even if `E` denotes a variable.
 
 The grammar for a *cast_expression* leads to certain syntactic ambiguities.
 
@@ -5208,7 +5209,7 @@
 If `ref` is not present, the second and third operands, `x` and `y`, of the `?:` operator control the type of the conditional expression:
 
 - If `x` has type `X` and `y` has type `Y` then,
-  - If an identity conversion exists between `X` and `Y`, then the result is the best common type of a set of expressions ([§12.6.3.16](expressions.md#126316-finding-the-best-common-type-of-a-set-of-expressions)). If either type is `dynamic`, type inference prefers `dynamic` ([§8.7](types.md#87-the-dynamic-type)). If either type is a tuple type ([§8.3.11](types.md#8311-tuple-types)), type inference includes the element names when the element names in the same ordinal position match in both tuples.
+  - If an identity conversion exists between `X` and `Y`, then the result is the best common type of a set of expressions ([§12.6.3.16](expressions.md#126316-finding-the-best-common-type-of-a-set-of-expressions)). ***placeholder for words somehow referring to “12.6.4.5 Better conversion from expression.”*** If either type is `dynamic`, type inference prefers `dynamic` ([§8.7](types.md#87-the-dynamic-type)). If either type is a tuple type ([§8.3.11](types.md#8311-tuple-types)), type inference includes the element names when the element names in the same ordinal position match in both tuples.
   - Otherwise, if an implicit conversion ([§10.2](conversions.md#102-implicit-conversions)) exists from `X` to `Y`, but not from `Y` to `X`, then `Y` is the type of the conditional expression.
   - Otherwise, if an implicit enumeration conversion ([§10.2.4](conversions.md#1024-implicit-enumeration-conversions)) exists from `X` to `Y`, then `Y` is the type of the conditional expression.
   - Otherwise, if an implicit enumeration conversion ([§10.2.4](conversions.md#1024-implicit-enumeration-conversions)) exists from `Y` to `X`, then `X` is the type of the conditional expression.