compiler: fix ranging over maps with particular map types

Some map keys are hard to compare, such as floats. They are stored as if
the map keys are of interface type instead of the key type itself. This
makes working with them in the runtime package easier: they are compared
as regular interfaces.

Iterating over maps didn't care about this special case though. It just
returns the key, value pair as it is stored in the map. This is buggy,
and this commit fixes this bug.

compiler/interface.go

@@ -31,6 +31,15 @@ func (b *builder) createMakeInterface(val llvm.Value, typ types.Type, pos token.
 	return itf
 }
 
+// extractValueFromInterface extract the value from an interface value
+// (runtime._interface) under the assumption that it is of the type given in
+// llvmType. The behavior is undefied if the interface is nil or llvmType
+// doesn't match the underlying type of the interface.
+func (b *builder) extractValueFromInterface(itf llvm.Value, llvmType llvm.Type) llvm.Value {
+	valuePtr := b.CreateExtractValue(itf, 1, "typeassert.value.ptr")
+	return b.emitPointerUnpack(valuePtr, []llvm.Type{llvmType})[0]
+}
+
 // getTypeCode returns a reference to a type code.
 // It returns a pointer to an external global which should be replaced with the
 // real type in the interface lowering pass.
@@ -416,8 +425,7 @@ func (b *builder) createTypeAssert(expr *ssa.TypeAssert) llvm.Value {
 	} else {
 		// Type assert on concrete type. Extract the underlying type from
 		// the interface (but only after checking it matches).
-		valuePtr := b.CreateExtractValue(itf, 1, "typeassert.value.ptr")
-		valueOk = b.emitPointerUnpack(valuePtr, []llvm.Type{assertedType})[0]
+		valueOk = b.extractValueFromInterface(itf, assertedType)
 	}
 	b.CreateBr(nextBlock)