[release-branch.go1.21] spec: update spec to version at tip

This updates the spec by copying over several recent CLs
describing the new type inference mechanisms.

Fixes #61659.

Change-Id: I750c901e73e0404f782a3632f5cd936e3775ae13
Reviewed-on: https://go-review.googlesource.com/c/go/+/514435
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

1 files changed, 381 insertions, 314 deletions

diff --git a/doc/go_spec.html b/doc/go_spec.html
index c2fa871eaa..d1b8bf2a91 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 14, 2023",
+	"Subtitle": "Version of July 31, 2023",
 	"Path": "/ref/spec"
 }-->
 
@@ -2511,7 +2511,7 @@ type (
 
 <p>
 A type definition creates a new, distinct type with the same
-<a href="#Types">underlying type</a> and operations as the given type
+<a href="#Underlying_types">underlying type</a> and operations as the given type
 and binds an identifier, the <i>type name</i>, to it.
 </p>
 
@@ -4343,7 +4343,7 @@ type parameter list    type arguments    after substitution
 When using a generic function, type arguments may be provided explicitly,
 or they may be partially or completely <a href="#Type_inference">inferred</a>
 from the context in which the function is used.
-Provided that they can be inferred, type arguments may be omitted entirely if the function is:
+Provided that they can be inferred, type argument lists may be omitted entirely if the function is:
 </p>
 
 <ul>
@@ -4351,7 +4351,7 @@ Provided that they can be inferred, type arguments may be omitted entirely if th
 	<a href="#Calls">called</a> with ordinary arguments,
 </li>
 <li>
-	<a href="#Assignment_statements">assigned</a> to a variable with an explicitly declared type,
+	<a href="#Assignment_statements">assigned</a> to a variable with a known type
 </li>
 <li>
 	<a href="#Calls">passed as an argument</a> to another function, or
@@ -4371,7 +4371,7 @@ must be inferrable from the context in which the function is used.
 // sum returns the sum (concatenation, for strings) of its arguments.
 func sum[T ~int | ~float64 | ~string](x... T) T { … }
 
-x := sum                       // illegal: sum must have a type argument (x is a variable without a declared type)
+x := sum                       // illegal: the type of x is unknown
 intSum := sum[int]             // intSum has type func(x... int) int
 a := intSum(2, 3)              // a has value 5 of type int
 b := sum[float64](2.0, 3)      // b has value 5.0 of type float64
@@ -4406,402 +4406,323 @@ For a generic type, all type arguments must always be provided explicitly.
 <h3 id="Type_inference">Type inference</h3>
 
 <p>
-<em>NOTE: This section is not yet up-to-date for Go 1.21.</em>
+A use of a generic function may omit some or all type arguments if they can be
+<i>inferred</i> from the context within which the function is used, including
+the constraints of the function's type parameters.
+Type inference succeeds if it can infer the missing type arguments
+and <a href="#Instantiations">instantiation</a> succeeds with the
+inferred type arguments.
+Otherwise, type inference fails and the program is invalid.
 </p>
 
 <p>
-Missing function type arguments may be <i>inferred</i> by a series of steps, described below.
-Each step attempts to use known information to infer additional type arguments.
-Type inference stops as soon as all type arguments are known.
-After type inference is complete, it is still necessary to substitute all type arguments
-for type parameters and verify that each type argument
-<a href="#Implementing_an_interface">implements</a> the relevant constraint;
-it is possible for an inferred type argument to fail to implement a constraint, in which
-case instantiation fails.
+Type inference uses the type relationships between pairs of types for inference:
+For instance, a function argument must be <a href="#Assignability">assignable</a>
+to its respective function parameter; this establishes a relationship between the
+type of the argument and the type of the parameter.
+If either of these two types contains type parameters, type inference looks for the
+type arguments to substitute the type parameters with such that the assignability
+relationship is satisfied.
+Similarly, type inference uses the fact that a type argument must
+<a href="#Satisfying_a_type_constraint">satisfy</a> the constraint of its respective
+type parameter.
 </p>
 
 <p>
-Type inference is based on
+Each such pair of matched types corresponds to a <i>type equation</i> containing
+one or multiple type parameters, from one or possibly multiple generic functions.
+Inferring the missing type arguments means solving the resulting set of type
+equations for the respective type parameters.
 </p>
 
