Fix tuple assignment and tuple element type inference

standard/conversions.md

@@ -345,7 +345,7 @@ In all cases, the rules ensure that a conversion is executed as a boxing convers
 
 ### 10.2.13 Implicit tuple conversions
 
-An implicit conversion exists from a tuple expression `E` to a tuple type `T` if `E` has the same arity as `T` and an implicit conversion exists from each element in `E` to the corresponding element type in `T`. The conversion is performed by creating an instance of `T`’s corresponding `System.ValueTuple<...>` type, and initializing each of its fields in order from left to right by evaluating the corresponding tuple element expression of `E`, converting it to the corresponding element type of `T` using the implicit conversion found, and initializing the field with the result.
+An implicit conversion exists from an expression `E` with a tuple type `S` to a tuple type `T` if `S` has the same arity as `T` and an implicit conversion exists from each element type in `S` to the corresponding element type in `T`.
 
 If an element name in the tuple expression does not match a corresponding element name in the tuple type, a warning shall be issued.
 

standard/expressions.md

@@ -33,12 +33,12 @@ An ***instance accessor*** is a property access on an instance, an event access
 
 ### 12.2.2 Values of expressions
 
-Most of the constructs that involve an expression ultimately require the expression to denote a ***value***. In such cases, if the actual expression denotes a namespace, a type, a method group, or nothing, a compile-time error occurs. However, if the expression denotes a property access, an indexer access, or a variable, the value of the property, indexer, or variable is implicitly substituted:
+Most of the constructs that involve an expression ultimately require the expression to denote a ***value***. In such cases, if the actual expression denotes a namespace, a type, a method group, or nothing, a compile-time error occurs. However, if the expression denotes a property access, an indexer access, a tuple, or a variable, the value of the property, indexer, tuple, or variable is implicitly substituted:
 
 - The value of a variable is simply the value currently stored in the storage location identified by the variable. A variable shall be considered definitely assigned ([§9.4](variables.md#94-definite-assignment)) before its value can be obtained, or otherwise a compile-time error occurs.
 - The value of a property access expression is obtained by invoking the get accessor of the property. If the property has no get accessor, a compile-time error occurs. Otherwise, a function member invocation ([§12.6.6](expressions.md#1266-function-member-invocation)) is performed, and the result of the invocation becomes the value of the property access expression.
 - The value of an indexer access expression is obtained by invoking the get accessor of the indexer. If the indexer has no get accessor, a compile-time error occurs. Otherwise, a function member invocation ([§12.6.6](expressions.md#1266-function-member-invocation)) is performed with the argument list associated with the indexer access expression, and the result of the invocation becomes the value of the indexer access expression.
-- The value of a tuple expression is obtained by applying an implicit tuple conversion ([§10.2.13](conversions.md#10213-implicit-tuple-conversions)) to the type of the tuple expression. It is an error to obtain the value of a tuple expression that does not have a type.
+- The value of a tuple expression is the value obtained by evaluating the tuple expression (§12.8.6). It is an error to obtain the value of a tuple expression that does not have a type.
 
 ## 12.3 Static and Dynamic Binding
 
@@ -763,14 +763,7 @@ Type inference takes place in phases. Each phase will try to infer type argument
 
 #### 12.6.3.2 The first phase
 
-For each of the method arguments `Eᵢ`:
-
-- If `Eᵢ` is an anonymous function, an *explicit parameter type inference* ([§12.6.3.8](expressions.md#12638-explicit-parameter-type-inferences)) is made *from* `Eᵢ` *to* `Tᵢ`
-- Otherwise, if `Eᵢ` has a type `U` and the corresponding parameter is a value parameter ([§15.6.2.2](classes.md#15622-value-parameters)) then a *lower-bound inference* ([§12.6.3.10](expressions.md#126310-lower-bound-inferences)) is made *from* `U` *to* `Tᵢ`.
-- Otherwise, if `Eᵢ` has a type `U` and the corresponding parameter is a reference parameter ([§15.6.2.3.3](classes.md#156233-reference-parameters)), or output parameter ([§15.6.2.3.4](classes.md#156234-output-parameters)) then an *exact inference* ([§12.6.3.9](expressions.md#12639-exact-inferences)) is made *from* `U` *to* `Tᵢ`.
-- Otherwise, if `Eᵢ` has a type `U` and the corresponding parameter is an input parameter ([§15.6.2.3.2](classes.md#156232-input-parameters)) and `Eᵢ` is an input argument, then an *exact inference* ([§12.6.3.9](expressions.md#12639-exact-inferences)) is made *from* `U` *to* `Tᵢ`.
-- Otherwise, if `Eᵢ` has a type `U` and the corresponding parameter is an input parameter ([§15.6.2.3.2](classes.md#156232-input-parameters)) then a *lower bound inference* ([§12.6.3.10](expressions.md#126310-lower-bound-inferences)) is made *from* `U` *to* `Tᵢ`.
-- Otherwise, no inference is made for this argument.
+For each of the method arguments `Eᵢ`, an input type inference is made from `Eᵢ` to the corresponding parameter type `Tᵢ`.
 
