go/types, types2: unify core types for unbound type parameters

go/types, types2: unify core types for unbound type parameters

NOTE: Should this change cause problems, the new functionality
can be disabled by setting the flag enableCoreTypeUnification
in unify.go to false.

In the code

func f1[M1 map[K1]int, K1 comparable](m1 M1) {}

func f2[M2 map[K2]int, K2 comparable](m2 M2) {
	f1(m2)
}

type inference attempts to unify the types of m1 and m2. This leads
to the unification attempt of M1 and M2. The result is that the type
argument for M1 is inferred to be M2. Since there is no furter function
argument to use, constraint type inference attempts to infer the type
for K1 which is still missing. Constraint type inference (inferB in
the trace below) compares the inferred type for M1 (i.e., M2) against
map[K1]int. M2 is bound to f2, not f1; with the existing algorithm
that means M2 is simply a named type without further information.
Unification fails and with that type inference, and the type checker
reports an error.

-- inferA [M1₁, K1₂] ➞ []
M1₁ ≡ M2₃
.  M1₁ ➞ M2₃
-- inferB [M1₁, K1₂] ➞ [M2₃, <nil>]
M1₁ ➞ M2₃
M1₁ ≡ map[K1₂]int
.  M2₃ ≡ map[K1₂]int
.  M2₃ ≢ map[K1₂]int
M1₁ ≢ map[K1₂]int
=> inferB [M1₁, K1₂] ➞ []
=> inferA [M1₁, K1₂] ➞ []

With this change, when attempting to unify M2 with map[K1]int,
rather than failing, the unifier now considers the core type of
M2 which is map[K2]int. This leads to the unification of K1 and
K2; so type inference successfully infers M2 for M1 and K2 for K1.

-- inferA [M1₁, K1₂] ➞ []
M1₁ ≡ M2₃
.  M1₁ ➞ M2₃
-- inferB [M1₁, K1₂] ➞ [M2₃, <nil>]
M1₁ ➞ M2₃
M1₁ ≡ map[K1₂]int
.  M2₃ ≡ map[K1₂]int
.  .  core M2₃ ≡ map[K1₂]int
.  .  map[K2₄]int ≡ map[K1₂]int
.  .  .  K2₄ ≡ K1₂
.  .  .  .  K1₂ ➞ K2₄
.  .  .  int ≡ int
=> inferB [M1₁, K1₂] ➞ [M2₃, K2₄]
=> inferA [M1₁, K1₂] ➞ [M2₃, K2₄]

The fix for this issue was provided by Rob Findley in CL 380375;
this change is a copy of that fix with some additional changes:

- Constraint type inference doesn't simply use a type parameter's
  core type. Instead, if the type parameter type set consists of
  a single, possibly named type, it uses that type. Factor out the
  existing code into a new function adjCoreType. This change is not
  strictly needed but makes it easier to think about the code.

- Tracing code is added for debugging type inference. All tracing
  code is guarded with the flag traceEnabled which is set to false
  by default.

- The change to the unification algorithm is guarded with the flag
  enableCoreTypeUnification.

- The sprintf function has a new type switch case for lists of
  type parameters. This is used for tracing output (and was also
  missing for a panic that was printing type parameter lists).

Fixes #50755.

Change-Id: Ie50c8f4540fcd446a71b07e2b451a95339b530ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/385354
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>