-<ul>
-<li>
-	a <a href="#Type_parameter_declarations">type parameter list</a>
-</li>
-<li>
-	a substitution map <i>M</i> initialized with the known type arguments, if any
-</li>
-<li>
-	a (possibly empty) list of ordinary function arguments (in case of a function call only)
-</li>
-</ul>
-
-<p>
-and then proceeds with the following steps:
-</p>
-
-<ol>
-<li>
-	apply <a href="#Function_argument_type_inference"><i>function argument type inference</i></a>
-	to all <i>typed</i> ordinary function arguments
-</li>
-<li>
-	apply <a href="#Constraint_type_inference"><i>constraint type inference</i></a>
-</li>
-<li>
-	apply function argument type inference to all <i>untyped</i> ordinary function arguments
-	using the default type for each of the untyped function arguments
-</li>
-<li>
-	apply constraint type inference
-</li>
-</ol>
-
 <p>
-If there are no ordinary or untyped function arguments, the respective steps are skipped.
-Constraint type inference is skipped if the previous step didn't infer any new type arguments,
-but it is run at least once if there are missing type arguments.
-</p>
-
-<p>
-The substitution map <i>M</i> is carried through all steps, and each step may add entries to <i>M</i>.
-The process stops as soon as <i>M</i> has a type argument for each type parameter or if an inference step fails.
-If an inference step fails, or if <i>M</i> is still missing type arguments after the last step, type inference fails.
-</p>
-
-<h4 id="Type_unification">Type unification</h4>
-
-<p>
-Type inference is based on <i>type unification</i>. A single unification step
-applies to a <a href="#Type_inference">substitution map</a> and two types, either
-or both of which may be or contain type parameters. The substitution map tracks
-the known (explicitly provided or already inferred) type arguments: the map
-contains an entry <code>P</code> &RightArrow; <code>A</code> for each type
-parameter <code>P</code> and corresponding known type argument <code>A</code>.
-During unification, known type arguments take the place of their corresponding type
-parameters when comparing types. Unification is the process of finding substitution
-map entries that make the two types equivalent.
-</p>
-
-<p>
-For unification, two types that don't contain any type parameters from the current type
-parameter list are <i>equivalent</i>
-if they are identical, or if they are channel types that are identical ignoring channel
-direction, or if their underlying types are equivalent.
-</p>
-
-<p>
-Unification works by comparing the structure of pairs of types: their structure
-disregarding type parameters must be identical, and types other than type parameters
-must be equivalent.
-A type parameter in one type may match any complete subtype in the other type;
-each successful match causes an entry to be added to the substitution map.
-If the structure differs, or types other than type parameters are not equivalent,
-unification fails.
-</p>
-
-<!--
-TODO(gri) Somewhere we need to describe the process of adding an entry to the
-          substitution map: if the entry is already present, the type argument
-	  values are themselves unified.
--->
-
-<p>
-For example, if <code>T1</code> and <code>T2</code> are type parameters,
-<code>[]map[int]bool</code> can be unified with any of the following:
+For example, given
 </p>
 
 <pre>
-[]map[int]bool   // types are identical
-T1               // adds T1 &RightArrow; []map[int]bool to substitution map
-[]T1             // adds T1 &RightArrow; map[int]bool to substitution map
-[]map[T1]T2      // adds T1 &RightArrow; int and T2 &RightArrow; bool to substitution map
-</pre>
-
-<p>
-On the other hand, <code>[]map[int]bool</code> cannot be unified with any of
-</p>
+// dedup returns a copy of the argument slice with any duplicate entries removed.
+func dedup[S ~[]E, E comparable](S) S { … }
 
-<pre>
-int              // int is not a slice
-struct{}         // a struct is not a slice
-[]struct{}       // a struct is not a map
-[]map[T1]string  // map element types don't match
+type Slice []int
+var s Slice
+s = dedup(s)   // same as s = dedup[Slice, int](s)
 </pre>
 
