[dev.typeparams] cmd/compile: force untyped constants from types2 to expected kind

Currently, types2 sometimes produces constant.Values with a Kind
different than the untyped constant type's Is{Integer,Float,Complex}
info, which irgen expects to always match.

While we mull how best to proceed in #43891, this CL adapts irgen to
types2's current behavior. In particular, fixedbugs/issue11945.go now
passes with -G=3.

Updates #43891.

Change-Id: I24823a32ff49af6045a032d3903dbb55cbec6bef
Reviewed-on: https://go-review.googlesource.com/c/go/+/286652
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/noder/decl.go

@@ -5,6 +5,8 @@
 package noder
 
 import (
+	"go/constant"
+
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/syntax"
@@ -58,7 +60,22 @@ func (g *irgen) constDecl(out *ir.Nodes, decl *syntax.ConstDecl) {
 
 	for _, name := range decl.NameList {
 		name, obj := g.def(name)
-		name.SetVal(obj.(*types2.Const).Val())
+
+		// For untyped numeric constants, make sure the value
+		// representation matches what the rest of the
+		// compiler (really just iexport) expects.
+		// TODO(mdempsky): Revisit after #43891 is resolved.
+		val := obj.(*types2.Const).Val()
+		switch name.Type() {
+		case types.UntypedInt, types.UntypedRune:
+			val = constant.ToInt(val)
+		case types.UntypedFloat:
+			val = constant.ToFloat(val)
+		case types.UntypedComplex:
+			val = constant.ToComplex(val)
+		}
+		name.SetVal(val)
+
 		out.Append(ir.NewDecl(g.pos(decl), ir.ODCLCONST, name))
 	}
 }

test/fixedbugs/issue11945.go

@@ -13,6 +13,10 @@ const (
 	_ = real(0) // from bug report
 	_ = imag(0) // from bug report
 
+	// same as above, but exported for #43891
+	Real0 = real(0)
+	Imag0 = imag(0)
+
 	// if the arguments are untyped, the results must be untyped
 	// (and compatible with types that can represent the values)
 	_ int = real(1)