 #### 12.6.3.3 The second phase
 
@@ -798,10 +791,23 @@ An *unfixed* type variable `Xᵢ` *depends directly on* an *unfixed* type varia
 
 `Xₑ` *depends on* `Xᵢ` if `Xₑ` *depends directly on* `Xᵢ` or if `Xᵢ` *depends directly on* `Xᵥ` and `Xᵥ` *depends on* `Xₑ`. Thus “*depends on*” is the transitive but not reflexive closure of “*depends directly on*”.
 
+#### §input-type-inference Input type inferences
+
+An *input type inference* is made *from* an expression `E` *to* a type `T` in the following way:
+
+- If `E` is a tuple expression (§12.8.6) with arity `N` and elements `Eᵢ`, and `T` is a tuple type with arity `N` with a corresponding element type `Tₑ` or `T` is a nullable value type `T0?` and `T0` is a tuple type with arity `N` that has a corresponding element type `Tₑ`, then for each `Eᵢ`, an input type inference is made from `Eᵢ` to `Tₑ`.
+- If `E` is an anonymous function, an *explicit parameter type inference* ([§12.6.3.8](expressions.md#12638-explicit-parameter-type-inferences)) is made *from* `E` *to* `T`
+- Otherwise, if `E` has a type `U` and the corresponding parameter is a value parameter ([§15.6.2.2](classes.md#15622-value-parameters)) then a *lower-bound inference* ([§12.6.3.10](expressions.md#126310-lower-bound-inferences)) is made *from* `U` *to* `T`.
+- Otherwise, if `E` has a type `U` and the corresponding parameter is a reference parameter ([§15.6.2.3.3](classes.md#156233-reference-parameters)), or output parameter ([§15.6.2.3.4](classes.md#156234-output-parameters)) then an *exact inference* ([§12.6.3.9](expressions.md#12639-exact-inferences)) is made *from* `U` *to* `T`.
+- Otherwise, if `E` has a type `U` and the corresponding parameter is an input parameter ([§15.6.2.3.2](classes.md#156232-input-parameters)) and `E` is an input argument, then an *exact inference* ([§12.6.3.9](expressions.md#12639-exact-inferences)) is made *from* `U` *to* `T`.
+- Otherwise, if `E` has a type `U` and the corresponding parameter is an input parameter ([§15.6.2.3.2](classes.md#156232-input-parameters)) then a *lower bound inference* ([§12.6.3.10](expressions.md#126310-lower-bound-inferences)) is made *from* `U` *to* `T`.
+- Otherwise, no inference is made for this argument.
+
 #### 12.6.3.7 Output type inferences
 
-An *output type inference* is made *from* an expression `E` *to* a type T in the following way:
+An *output type inference* is made *from* an expression `E` *to* a type `T` in the following way:
 
+- If `E` is a tuple expression with arity `N` and elements `Eᵢ`, and `T` is a tuple type with arity `N` a corresponding element type `Tₑ` or `T` is a nullable value type `T0?` and `T0` is a tuple type with arity `N` that has a corresponding element type `Tₑ`, then for each `Eᵢ` an output type inference is made from `Eᵢ` to `Tₑ`.
 - If `E` is an anonymous function with inferred return type `U` ([§12.6.3.13](expressions.md#126313-inferred-return-type)) and `T` is a delegate type or expression tree type with return type `Tₓ`, then a *lower-bound inference* ([§12.6.3.10](expressions.md#126310-lower-bound-inferences)) is made *from* `U` *to* `Tₓ`.
 - Otherwise, if `E` is a method group and `T` is a delegate type or expression tree type with parameter types `T₁...Tᵥ` and return type `Tₓ`, and overload resolution of `E` with the types `T₁...Tᵥ` yields a single method with return type `U`, then a *lower-bound inference* is made *from* `U` *to* `Tₓ`.
 - Otherwise, if `E` is an expression with type `U`, then a *lower-bound inference* is made *from* `U` *to* `T`.
@@ -1611,7 +1617,7 @@ A tuple expression has a type if and only if each of its element expressions `Ei
 
 A tuple expression is evaluated by evaluating each of its element expressions in order from left to right.
 
-A tuple value can be obtained from a tuple expression by converting it to a tuple type ([§10.2.13](conversions.md#10213-implicit-tuple-conversions)), by reclassifying it as a value ([§12.2.2](expressions.md#1222-values-of-expressions))) or by making it the target of a deconstructing assignment ([§12.21.2](expressions.md#12212-simple-assignment)).
+A tuple value is obtained from a tuple expression by evaluating it and storing the result in corresponding `System.ValueTuple<...>` type, and initializing each of its fields in order from left to right by evaluating the corresponding tuple element expression of `E`, converting it to the corresponding element type of `T` using the implicit conversion found, and initializing the field with the result.
 
 > *Example*:
 >