 <p>
-As an exception to this general rule, because a <a href="#Type_definitions">defined type</a>
-<code>D</code> and a type literal <code>L</code> are never equivalent,
-unification compares the underlying type of <code>D</code> with <code>L</code> instead.
-For example, given the defined type
+the variable <code>s</code> of type <code>Slice</code> must be assignable to
+the function parameter type <code>S</code> for the program to be valid.
+To reduce complexity, type inference ignores the directionality of assignments,
+so the type relationship between <code>Slice</code> and <code>S</code> can be
+expressed via the (symmetric) type equation <code>Slice ≡<sub>A</sub> S</code>
+(or <code>S ≡<sub>A</sub> Slice</code> for that matter),
+where the <code><sub>A</sub></code> in <code>≡<sub>A</sub></code>
+indicates that the LHS and RHS types must match per assignability rules
+(see the section on <a href="#Type_unification">type unification</a> for
+details).
+Similarly, the type parameter <code>S</code> must satisfy its constraint
+<code>~[]E</code>. This can be expressed as <code>S ≡<sub>C</sub> ~[]E</code>
+where <code>X ≡<sub>C</sub> Y</code> stands for
+"<code>X</code> satisfies constraint <code>Y</code>".
+These observations lead to a set of two equations
 </p>
 
 <pre>
-type Vector []float64
+	Slice ≡<sub>A</sub> S      (1)
+	S     ≡<sub>C</sub> ~[]E   (2)
 </pre>
 
 <p>
-and the type literal <code>[]E</code>, unification compares <code>[]float64</code> with
-<code>[]E</code> and adds an entry <code>E</code> &RightArrow; <code>float64</code> to
-the substitution map.
-</p>
-
-<h4 id="Function_argument_type_inference">Function argument type inference</h4>
-
-<!-- In this section and the section on constraint type inference we start with examples
-rather than have the examples follow the rules as is customary elsewhere in spec.
-Hopefully this helps building an intuition and makes the rules easier to follow. -->
-
-<p>
-Function argument type inference infers type arguments from function arguments:
-if a function parameter is declared with a type <code>T</code> that uses
-type parameters,
-<a href="#Type_unification">unifying</a> the type of the corresponding
-function argument with <code>T</code> may infer type arguments for the type
-parameters used by <code>T</code>.
-</p>
-
-<p>
-For instance, given the generic function
+which now can be solved for the type parameters <code>S</code> and <code>E</code>.
+From (1) a compiler can infer that the type argument for <code>S</code> is <code>Slice</code>.
+Similarly, because the underlying type of <code>Slice</code> is <code>[]int</code>
+and <code>[]int</code> must match <code>[]E</code> of the constraint,
+a compiler can infer that <code>E</code> must be <code>int</code>.
+Thus, for these two equations, type inference infers
 </p>
 
 <pre>
-func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number
+	S ➞ Slice
+	E ➞ int
 </pre>
 
 <p>
-and the call
-</p>
-
-<pre>
-var vector []float64
-scaledVector := scale(vector, 42)
-</pre>
-
-<p>
-the type argument for <code>Number</code> can be inferred from the function argument
-<code>vector</code> by unifying the type of <code>vector</code> with the corresponding
-parameter type: <code>[]float64</code> and <code>[]Number</code>
-match in structure and <code>float64</code> matches with <code>Number</code>.
-This adds the entry <code>Number</code> &RightArrow; <code>float64</code> to the
-<a href="#Type_unification">substitution map</a>.
-Untyped arguments, such as the second function argument <code>42</code> here, are ignored
-in the first round of function argument type inference and only considered if there are
-unresolved type parameters left.
+Given a set of type equations, the type parameters to solve for are
+the type parameters of the functions that need to be instantiated
+and for which no explicit type arguments is provided.
+These type parameters are called <i>bound</i> type parameters.
+For instance, in the <code>dedup</code> example above, the type parameters
+<code>P</code> and <code>E</code> are bound to <code>dedup</code>.
+An argument to a generic function call may be a generic function itself.
+The type parameters of that function are included in the set of bound
+type parameters.
+The types of function arguments may contain type parameters from other
+functions (such as a generic function enclosing a function call).
+Those type parameters may also appear in type equations but they are
+not bound in that context.
+Type equations are always solved for the bound type parameters only.
 </p>
 
 <p>
-Inference happens in two separate phases; each phase operates on a specific list of
-(parameter, argument) pairs:
+Type inference supports calls of generic functions and assignments
+of generic functions to (explicitly function-typed) variables.
+This includes passing generic functions as arguments to other
+(possibly also generic) functions, and returning generic functions
+as results.
+Type inference operates on a set of equations specific to each of
+these cases.
+The equations are as follows (type argument lists are omitted for clarity):
 </p>
 
-<ol>
+<ul>
 <li>
-	The list <i>Lt</i> contains all (parameter, argument) pairs where the parameter
-	type uses type parameters and where the function argument is <i>typed</i>.
+	<p>
+	For a function call <code>f(a<sub>0</sub>, a<sub>1</sub>, …)</code> where
+	<code>f</code> or a function argument <code>a<sub>i</sub></code> is
+	a generic function:
+	<br>
+	Each pair <code>(a<sub>i</sub>, p<sub>i</sub>)</code> of corresponding
+	function arguments and parameters where <code>a<sub>i</sub></code> is not an
+	<a href="#Constants">untyped constant</a> yields an equation
+	<code>typeof(p<sub>i</sub>) ≡<sub>A</sub> typeof(a<sub>i</sub>)</code>.
+	<br>
+	If <code>a<sub>i</sub></code> is an untyped constant <code>c<sub>j</sub></code>,
+	and <code>typeof(p<sub>i</sub>)</code> is a bound type parameter <code>P<sub>k</sub></code>,
+	the pair <code>(c<sub>j</sub>, P<sub>k</sub>)</code> is collected separately from
+	the type equations.
+	</p>
 </li>
 <li>
-	The list <i>Lu</i> contains all remaining pairs where the parameter type is a single
-	type parameter. In this list, the respective function arguments are untyped.
+	<p>
+	For an assignment <code>v = f</code> of a generic function <code>f</code> to a
+	(non-generic) variable <code>v</code> of function type:
+	<br>
+	<code>typeof(v) ≡<sub>A</sub> typeof(f)</code>.
+	</p>
 </li>
-</ol>
-
-<p>
-Any other (parameter, argument) pair is ignored.
-</p>
+<li>
+	<p>
+	For a return statement <code>return …, f, … </code> where <code>f</code> is a
+	generic function returned as a result to a (non-generic) result variable
+	<code>r</code> of function type:
+	<br>
+	<code>typeof(r) ≡<sub>A</sub> typeof(f)</code>.
+	</p>
+</li>
+</ul>
 
 <p>
-By construction, the arguments of the pairs in <i>Lu</i> are <i>untyped</i> constants
-(or the untyped boolean result of a comparison). And because <a href="#Constants">default types</a>
-of untyped values are always predeclared non-composite types, they can never match against
-a composite type, so it is sufficient to only consider parameter types that are single type
-parameters.
+Additionally, each type parameter <code>P<sub>k</sub></code> and corresponding type constraint
+<code>C<sub>k</sub></code> yields the type equation
+<code>P<sub>k</sub> ≡<sub>C</sub> C<sub>k</sub></code>.
 </p>
 
 <p>
-Each list is processed in a separate phase:
+Type inference gives precedence to type information obtained from typed operands
+before considering untyped constants.
+Therefore, inference proceeds in two phases:
 </p>
 
 <ol>
 <li>
-	In the first phase, the parameter and argument types of each pair in <i>Lt</i>
-	are unified. If unification succeeds for a pair, it may yield new entries that
-	are added to the substitution map <i>M</i>. If unification fails, type inference
-	fails.
+	<p>
+	The type equations are solved for the bound
+	type parameters using <a href="#Type_unification">type unification</a>.
+	If unification fails, type inference fails.
+	</p>
 </li>
 <li>
-	The second phase considers the entries of list <i>Lu</i>. Type parameters for
-	which the type argument has already been determined are ignored in this phase.
-	For each remaining pair, the parameter type (which is a single type parameter) and
-	the <a href="#Constants">default type</a> of the corresponding untyped argument is
-	unified. If unification fails, type inference fails.
+	<p>
+	For each bound type parameter <code>P<sub>k</sub></code> for which no type argument
+	has been inferred yet and for which one or more pairs
+	<code>(c<sub>j</sub>, P<sub>k</sub>)</code> with that same type parameter
+	were collected, determine the <a href="#Constant_expressions">constant kind</a>
+	of the constants <code>c<sub>j</sub></code> in all those pairs the same way as for
+	<a href="#Constant_expressions">constant expressions</a>.
+	The type argument for <code>P<sub>k</sub></code> is the
+	<a href="#Constants">default type</a> for the determined constant kind.
+	If a constant kind cannot be determined due to conflicting constant kinds,
+	type inference fails.
+	</p>
 </li>
 </ol>
 
 <p>
-While unification is successful, processing of each list continues until all list elements
-are considered, even if all type arguments are inferred before the last list element has
-been processed.
+If not all type arguments have been found after these two phases, type inference fails.
 </p>
 
 <p>
-Example:
+If the two phases are successful, type inference determined a type argument for each
+bound type parameter:
 </p>
 
 <pre>
-func min[T ~int|~float64](x, y T) T
-
-var x int
-min(x, 2.0)    // T is int, inferred from typed argument x; 2.0 is assignable to int
-min(1.0, 2.0)  // T is float64, inferred from default type for 1.0 and matches default type for 2.0
-min(1.0, 2)    // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)
+	P<sub>k</sub> ➞ A<sub>k</sub>
 </pre>
 
 <p>
-In the example <code>min(1.0, 2)</code>, processing the function argument <code>1.0</code>
-yields the substitution map entry <code>T</code> &RightArrow; <code>float64</code>. Because
-processing continues until all untyped arguments are considered, an error is reported. This
-ensures that type inference does not depend on the order of the untyped arguments.
-</p>
-
-<h4 id="Constraint_type_inference">Constraint type inference</h4>
-
-<p>
-Constraint type inference infers type arguments by considering type constraints.
-If a type parameter <code>P</code> has a constraint with a
-<a href="#Core_types">core type</a> <code>C</code>,
-<a href="#Type_unification">unifying</a> <code>P</code> with <code>C</code>
-may infer additional type arguments, either the type argument for <code>P</code>,
-or if that is already known, possibly the type arguments for type parameters
-used in <code>C</code>.
+A type argument <code>A<sub>k</sub></code> may be a composite type,
+containing other bound type parameters <code>P<sub>k</sub></code> as element types
+(or even be just another bound type parameter).
+In a process of repeated simplification, the bound type parameters in each type
+argument are substituted with the respective type arguments for those type
+parameters until each type argument is free of bound type parameters.
 </p>
 
 <p>
-For instance, consider the type parameter list with type parameters <code>List</code> and
-<code>Elem</code>:
+If type arguments contain cyclic references to themselves
+through bound type parameters, simplification and thus type
+inference fails.
+Otherwise, type inference succeeds.
 </p>
 
-<pre>
-[List ~[]Elem, Elem any]
-</pre>
+<h4 id="Type_unification">Type unification</h4>
 
 <p>
-Constraint type inference can deduce the type of <code>Elem</code> from the type argument
-for <code>List</code> because <code>Elem</code> is a type parameter in the core type
-<code>[]Elem</code> of <code>List</code>.
-If the type argument is <code>Bytes</code>:
+Type inference solves type equations through <i>type unification</i>.
+Type unification recursively compares the LHS and RHS types of an
+equation, where either or both types may be or contain bound type parameters,
+and looks for type arguments for those type parameters such that the LHS
+and RHS match (become identical or assignment-compatible, depending on
+context).
+To that effect, type inference maintains a map of bound type parameters
+to inferred type arguments; this map is consulted and updated during type unification.
+Initially, the bound type parameters are known but the map is empty.
+During type unification, if a new type argument <code>A</code> is inferred,
+the respective mapping <code>P ➞ A</code> from type parameter to argument
+is added to the map.
+Conversely, when comparing types, a known type argument
+(a type argument for which a map entry already exists)
+takes the place of its corresponding type parameter.
+As type inference progresses, the map is populated more and more
+until all equations have been considered, or until unification fails.
+Type inference succeeds if no unification step fails and the map has
+an entry for each type parameter.
 </p>
 
-<pre>
-type Bytes []byte
 </pre>
-
-<p>
-unifying the underlying type of <code>Bytes</code> with the core type means
-unifying <code>[]byte</code> with <code>[]Elem</code>. That unification succeeds and yields
-the <a href="#Type_unification">substitution map</a> entry
-<code>Elem</code> &RightArrow; <code>byte</code>.
-Thus, in this example, constraint type inference can infer the second type argument from the
-first one.
-</p>
-
-<p>
-Using the core type of a constraint may lose some information: In the (unlikely) case that
-the constraint's type set contains a single <a href="#Type_definitions">defined type</a>
-<code>N</code>, the corresponding core type is <code>N</code>'s underlying type rather than
-<code>N</code> itself. In this case, constraint type inference may succeed but instantiation
-will fail because the inferred type is not in the type set of the constraint.
-Thus, constraint type inference uses the <i>adjusted core type</i> of
-a constraint: if the type set contains a single type, use that type; otherwise use the
-constraint's core type.
-</p>
-
-<p>
-Generally, constraint type inference proceeds in two phases: Starting with a given
-substitution map <i>M</i>
-</p>
-
-<ol>
-<li>
-For all type parameters with an adjusted core type, unify the type parameter with that
-type. If any unification fails, constraint type inference fails.
-</li>
-
-<li>
-At this point, some entries in <i>M</i> may map type parameters to other
-type parameters or to types containing type parameters. For each entry
-<code>P</code> &RightArrow; <code>A</code> in <i>M</i> where <code>A</code> is or
-contains type parameters <code>Q</code> for which there exist entries
-<code>Q</code> &RightArrow; <code>B</code> in <i>M</i>, substitute those
-<code>Q</code> with the respective <code>B</code> in <code>A</code>.
-Stop when no further substitution is possible.
-</li>
-</ol>
-
-<p>
-The result of constraint type inference is the final substitution map <i>M</i> from type
-parameters <code>P</code> to type arguments <code>A</code> where no type parameter <code>P</code>
-appears in any of the <code>A</code>.
-</p>
-
-<p>
-For instance, given the type parameter list
+For example, given the type equation with the bound type parameter
+<code>P</code>
 </p>
 
 <pre>
-[A any, B []C, C *A]
+	[10]struct{ elem P, list []P } ≡<sub>A</sub> [10]struct{ elem string; list []string }
 </pre>
 
 <p>
-and the single provided type argument <code>int</code> for type parameter <code>A</code>,
-the initial substitution map <i>M</i> contains the entry <code>A</code> &RightArrow; <code>int</code>.
-</p>
-
-<p>
-In the first phase, the type parameters <code>B</code> and <code>C</code> are unified
-with the core type of their respective constraints. This adds the entries
-<code>B</code> &RightArrow; <code>[]C</code> and <code>C</code> &RightArrow; <code>*A</code>
-to <i>M</i>.
-
-<p>
-At this point there are two entries in <i>M</i> where the right-hand side
-is or contains type parameters for which there exists other entries in <i>M</i>:
-<code>[]C</code> and <code>*A</code>.
-In the second phase, these type parameters are replaced with their respective
-types. It doesn't matter in which order this happens. Starting with the state
-of <i>M</i> after the first phase:
-</p>
-
-<p>
-<code>A</code> &RightArrow; <code>int</code>,
-<code>B</code> &RightArrow; <code>[]C</code>,
-<code>C</code> &RightArrow; <code>*A</code>
+type inference starts with an empty map.
+Unification first compares the top-level structure of the LHS and RHS
+types.
+Both are arrays of the same length; they unify if the element types unify.
+Both element types are structs; they unify if they have
+the same number of fields with the same names and if the
+field types unify.
+The type argument for <code>P</code> is not known yet (there is no map entry),
+so unifying <code>P</code> with <code>string</code> adds
+the mapping <code>P ➞ string</code> to the map.
+Unifying the types of the <code>list</code> field requires
+unifying <code>[]P</code> and <code>[]string</code> and
+thus <code>P</code> and <code>string</code>.
+Since the type argument for <code>P</code> is known at this point
+(there is a map entry for <code>P</code>), its type argument
+<code>string</code> takes the place of <code>P</code>.
+And since <code>string</code> is identical to <code>string</code>,
+this unification step succeeds as well.
+Unification of the LHS and RHS of the equation is now finished.
+Type inference succeeds because there is only one type equation,
+no unification step failed, and the map is fully populated.
 </p>
 
 <p>
-Replace <code>A</code> on the right-hand side of &RightArrow; with <code>int</code>:
+Unification uses a combination of <i>exact</i> and <i>loose</i>
+unification depending on whether two types have to be
+<a href="#Type_identity">identical</a>,
+<a href="#Assignability">assignment-compatible</a>, or
+only structurally equal.
+The respective <a href="#Type_unification_rules">type unification rules</a>
+are spelled out in detail in the <a href="#Appendix">Appendix</a>.
 </p>
 
 <p>
-<code>A</code> &RightArrow; <code>int</code>,
-<code>B</code> &RightArrow; <code>[]C</code>,
-<code>C</code> &RightArrow; <code>*int</code>
+For an equation of the form <code>X ≡<sub>A</sub> Y</code>,
+where <code>X</code> and <code>Y</code> are types involved
+in an assignment (including parameter passing and return statements),
+the top-level type structures may unify loosely but element types
+must unify exactly, matching the rules for assignments.
 </p>
 
 <p>
-Replace <code>C</code> on the right-hand side of &RightArrow; with <code>*int</code>:
+For an equation of the form <code>P ≡<sub>C</sub> C</code>,
+where <code>P</code> is a type parameter and <code>C</code>
+its corresponding constraint, the unification rules are bit
+more complicated:
 </p>
 
-<p>
-<code>A</code> &RightArrow; <code>int</code>,
-<code>B</code> &RightArrow; <code>[]*int</code>,
-<code>C</code> &RightArrow; <code>*int</code>
-</p>
+<ul>
+<li>
+	If <code>C</code> has a <a href="#Core_types">core type</a>
+	<code>core(C)</code>
+	and <code>P</code> has a known type argument <code>A</code>,
+	<code>core(C)</code> and <code>A</code> must unify loosely.
+	If <code>P</code> does not have a known type argument
+	and <code>C</code> contains exactly one type term <code>T</code>
+	that is not an underlying (tilde) type, unification adds the
+	mapping <code>P ➞ T</code> to the map.
+</li>
+<li>
+	If <code>C</code> does not have a core type
+	and <code>P</code> has a known type argument <code>A</code>,
+	<code>A</code> must have all methods of <code>C</code>, if any,
+	and corresponding method types must unify exactly.
+</li>
+</ul>
 
 <p>
-At this point no further substitution is possible and the map is full.
-Therefore, <code>M</code> represents the final map of type parameters
-to type arguments for the given type parameter list.
+When solving type equations from type constraints,
+solving one equation may infer additional type arguments,
+which in turn may enable solving other equations that depend
+on those type arguments.
+Type inference repeats type unification as long as new type
+arguments are inferred.
 </p>
 
 <h3 id="Operators">Operators</h3>
@@ -5479,7 +5400,7 @@ in any of these cases:
 	ignoring struct tags (see below),
 	<code>x</code>'s type and <code>T</code> are not
 	<a href="#Type_parameter_declarations">type parameters</a> but have
-	<a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
+	<a href="#Type_identity">identical</a> <a href="#Underlying_types">underlying types</a>.
 	</li>
 	<li>
 	ignoring struct tags (see below),
@@ -7324,7 +7245,8 @@ clear(t)    type parameter    see below
 </pre>
 
 <p>
-If the argument type is a <a href="#Type_parameter_declarations">type parameter</a>,
+If the type of the argument to <code>clear</code> is a
+<a href="#Type_parameter_declarations">type parameter</a>,
 all types in its type set must be maps or slices, and <code>clear</code>
 performs the operation corresponding to the actual type argument.
 </p>
@@ -8290,7 +8212,7 @@ of if the general conversion rules take care of this.
 <p>
 A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code>
 value may not be <a href="#Address_operators">dereferenced</a>.
-Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be
+Any pointer or value of <a href="#Underlying_types">underlying type</a> <code>uintptr</code> can be
 <a href="#Conversions">converted</a> to a type of underlying type <code>Pointer</code> and vice versa.
 The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined.
 </p>
@@ -8438,3 +8360,148 @@ The following minimal alignment properties are guaranteed:
 <p>
 A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory.
 </p>
+
+<h2 id="Appendix">Appendix</h2>
+
+<h3 id="Type_unification_rules">Type unification rules</h3>
+
+<p>
+The type unification rules describe if and how two types unify.
+The precise details are relevant for Go implementations,
+affect the specifics of error messages (such as whether
+a compiler reports a type inference or other error),
+and may explain why type inference fails in unusual code situations.
+But by and large these rules can be ignored when writing Go code:
+type inference is designed to mostly "work as expected",
+and the unification rules are fine-tuned accordingly.
+</p>
+
+<p>
+Type unification is controlled by a <i>matching mode</i>, which may
+be <i>exact</i> or <i>loose</i>.
+As unification recursively descends a composite type structure,
+the matching mode used for elements of the type, the <i>element matching mode</i>,
+remains the same as the matching mode except when two types are unified for
+<a href="#Assignability">assignability</a> (<code>≡<sub>A</sub></code>):
+in this case, the matching mode is <i>loose</i> at the top level but
+then changes to <i>exact</i> for element types, reflecting the fact
+that types don't have to be identical to be assignable.
+</p>
+
+<p>
+Two types that are not bound type parameters unify exactly if any of
+following conditions is true:
+</p>
+
+<ul>
+<li>
+	Both types are <a href="#Type_identity">identical</a>.
+</li>
+<li>
+	Both types have identical structure and their element types
+	unify exactly.
+</li>
+<li>
+	Exactly one type is an <a href="#Type_inference">unbound</a>
+	type parameter with a <a href="#Core_types">core type</a>,
+	and that core type unifies with the other type per the
+	unification rules for <code>≡<sub>A</sub></code>
+	(loose unification at the top level and exact unification
+	for element types).
+</li>
+</ul>
+
+<p>
+If both types are bound type parameters, they unify per the given
+matching modes if:
+</p>
+
+<ul>
+<li>
+	Both type parameters are identical.
+</li>
+<li>
+	At most one of the type parameters has a known type argument.
+	In this case, the type parameters are <i>joined</i>:
+	they both stand for the same type argument.
+	If neither type parameter has a known type argument yet,
+	a future type argument inferred for one the type parameters
+	is simultaneously inferred for both of them.
+</li>
+<li>
+	Both type parameters have a known type argument
+	and the type arguments unify per the given matching modes.
+</li>
+</ul>
+
+<p>
+A single bound type parameter <code>P</code> and another type <code>T</code> unify
+per the given matching modes if:
+</p>
+
+<ul>
+<li>
+	<code>P</code> doesn't have a known type argument.
+	In this case, <code>T</code> is inferred as the type argument for <code>P</code>.
+</li>
+<li>
+	<code>P</code> does have a known type argument <code>A</code>,
+	<code>A</code> and <code>T</code> unify per the given matching modes,
+	and one of the following conditions is true:
+	<ul>
+	<li>
+		Both <code>A</code> and <code>T</code> are interface types:
+		In this case, if both <code>A</code> and <code>T</code> are
+		also <a href="#Type_definitions">defined</a> types,
+		they must be <a href="#Type_identity">identical</a>.
+		Otherwise, if neither of them is a defined type, they must
+		have the same number of methods
+		(unification of <code>A</code> and <code>T</code> already
+		established that the methods match).
+	</li>
+	<li>
+		Neither <code>A</code> nor <code>T</code> are interface types:
+		In this case, if <code>T</code> is a defined type, <code>T</code>
+		replaces <code>A</code> as the inferred type argument for <code>P</code>.
+	</li>
+	<li>
+		In all other cases unification of <code>P</code> and <code>T</code> fails.
+	</li>
+	</ul>
+</li>
+</ul>
+
+<p>
+Finally, two types that are not bound type parameters unify loosely
+(and per the element matching mode) if:
+</p>
+
+<ul>
+<li>
+	Both types unify exactly.
+</li>
+<li>
+	One type is a <a href="#Type_definitions">defined type</a>,
+	the other type is a type literal, but not an interface,
+	and their underlying types unify per the element matching mode.
+</li>
+<li>
+	Both types are interfaces (but not type parameters) with
+	identical <a href="#Interface_types">type terms</a>,
+	both or neither embed the predeclared type
+	<a href="#Predeclared_identifiers">comparable</a>,
+	corresponding method types unify per the element matching mode,
+	and the method set of one of the interfaces is a subset of
+	the method set of the other interface.
+</li>
+<li>
+	Only one type is an interface (but not a type parameter),
+	corresponding methods of the two types unify per the element matching mode,
+	and the method set of the interface is a subset of
+	the method set of the other type.
+</li>
+<li>
+	Both types have the same structure and their element types
+	unify per the element matching mode.
+</li>
+</ul>